A tungsten boride cermet and the preparing thereof

Abstract

There is provided a cermet, and method for preparing a cermet comprising precipitates of tungsten boride in a metal binder, the method comprising: reacting elemental tungsten with elemental boron in an exothermic reaction within a melt of a metallic flux to form the precipitates of tungsten boride. The metallic flux mediates a self-propagating exothermic reaction between the elemental tungsten and the elemental boron during said reacting. The metallic flux is a metal selected from the group consisting of: Fe, Sn, Cu, Cr, Ta, Hf, and Ti, or a combination thereof, and the metal binder of the cermet is formed from the metal of the metallic flux.

Description

TECHNICAL FIELD

[0001] The present invention relates to a cermet comprising tungsten diboride in a metal matrix, a method for preparing the cermet, and a use of the cermet.BACKGOUND

[0002] Cermets combines the attractive properties of ceramics, such as high temperature resistance and hardness, with those of metals, such as the ability to undergo plastic deformation.

[0003] Cermets comprising tungsten borides have for a long time been recognized as an excellent material for the use in radiation shields as they offer both excellent heat and oxidation resistance as well as absorption of both neutron and high energy electromagnetic radiation. Radiation shields are typically comprised of relatively large cermet plates. The manufacturing of such bulk objects is both difficult and costly. Furthermore, the current manufacturing route known in the art involves the intermixing of pre-synthesized tungsten boride powders with powders of a metal and a subsequent consolidation thereof. These manufacturing methods are not only costly, but further, they provide radiation shielding materials which are susceptible to neutron induced swelling and associated embrittlement.

[0004] Hence, there is a need for new technology for the manufacturing of cermet materials which are more economically viable, and further provides cermets which are less susceptible to neutron induced swelling.SUMMARY

[0005] It is an object of the present invention to provide a solution to at least some of the problems associated with the manufacturing of cermets comprising tungsten boride in a metal binder.

[0006] It is a further object of the invention to provide a tungsten boride cermet which is less susceptible to neutron swelling.

[0007] The invention is set out in the appended set of claims. This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description.

[0008] According to a first aspect, the present invention relates to a method for preparing a cermet comprising precipitates of tungsten boride in a metal binder, the method comprising: reacting elemental tungsten with elemental boron in an exothermic reaction within a melt of a metallic flux to form the precipitates of tungsten boride, wherein the metallic flux mediates a self-propagating exothermic reaction between the elemental tungsten and the elemental boron during said reacting; wherein the metallic flux is a metal selected from the group consisting of: Fe, Sn, Cu, Cr, Ta, Hf, and Ti, or a combination thereof; and wherein the metal binder of the cermet is formed from the metal of the metallic flux.

[0009] Hence, there is provided a method wherein tungsten and boron react with one another to form tungsten boride, which formed tungsten boride is simultaneously bound in a metal binder formed from the metal of the metallic flux. To the inventors' surprise, the method further provides a manufacturing route involving a self-propagating exothermic reaction between tungsten and boron, which reaction is enabled by allowing the tungsten and the boron to react and form tungsten borides within the melt of the metallic flux. The dynamics of this self-propagating exothermic reaction may be explained as follows: First, the metallic flux is at least locally molten. Then, boron dissolves into the molten metallic flux, and form a solution with the molten metallic flux. This results in lowering of the solidus / liquidus of the molten metallic flux. Next, the molten metallic flux, now in solution with boron, wets the tungsten, thereby initiating an exothermic reaction between tungsten and boron and forming tungsten boride. The heat generated by the reaction between the tungsten and the boron in solution with the metallic flux is sufficient to cause a further melting of adjacent unmolten metallic flux, hence the reaction propagates. A reaction between boron and tungsten alone, i.e., without the metallic flux, is not sufficiently exothermic to cause a self-propagating reaction. Hence, it is only by the presence of the metallic flux, and by the forming of a solution between the boron and the metallic flux according to the above, that a self-propagating exothermic reaction can be achieved. Accordingly, it is essential that the metallic flux allows for enough boron to be solved therein, i.e., the metallic flux must have a suitable solubility of boron for the sustaining of the self-propagating exothermic reaction. Correspondingly, it is essential that a sufficient amount of boron is available to form such solution. The relative amount of elemental tungsten to elemental boron may be in a range of 80:20 - 95:5, preferably 85:15 - 90:10, most preferably 87:13, for tungsten and boron respectively, in wt%. Meanwhile, the amount of metallic flux with respect to the total amount of tungsten and boron may be in a range of 1-20 wt%, preferably 1-10 wt%, most preferred 1-5 wt%.

[0010] A further surprising effect realised by the inventors was that the cermet derived from the method according to the first aspect displayed a microstructure comprising sub-micron precipitates of tungsten borides. By the meaning of sub-micron, it is here meant that the precipitates have at least one extension being in the sub-micron range. While the specifics of the microstructure of the cermet will be discussed later, it is worth to emphasise the reason for its formation in the contexts of the inventive method. The reaction between the tungsten and the boron being in solution with the metallic flux results in a rapidly advancing reaction front during the step of reacting. The term reaction front may be defined as a region between mostly unreacted material and mostly reacted material. An associated rapid cooling of the material after the reaction front has passed, sets the conditions for the formation of tungsten boride precipitates in the sub-micron range.

[0011] The method for preparing the cermet may further comprise a step of obtaining a reaction mixture by mixing powders of the elemental tungsten, the elemental boron, and the metallic flux. The step of preparing the reaction mixture may comprise the step of providing and weighing a respective amount of elemental tungsten, and the elemental boron. The respective amounts of the provided elemental tungsten and elemental boron may be determined by a stoichiometric relationship. This stochiometric relationship may be based on a desired formation of specific tungsten boride phases, for example, WB 4 and / or WB 2-x may stoichiometrically correspond to relative amounts of elemental tungsten and boron according to a ratio of 87:13, for tungsten and boron respectively, in wt%. As stated earlier, the relative amount of elemental tungsten to elemental boron may be in a range of 80:20 - 95:5, preferably 85:15 - 90:10, most preferably 87:13, for tungsten and boron respectively, in wt%. Meanwhile the relative amount of metallic flux of the reaction mixture, taken as a whole, may be in a range of 1-20 wt%, preferably 1-10 wt%, most preferred 1-5 wt%. The mixing may be achieved by any means known in the art. Said means of mixing may include but is not limited to the use of a ball mill, tumbler or a mortar and pestle. Generally, mixing of the boron, the tungsten and the metallic flux is beneficial for achieving a complete and uniform reaction.

[0012] The method for preparing the cermet may further comprise a step of compressing the reaction mixture into a compact, preferably the compressing is performed at a pressure in a range of 10-200 MPa. This step may also be referred to as briquetting of the reaction mixture. The compact will typically have a cylindrical shape. However, other shapes are possible. Said step of compressing the reaction mixture may be achieved by any means known to the person skilled in the art including but not limited to the use of a die and a plunger together with a hydraulic press. Compressing the reaction mixture into a compact may be beneficial as the associated consolidation of the reaction mixture may promote a more stable propagation of the reaction front during reacting.

[0013] The step of reacting may be performed by heating said reaction mixture and / or compact, thereby igniting said self-propagating reaction. The heating may be performed at a temperature in a range of 1000-1200°C, preferably in a range of 1050-1150°C, more preferably around 1100°C. In some embodiments said heating may be uniform, such that the entire reaction mixture or compact is uniformly heated to ignite said reacting. However, the heating can also be local, such that only a portion of the reaction mixture or compact is heated. Local heating may be achieved by, for example, a focused laser. In the case of local heating, the reaction will propagate throughout the reaction mixture or compact, i.e., self-propagate, due to the self-sustaining exothermic nature of the reaction. Heating may further be achieved by Vacuum Induction Melting (VIM). In a preferred embodiment, said step of reacting may be achieved by heating said reaction mixture and / or compact in an inert or evacuated atmosphere, thereby igniting said self-propagating exothermic reaction. Heating in an inert and / or evacuated atmosphere may also, for example, be achieved by Vacuum Induction Melting (VIM). Preforming the step of heating in an inter and / or evacuated atmosphere reduce the risk for the formation of oxides or nitrides during said step of reacting.

[0014] In some embodiments of the method, the elemental boron is isotopically enriched in 10< B. For example, the elemental boron may be isotopically enriched to comprise 50%, 75%, 95%, or 100% 10< B. Isotopically enriched boron is particularly desirable when preparing a cermet which is to be used as a neutron radiation shielding material.

[0015] As a subsequent step to the step of reacting, the method for preparing the cermet may further comprise a step of crushing the prepared cermet into a cermet powder. Said crushing may be performed by any means known to the skilled person, including but not limited to the use of grinders, bead mills, ball mills, mortar / pestles, jet-milling, or planetary milling. The cermet may be crushed into a powder, having for example a mean particle size in the range of 10-500 µm, 10-250 µm, or 10-100 µm.

[0016] Some embodiments of the method may further comprise the step of consolidating the cermet powder using sintering or hot-pressing. The consolidating may be achieved by any means known to the skilled person, including but not limited to sintering, spark plasma sintering, hot pressing, or hot isostatic pressing. The step of consolidating allows large, bulk cermets, for example radiation shields to be prepared. The sub-micron precipitates of the cermet powder are preserved during the consolidation step. Accordingly, engineering objects with dimensions of 10-100 cm can be manufactured, which have 10-1000 nm-sized nanostructures uniformly distributed throughout their cross-section.

[0017] According to a second aspect of the invention, there is provided a cermet comprising tungsten boride precipitates in a metal binder, wherein the tungsten boride precipitates display a needle-like morphology, and wherein the metal binder is selected from the group consisting of: Fe, Sn, Cu, Cr, Ta, Hf, and Ti, or a combination thereof. The tungsten boride precipitates may be at least one of WB 2-x , WB 1.75 or WB 4 . It should be understood that WB 2-x , which is the now scientifically recognized formula for phases previously referred to as W 2 B 5 , W 2 B 4 or W 2 B 4+x , is to be interpreted as equivalent to W 2 B 5 , W 2 B 4 and W 2 B 4+x . The cermet may comprise tungsten boride precipitates in an amount of 80-99 wt%, preferably 90-99 wt%, most preferably 95-99 wt%, the rest being metal binder. Preferably, the cermet material is free of any elemental tungsten or any nitride. Accordingly, it is preferred that the respective amounts of elemental boron and tungsten provided to the reaction mixture has a stoichiometric relationship corresponding to at least one of WB 2-x , WB 1.75 or WB 4 or a combination thereof.

[0018] The tungsten boride precipitates may have a needle-like morphology. By the meaning of needle-like, it is here meant that a primary extension of a precipitate, i.e., the longest extension of the precipitate, is much longer than an associated secondary extension of the precipitate, i.e., the shortest extension of the precipitate. Typically, the secondary extension is perpendicular to the primary extension. Hence, the word needle-like is used to emphasise the large aspect ratios between the primary and secondary extensions of the tungsten boride precipitates of the cermet. A majority of the tungsten boride precipitates may have aspect ratios between their respective primary and associated secondary extensions being in the range of 1:3-1:50, 1:6-1:50, or 1:10-1:50. The secondary extension of the majority of the precipitates may for example be in a range of 10-500 nm, or 10-1000 nm. For example, a tungsten boride precipitate may have a primary extension of 800 nm and have secondary extension of 40 nm, resulting in an aspect ratio of 1:20. Said needle-like morphology of the tungsten boride precipitates is a direct consequence of the reaction kinetics resulting from the inventive method according to the first aspect of the invention. The needle-like morphology just described strongly influences the material properties of the cermet. For example, the sub-micron tungsten boride precipitates, i.e., the nano-structured needles of tungsten boride, vastly increases the number of grain boundaries in the material. This significantly increases the cermet's resistance to neutron induced radiation swelling. Further to the increase in resistance to radiation swelling, the needle like morphology result in a material with superior mechanical properties such as hardness. In some embodiments of the provided cermet, the tungsten boride precipitates are comprised of boron which is isotopically enriched in 10< B. Such isotopic enrichment in 10< B improves the radiation shielding properties of the cermet. The cermet may be tailored to comprise mostly one of WB 2-x , WB 1.75 or WB 4 , of which WB 2-x is preferred for at least radiation shielding applications. This is because WB 2-x offers one of the best ratios between the respective number of tungsten and boron atoms. Said tailoring may be achieved controlling the respective amounts of tungsten and boron which are available to react with one another during the preparing of the cermet. Formation of WB 2-x is associated with a relative amount of tungsten and boron according to a ratio of 87:13, for tungsten and boron respectively, in wt%.

[0019] According to a third aspect of the invention, there is provided a use of a cermet according to the second aspect as a radiation shielding material. However, the cermet according to the second aspect may have further applications of use, for example as a hard coating, a wear-resistant tool or die, and as bulletproof armour.

[0020] A further scope of applicability will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples are given by way of illustration only.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The above and other aspects will now be described in more detail, with reference to appended figures. The figures should not be considered limiting; instead, they are used for explaining and understanding. Fig. 1 Shows a flow diagram of the method according to the first aspect of the invention. Fig. 2 Shows a scanning electron micrograph of a cermet obtained after a step of reacting according to the first aspect of the invention. Fig. 3 Shows a scanning electron micrograph of a cermet obtained after a step of consolidating according to the first aspect of the invention. DETAILED DESCRIPTION

[0022] It is to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It must be noted that, as used in the specification and the appended claims, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements unless the context clearly dictates otherwise. Thus, for example, reference to "precipitate" or "the precipitate" may include several precipitates, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps.

[0023] Fig. 1 shows the method for preparing a cermet comprising precipitates of tungsten boride in a metal binder according to the first aspect of the invention. The method comprises a step of reacting S1 elemental tungsten with elemental boron in an exothermic reaction within a melt of a metallic flux to form the precipitates of tungsten boride. The metallic flux mediates a self-propagating exothermic reaction between the elemental tungsten and the elemental boron during said step of reacting S1. By the meaning of self propagating, it is here meant that the reaction, once started, propagates at its own volition and without the requirement of external heating.

[0024] The metallic flux is a metal selected from the group consisting of: Fe, Sn, Cu, Cr, Ta, Hf, and Ti, or a combination thereof, and wherein the metal binder of the cermet is formed from the metal of the metallic flux. The metallic flux may be present in the reaction S1 in elemental form, i.e., unalloyed, or it may be alloyed. For example, the metallic flux may be an alloy between Cu and Sn, i.e., bronze.

[0025] The reacting S1 between tungsten and boron forms tungsten boride precipitates in the melt of the metallic flux, hence the precipitates of tungsten boride are both formed and bound in the metal binder formed from the metal of the metallic flux in one and the same step of the method, i.e., the step of reacting S1.

[0026] A reaction between boron and tungsten alone, i.e., without the presence of the metallic flux is not sufficiently exothermic to cause a self-propagating reaction. Instead, the reacting in the art between tungsten and boron typically involves external heating of the tungsten and boron for prolonged periods of time. Accordingly, it is only by the presence of the metallic flux in the step of reacting S1 according to the invention that a self-propagating reaction can be achieved.

[0027] Accordingly, the metallic flux serves at least two purposes, first the metallic flux mediates a self-propagating exothermic reaction between tungsten and boron, and then the metallic flux forms the binder of the cermet. By the meaning of the word 'mediates' it is here meant that, it is only by the presence of metallic flux that a self-propagating reaction can be achieved in the step of reacting S1. Hence the metallic flux can also be said to enable a self-propagating reaction between the tungsten and the boron.

[0028] The dynamics of the self-propagating exothermic reaction in the step of reacting S1 may be explained as follows: First, the metallic flux is at least locally molten. Boron dissolves into the molten metallic flux and forms a solution with the metallic flux. This forming of a solution with boron lowers the solidus / liquidus of the molten metallic flux. Furthermore, the molten metallic flux, now in a solution with boron, wets the tungsten, thereby initiating an exothermic reaction between tungsten and boron, thereby forming tungsten boride. The heat generated by the reaction between the tungsten and the boron in solution with the metallic flux is sufficient to cause a further melting of adjacent unmolten metallic flux, hence the reaction propagates. Hence, the self propagating nature of the reaction is enabled by a forming of a solution between the metallic flux and the boron, and a subsequent wetting of the tungsten by the formed solution. Accordingly, it is essential that the metallic flux in the step of reacting S1 allows for enough boron to be solved therein, i.e., the metallic flux must have a suitable solubility of boron for the sustaining of the self-propagating exothermal reaction. Correspondingly, it is essential that a sufficient amount of boron, relative to the amount of tungsten, is available to form such solution. For example, a ratio between tungsten and boron may be at least 5:95, for boron and tungsten respectively, in wt%. The relative amount of elemental tungsten to elemental boron in the step of reacting S1 may be in a range of 80:20 - 95:5, preferably 85:15 - 90:10, most preferably 87:13, for tungsten and boron respectively, in wt%. Meanwhile, the amount of metallic flux with respect to the tungsten and the boron may be in a range of 1-20 wt%, preferably 1-10 wt%, most preferred 1-5 wt%.

[0029] In some embodiments of the method, the elemental boron is isotopically enriched in 10< B. For example, the elemental boron may be isotopically enriched to comprise 50%, 75%, 95%, or 100% 10< B.

[0030] Fig 1. also shows that the method according to the invention may comprise a step of obtaining a reaction mixture S0 by mixing powders of the elemental tungsten, the elemental boron, and the metallic flux. The step of preparing the reaction mixture S0 may comprise the step of providing and weighing a respective amount of elemental tungsten, and the elemental boron. The respective amounts of the provided elemental tungsten and elemental boron may be determined by a stoichiometric relationship. This stochiometric relationship may be based on a desired formation of specific tungsten boride phases. For example, WB 4 and / or WB 2-x may stoichiometrically correspond relative amounts of elemental tungsten and boron according to a ratio of 87:13, for tungsten and boron respectively, in wt%. As stated earlier, the relative amount of elemental tungsten to elemental boron may be in a range of 80:20 - 95:5, preferably 85:15 - 90:10, most preferably 87:13, for tungsten and boron respectively, in wt%. Meanwhile the relative amount of metallic flux of the reaction mixture, taken as a whole, may be in a range of 1-20 wt%, preferably 1-10 wt%, most preferred 1-5 wt%. The mixing may be achieved by any means known in the art. Said means of mixing may include but is not limited to the use of a ball mill, tumbler or a mortar and pestle. Generally, mixing of the boron, the tungsten and the metallic flux is beneficial for achieving a complete and uniform reaction.

[0031] Fig. 1 also shows that the method for preparing the cermet may further comprise a step of compressing S0a the reaction mixture into a compact, preferably the compressing S0a is performed at a pressure in a range of 10-200 MPa. The step of compressing S0a may also be referred to as briquetting of the reaction mixture. The compact will typically have a cylindrical shape. However, other shapes are possible. Said step of compressing S0a the reaction mixture may be achieved by any means known to the person skilled in the art including but not limited to the use of a die and a plunger together with a hydraulic press.

[0032] Fig. 1 also shows that the method according to the invention may further comprise a step of heating S0b the reaction mixture. Said heating S0b may be performed by heating S0b said reaction mixture and / or compact in an inert or evacuated atmosphere, thereby igniting said self-propagating reaction by melting at least a portion of the metallic flux, thereby forming a solution with boron, which enables the reacting S1 between tungsten and boron. Typically, it is to be expected that a melting point of the metallic flux is lowered by the presence of the elemental boron due to a diffusion of boron into the metallic flux prior to its melting. As such the melting of at least said portion of the metallic flux may occur at temperatures which is lower than a temperature which is typically associated with the melting of the element or alloy from which the metallic flux is comprised.

[0033] Preferably the heating S0b is performed at a temperature in a range of 1000-1200°C, preferably in a range of 1050-1150°C, preferably around 1100°C. In some embodiments said heating S0b may be uniform, such that the entire reaction mixture or compact is uniformly heated to ignite said reacting. However, the heating S0b may also be local, such that only a portion of the reaction mixture or compact is heated. Local heating S0b may be achieved by, for example, a focused laser. In the case of local heating, the reaction will propagate throughout the reaction mixture or compact, i.e., self-propagate, due to the self-sustaining exothermic nature of the reaction. Heating S0b may for example be achieved by Vacuum Induction Melting (VIM). In a preferred embodiment, said step of reacting S1 is achieved by heating S0b said reaction mixture or compact in an inert or evacuated atmosphere, thereby igniting said self-propagating exothermic reaction. Heating S0b in an inert or evacuated atmosphere may also, for example, be achieved by Vacuum Induction Melting (VIM).

[0034] Fig. 1 also shows that the method for preparing the cermet may further comprise a step of crushing S2 the prepared cermet into a cermet powder. Said crushing S2 may be performed by the any means known to the skilled person, including but not limited to the use of grinders, bead mills, ball mills, mortar / pestles, jet-milling, or planetary milling. The cermet may be crushed into a powder, having for example a mean particle size in the range of 10-500 µm, 10-250 µm, or 10-100 µm.

[0035] Some embodiments of the method may further comprise the step of consolidating S3 the cermet powder using sintering or hot-pressing. The consolidating S3 may be achieved by any means known to the skilled person, including but not limited to sintering, spark plasma sintering, hot pressing, or hot isostatic pressing. The step of consolidating S3 allows large, bulk cermets, for example radiation shields to be prepared. The sub-micron precipitates of the tungsten borides, including their needle-like morphology, derived from the strep of reacting S1, are preserved during the step of consolidation S3. Accordingly, engineering objects with dimensions of 10-100 cm can be manufactured, which have 10-1000 nm-sized nanostructures uniformly distributed throughout their cross-section.

[0036] Fig. 2 shows a cermet powder particle after the step of crushing S3 comprising tungsten boride precipitates in a metal binder according to the second aspect of the invention. The metal binder may be selected from the group consisting of: Fe, Sn, Cu, Cr, Ta, Hf, and Ti, or a combination thereof, in the present example Cu-Sn (bronze).

[0037] Fig. 2 further shows that a majority of the tungsten boride precipitates display a needle-like morphology when viewed in a two-dimensional plane, i.e., in a cross-sectional image. By the meaning of needle-like, it is here meant that a primary extension of a precipitate, i.e., the longest extension of the precipitate, is much longer than an associated secondary extension of the precipitate, i.e., the shortest extension of the precipitate. Hence, the word needle-like is used to emphasize the large aspect ratios between the primary and secondary extensions of the tungsten boride precipitates of the cermet. A majority of the tungsten boride precipitates may have aspect ratios between their respective primary and associated secondary extensions being in the range of 1:3-1:50, 1:6-1:50, or 1:10-1:50. The secondary extension of the majority of the precipitates may for example be in a range of 10-500 nm, or 10-1000 nm. For example, a tungsten boride precipitate may have a primary extension of 800 nm and have secondary extension of 40 nm, resulting in an aspect ratio of 1:20.

[0038] Said needle-like morphology of the tungsten boride precipitates is a direct consequence of the reaction kinetics resulting from the inventive method according to the first aspect of the invention. The needle-like morphology shown in Fig. 2 strongly influences the material properties of the cermet. For example, the sub-micron tungsten boride precipitates, i.e., the nano-structured needles of tungsten boride, vastly increases the number of grain boundaries in the material. This significantly increases the cermet's resistance to neutron induced radiation swelling.

[0039] The tungsten boride precipitates may be at least one of WB 2-x , WB 1.75 or WB 4 . Furthermore, the cermet may comprise tungsten boride precipitates in an amount of 80-99 wt%, preferably 90-99 wt%, most preferably 95-99 wt%, the rest being metal binder. Preferably, the cermet material is free of any elemental tungsten or any nitride. Accordingly, it is preferred that the respective amounts of elemental boron and tungsten provided to the reaction mixture has a stoichiometric relationship corresponding to at least one of WB 2-x , WB 1.75 or WB 4 or a combination thereof. In some embodiments of the provided cermet, the tungsten boride precipitates are comprised of boron which is isotopically enriched in 10< B. Such isotopic enrichment in 10< B improves the radiation shielding properties of the cermet.

[0040] According to the third aspect of the invention, there is provided a use of a cermet according to the second aspect as a radiation shielding material. However, the cermet according to the second aspect may have further applications of use, for example as a hard coating, a wear-resistant tool or die, and as bulletproof armour.EXAMPLESExample 1

[0041] Powders of tungsten and boron, in a ratio of 87 wt% W and 13 wt% B were mixed with a metallic flux powder, in the present example bronze (Cu-Sn) powder, according to a ratio of 90 wt% (W+B) and 10 wt% bronze powder, thereby obtaining a reaction mixture.

[0042] The reaction mixture was heated in a graphite crucible inside a VIM at 1100°C to trigger a self-propagating exothermic reaction. Fig 2 shows a micrograph of a cermet particle acquired from the step of reacting. The cermet particles comprised sub-micron structures i.e., needles, laths and particles of tungsten boride, within a bronze matrix / binder. The acquired cermet was then consolidated by hot pressing and sintering at 1100°C to form a consolidated cermet. Fig. 3 shows a cross-sectional micrograph of the consolidated cermet, which micrograph shows that the sub-micron structure of the tungsten boride was retained during the step of consolidating. The consolidated cermet material had a high hardness (>1500 HV10), good oxidation resistance at 800°C in air, good corrosion resistance in salt water, and non-magnetic properties.Example 2

[0043] Powders of tungsten and boron, in a ratio of 87 wt% W and 13 wt% B were mixed with a metallic flux powder, in the present example a Fe-Cr-C alloy powder, according to a ratio of 90 wt% (W+B) and 10 wt% Fe-Cr-C alloy powder, thereby obtaining a reaction mixture.

[0044] The reaction mixture was heated in a graphite crucible inside a VIM at 1100°C to trigger a self-propagating exothermic reaction. Cermet particles acquired from the reaction comprised sub-micron structures i.e., needles, laths and particles of tungsten boride, within a Fe-Cr-C alloy matrix / binder. The acquired cermet was then consolidated by hot pressing and sintering at 1100°C to form a consolidated cermet. The consolidated cermet material had a high hardness (>1500 HV 10 ), good oxidation resistance at 800°C in air, good corrosion resistance in salt water, and non-magnetic properties.

Examples

example 1

Example 1

[0041]Powders of tungsten and boron, in a ratio of 87 wt% W and 13 wt% B were mixed with a metallic flux powder, in the present example bronze (Cu-Sn) powder, according to a ratio of 90 wt% (W+B) and 10 wt% bronze powder, thereby obtaining a reaction mixture.

[0042]The reaction mixture was heated in a graphite crucible inside a VIM at 1100°C to trigger a self-propagating exothermic reaction. Fig 2 shows a micrograph of a cermet particle acquired from the step of reacting. The cermet particles comprised sub-micron structures i.e., needles, laths and particles of tungsten boride, within a bronze matrix / binder. The acquired cermet was then consolidated by hot pressing and sintering at 1100°C to form a consolidated cermet. Fig. 3 shows a cross-sectional micrograph of the consolidated cermet, which micrograph shows that the sub-micron structure of the tungsten boride was retained during the step of consolidating. The consolidated cermet material had a high hardness (>1500 HV10), ...

example 2

Example 2

[0043]Powders of tungsten and boron, in a ratio of 87 wt% W and 13 wt% B were mixed with a metallic flux powder, in the present example a Fe-Cr-C alloy powder, according to a ratio of 90 wt% (W+B) and 10 wt% Fe-Cr-C alloy powder, thereby obtaining a reaction mixture.

[0044]The reaction mixture was heated in a graphite crucible inside a VIM at 1100°C to trigger a self-propagating exothermic reaction. Cermet particles acquired from the reaction comprised sub-micron structures i.e., needles, laths and particles of tungsten boride, within a Fe-Cr-C alloy matrix / binder. The acquired cermet was then consolidated by hot pressing and sintering at 1100°C to form a consolidated cermet. The consolidated cermet material had a high hardness (>1500 HV 10 ), good oxidation resistance at 800°C in air, good corrosion resistance in salt water, and non-magnetic properties.

TECHNICAL FIELD

[0001] The present invention relates to a cermet comprising tungsten diboride in a metal matrix, a method for preparing the cermet, and a use of the cermet.BACKGOUND

[0002] Cermets combines the attractive properties of ceramics, such as high temperature resistance and hardness, with those of metals, such as the ability to undergo plastic deformation.

[0003] Cermets comprising tungsten borides have for a long time been recognized as an excellent material for the use in radiation shields as they offer both excellent heat and oxidation resistance as well as absorption of both neutron and high energy electromagnetic radiation. Radiation shields are typically comprised of relatively large cermet plates. The manufacturing of such bulk objects is both difficult and costly. Furthermore, the current manufacturing route known in the art involves the intermixing of pre-synthesized tungsten boride powders with powders of a metal and a subsequent consolidation thereof. These manufacturing methods are not only costly, but further, they provide radiation shielding materials which are susceptible to neutron induced swelling and associated embrittlement.

[0004] Hence, there is a need for new technology for the manufacturing of cermet materials which are more economically viable, and further provides cermets which are less susceptible to neutron induced swelling.SUMMARY

[0005] It is an object of the present invention to provide a solution to at least some of the problems associated with the manufacturing of cermets comprising tungsten boride in a metal binder.

[0006] It is a further object of the invention to provide a tungsten boride cermet which is less susceptible to neutron swelling.

[0007] The invention is set out in the appended set of claims. This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description.

[0008] According to a first aspect, the present invention relates to a method for preparing a cermet comprising precipitates of tungsten boride in a metal binder, the method comprising: reacting elemental tungsten with elemental boron in an exothermic reaction within a melt of a metallic flux to form the precipitates of tungsten boride, wherein the metallic flux mediates a self-propagating exothermic reaction between the elemental tungsten and the elemental boron during said reacting; wherein the metallic flux is a metal selected from the group consisting of: Fe, Sn, Cu, Cr, Ta, Hf, and Ti, or a combination thereof; and wherein the metal binder of the cermet is formed from the metal of the metallic flux.

[0009] Hence, there is provided a method wherein tungsten and boron react with one another to form tungsten boride, which formed tungsten boride is simultaneously bound in a metal binder formed from the metal of the metallic flux. To the inventors' surprise, the method further provides a manufacturing route involving a self-propagating exothermic reaction between tungsten and boron, which reaction is enabled by allowing the tungsten and the boron to react and form tungsten borides within the melt of the metallic flux. The dynamics of this self-propagating exothermic reaction may be explained as follows: First, the metallic flux is at least locally molten. Then, boron dissolves into the molten metallic flux, and form a solution with the molten metallic flux. This results in lowering of the solidus / liquidus of the molten metallic flux. Next, the molten metallic flux, now in solution with boron, wets the tungsten, thereby initiating an exothermic reaction between tungsten and boron and forming tungsten boride. The heat generated by the reaction between the tungsten and the boron in solution with the metallic flux is sufficient to cause a further melting of adjacent unmolten metallic flux, hence the reaction propagates. A reaction between boron and tungsten alone, i.e., without the metallic flux, is not sufficiently exothermic to cause a self-propagating reaction. Hence, it is only by the presence of the metallic flux, and by the forming of a solution between the boron and the metallic flux according to the above, that a self-propagating exothermic reaction can be achieved. Accordingly, it is essential that the metallic flux allows for enough boron to be solved therein, i.e., the metallic flux must have a suitable solubility of boron for the sustaining of the self-propagating exothermic reaction. Correspondingly, it is essential that a sufficient amount of boron is available to form such solution. The relative amount of elemental tungsten to elemental boron may be in a range of 80:20 - 95:5, preferably 85:15 - 90:10, most preferably 87:13, for tungsten and boron respectively, in wt%. Meanwhile, the amount of metallic flux with respect to the total amount of tungsten and boron may be in a range of 1-20 wt%, preferably 1-10 wt%, most preferred 1-5 wt%.

[0010] A further surprising effect realised by the inventors was that the cermet derived from the method according to the first aspect displayed a microstructure comprising sub-micron precipitates of tungsten borides. By the meaning of sub-micron, it is here meant that the precipitates have at least one extension being in the sub-micron range. While the specifics of the microstructure of the cermet will be discussed later, it is worth to emphasise the reason for its formation in the contexts of the inventive method. The reaction between the tungsten and the boron being in solution with the metallic flux results in a rapidly advancing reaction front during the step of reacting. The term reaction front may be defined as a region between mostly unreacted material and mostly reacted material. An associated rapid cooling of the material after the reaction front has passed, sets the conditions for the formation of tungsten boride precipitates in the sub-micron range.

[0011] The method for preparing the cermet may further comprise a step of obtaining a reaction mixture by mixing powders of the elemental tungsten, the elemental boron, and the metallic flux. The step of preparing the reaction mixture may comprise the step of providing and weighing a respective amount of elemental tungsten, and the elemental boron. The respective amounts of the provided elemental tungsten and elemental boron may be determined by a stoichiometric relationship. This stochiometric relationship may be based on a desired formation of specific tungsten boride phases, for example, WB 4 and / or WB 2-x may stoichiometrically correspond to relative amounts of elemental tungsten and boron according to a ratio of 87:13, for tungsten and boron respectively, in wt%. As stated earlier, the relative amount of elemental tungsten to elemental boron may be in a range of 80:20 - 95:5, preferably 85:15 - 90:10, most preferably 87:13, for tungsten and boron respectively, in wt%. Meanwhile the relative amount of metallic flux of the reaction mixture, taken as a whole, may be in a range of 1-20 wt%, preferably 1-10 wt%, most preferred 1-5 wt%. The mixing may be achieved by any means known in the art. Said means of mixing may include but is not limited to the use of a ball mill, tumbler or a mortar and pestle. Generally, mixing of the boron, the tungsten and the metallic flux is beneficial for achieving a complete and uniform reaction.

[0012] The method for preparing the cermet may further comprise a step of compressing the reaction mixture into a compact, preferably the compressing is performed at a pressure in a range of 10-200 MPa. This step may also be referred to as briquetting of the reaction mixture. The compact will typically have a cylindrical shape. However, other shapes are possible. Said step of compressing the reaction mixture may be achieved by any means known to the person skilled in the art including but not limited to the use of a die and a plunger together with a hydraulic press. Compressing the reaction mixture into a compact may be beneficial as the associated consolidation of the reaction mixture may promote a more stable propagation of the reaction front during reacting.

[0013] The step of reacting may be performed by heating said reaction mixture and / or compact, thereby igniting said self-propagating reaction. The heating may be performed at a temperature in a range of 1000-1200°C, preferably in a range of 1050-1150°C, more preferably around 1100°C. In some embodiments said heating may be uniform, such that the entire reaction mixture or compact is uniformly heated to ignite said reacting. However, the heating can also be local, such that only a portion of the reaction mixture or compact is heated. Local heating may be achieved by, for example, a focused laser. In the case of local heating, the reaction will propagate throughout the reaction mixture or compact, i.e., self-propagate, due to the self-sustaining exothermic nature of the reaction. Heating may further be achieved by Vacuum Induction Melting (VIM). In a preferred embodiment, said step of reacting may be achieved by heating said reaction mixture and / or compact in an inert or evacuated atmosphere, thereby igniting said self-propagating exothermic reaction. Heating in an inert and / or evacuated atmosphere may also, for example, be achieved by Vacuum Induction Melting (VIM). Preforming the step of heating in an inter and / or evacuated atmosphere reduce the risk for the formation of oxides or nitrides during said step of reacting.

[0014] In some embodiments of the method, the elemental boron is isotopically enriched in 10< B. For example, the elemental boron may be isotopically enriched to comprise 50%, 75%, 95%, or 100% 10< B. Isotopically enriched boron is particularly desirable when preparing a cermet which is to be used as a neutron radiation shielding material.

[0015] As a subsequent step to the step of reacting, the method for preparing the cermet may further comprise a step of crushing the prepared cermet into a cermet powder. Said crushing may be performed by any means known to the skilled person, including but not limited to the use of grinders, bead mills, ball mills, mortar / pestles, jet-milling, or planetary milling. The cermet may be crushed into a powder, having for example a mean particle size in the range of 10-500 µm, 10-250 µm, or 10-100 µm.

[0016] Some embodiments of the method may further comprise the step of consolidating the cermet powder using sintering or hot-pressing. The consolidating may be achieved by any means known to the skilled person, including but not limited to sintering, spark plasma sintering, hot pressing, or hot isostatic pressing. The step of consolidating allows large, bulk cermets, for example radiation shields to be prepared. The sub-micron precipitates of the cermet powder are preserved during the consolidation step. Accordingly, engineering objects with dimensions of 10-100 cm can be manufactured, which have 10-1000 nm-sized nanostructures uniformly distributed throughout their cross-section.

[0017] According to a second aspect of the invention, there is provided a cermet comprising tungsten boride precipitates in a metal binder, wherein the tungsten boride precipitates display a needle-like morphology, and wherein the metal binder is selected from the group consisting of: Fe, Sn, Cu, Cr, Ta, Hf, and Ti, or a combination thereof. The tungsten boride precipitates may be at least one of WB 2-x , WB 1.75 or WB 4 . It should be understood that WB 2-x , which is the now scientifically recognized formula for phases previously referred to as W 2 B 5 , W 2 B 4 or W 2 B 4+x , is to be interpreted as equivalent to W 2 B 5 , W 2 B 4 and W 2 B 4+x . The cermet may comprise tungsten boride precipitates in an amount of 80-99 wt%, preferably 90-99 wt%, most preferably 95-99 wt%, the rest being metal binder. Preferably, the cermet material is free of any elemental tungsten or any nitride. Accordingly, it is preferred that the respective amounts of elemental boron and tungsten provided to the reaction mixture has a stoichiometric relationship corresponding to at least one of WB 2-x , WB 1.75 or WB 4 or a combination thereof.

[0018] The tungsten boride precipitates may have a needle-like morphology. By the meaning of needle-like, it is here meant that a primary extension of a precipitate, i.e., the longest extension of the precipitate, is much longer than an associated secondary extension of the precipitate, i.e., the shortest extension of the precipitate. Typically, the secondary extension is perpendicular to the primary extension. Hence, the word needle-like is used to emphasise the large aspect ratios between the primary and secondary extensions of the tungsten boride precipitates of the cermet. A majority of the tungsten boride precipitates may have aspect ratios between their respective primary and associated secondary extensions being in the range of 1:3-1:50, 1:6-1:50, or 1:10-1:50. The secondary extension of the majority of the precipitates may for example be in a range of 10-500 nm, or 10-1000 nm. For example, a tungsten boride precipitate may have a primary extension of 800 nm and have secondary extension of 40 nm, resulting in an aspect ratio of 1:20. Said needle-like morphology of the tungsten boride precipitates is a direct consequence of the reaction kinetics resulting from the inventive method according to the first aspect of the invention. The needle-like morphology just described strongly influences the material properties of the cermet. For example, the sub-micron tungsten boride precipitates, i.e., the nano-structured needles of tungsten boride, vastly increases the number of grain boundaries in the material. This significantly increases the cermet's resistance to neutron induced radiation swelling. Further to the increase in resistance to radiation swelling, the needle like morphology result in a material with superior mechanical properties such as hardness. In some embodiments of the provided cermet, the tungsten boride precipitates are comprised of boron which is isotopically enriched in 10< B. Such isotopic enrichment in 10< B improves the radiation shielding properties of the cermet. The cermet may be tailored to comprise mostly one of WB 2-x , WB 1.75 or WB 4 , of which WB 2-x is preferred for at least radiation shielding applications. This is because WB 2-x offers one of the best ratios between the respective number of tungsten and boron atoms. Said tailoring may be achieved controlling the respective amounts of tungsten and boron which are available to react with one another during the preparing of the cermet. Formation of WB 2-x is associated with a relative amount of tungsten and boron according to a ratio of 87:13, for tungsten and boron respectively, in wt%.

[0019] According to a third aspect of the invention, there is provided a use of a cermet according to the second aspect as a radiation shielding material. However, the cermet according to the second aspect may have further applications of use, for example as a hard coating, a wear-resistant tool or die, and as bulletproof armour.

[0020] A further scope of applicability will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples are given by way of illustration only.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The above and other aspects will now be described in more detail, with reference to appended figures. The figures should not be considered limiting; instead, they are used for explaining and understanding. Fig. 1 Shows a flow diagram of the method according to the first aspect of the invention. Fig. 2 Shows a scanning electron micrograph of a cermet obtained after a step of reacting according to the first aspect of the invention. Fig. 3 Shows a scanning electron micrograph of a cermet obtained after a step of consolidating according to the first aspect of the invention. DETAILED DESCRIPTION

[0022] It is to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It must be noted that, as used in the specification and the appended claims, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements unless the context clearly dictates otherwise. Thus, for example, reference to "precipitate" or "the precipitate" may include several precipitates, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps.

[0023] Fig. 1 shows the method for preparing a cermet comprising precipitates of tungsten boride in a metal binder according to the first aspect of the invention. The method comprises a step of reacting S1 elemental tungsten with elemental boron in an exothermic reaction within a melt of a metallic flux to form the precipitates of tungsten boride. The metallic flux mediates a self-propagating exothermic reaction between the elemental tungsten and the elemental boron during said step of reacting S1. By the meaning of self propagating, it is here meant that the reaction, once started, propagates at its own volition and without the requirement of external heating.

[0024] The metallic flux is a metal selected from the group consisting of: Fe, Sn, Cu, Cr, Ta, Hf, and Ti, or a combination thereof, and wherein the metal binder of the cermet is formed from the metal of the metallic flux. The metallic flux may be present in the reaction S1 in elemental form, i.e., unalloyed, or it may be alloyed. For example, the metallic flux may be an alloy between Cu and Sn, i.e., bronze.

[0025] The reacting S1 between tungsten and boron forms tungsten boride precipitates in the melt of the metallic flux, hence the precipitates of tungsten boride are both formed and bound in the metal binder formed from the metal of the metallic flux in one and the same step of the method, i.e., the step of reacting S1.

[0026] A reaction between boron and tungsten alone, i.e., without the presence of the metallic flux is not sufficiently exothermic to cause a self-propagating reaction. Instead, the reacting in the art between tungsten and boron typically involves external heating of the tungsten and boron for prolonged periods of time. Accordingly, it is only by the presence of the metallic flux in the step of reacting S1 according to the invention that a self-propagating reaction can be achieved.

[0027] Accordingly, the metallic flux serves at least two purposes, first the metallic flux mediates a self-propagating exothermic reaction between tungsten and boron, and then the metallic flux forms the binder of the cermet. By the meaning of the word 'mediates' it is here meant that, it is only by the presence of metallic flux that a self-propagating reaction can be achieved in the step of reacting S1. Hence the metallic flux can also be said to enable a self-propagating reaction between the tungsten and the boron.

[0028] The dynamics of the self-propagating exothermic reaction in the step of reacting S1 may be explained as follows: First, the metallic flux is at least locally molten. Boron dissolves into the molten metallic flux and forms a solution with the metallic flux. This forming of a solution with boron lowers the solidus / liquidus of the molten metallic flux. Furthermore, the molten metallic flux, now in a solution with boron, wets the tungsten, thereby initiating an exothermic reaction between tungsten and boron, thereby forming tungsten boride. The heat generated by the reaction between the tungsten and the boron in solution with the metallic flux is sufficient to cause a further melting of adjacent unmolten metallic flux, hence the reaction propagates. Hence, the self propagating nature of the reaction is enabled by a forming of a solution between the metallic flux and the boron, and a subsequent wetting of the tungsten by the formed solution. Accordingly, it is essential that the metallic flux in the step of reacting S1 allows for enough boron to be solved therein, i.e., the metallic flux must have a suitable solubility of boron for the sustaining of the self-propagating exothermal reaction. Correspondingly, it is essential that a sufficient amount of boron, relative to the amount of tungsten, is available to form such solution. For example, a ratio between tungsten and boron may be at least 5:95, for boron and tungsten respectively, in wt%. The relative amount of elemental tungsten to elemental boron in the step of reacting S1 may be in a range of 80:20 - 95:5, preferably 85:15 - 90:10, most preferably 87:13, for tungsten and boron respectively, in wt%. Meanwhile, the amount of metallic flux with respect to the tungsten and the boron may be in a range of 1-20 wt%, preferably 1-10 wt%, most preferred 1-5 wt%.

[0029] In some embodiments of the method, the elemental boron is isotopically enriched in 10< B. For example, the elemental boron may be isotopically enriched to comprise 50%, 75%, 95%, or 100% 10< B.

[0030] Fig 1. also shows that the method according to the invention may comprise a step of obtaining a reaction mixture S0 by mixing powders of the elemental tungsten, the elemental boron, and the metallic flux. The step of preparing the reaction mixture S0 may comprise the step of providing and weighing a respective amount of elemental tungsten, and the elemental boron. The respective amounts of the provided elemental tungsten and elemental boron may be determined by a stoichiometric relationship. This stochiometric relationship may be based on a desired formation of specific tungsten boride phases. For example, WB 4 and / or WB 2-x may stoichiometrically correspond relative amounts of elemental tungsten and boron according to a ratio of 87:13, for tungsten and boron respectively, in wt%. As stated earlier, the relative amount of elemental tungsten to elemental boron may be in a range of 80:20 - 95:5, preferably 85:15 - 90:10, most preferably 87:13, for tungsten and boron respectively, in wt%. Meanwhile the relative amount of metallic flux of the reaction mixture, taken as a whole, may be in a range of 1-20 wt%, preferably 1-10 wt%, most preferred 1-5 wt%. The mixing may be achieved by any means known in the art. Said means of mixing may include but is not limited to the use of a ball mill, tumbler or a mortar and pestle. Generally, mixing of the boron, the tungsten and the metallic flux is beneficial for achieving a complete and uniform reaction.

[0031] Fig. 1 also shows that the method for preparing the cermet may further comprise a step of compressing S0a the reaction mixture into a compact, preferably the compressing S0a is performed at a pressure in a range of 10-200 MPa. The step of compressing S0a may also be referred to as briquetting of the reaction mixture. The compact will typically have a cylindrical shape. However, other shapes are possible. Said step of compressing S0a the reaction mixture may be achieved by any means known to the person skilled in the art including but not limited to the use of a die and a plunger together with a hydraulic press.

[0032] Fig. 1 also shows that the method according to the invention may further comprise a step of heating S0b the reaction mixture. Said heating S0b may be performed by heating S0b said reaction mixture and / or compact in an inert or evacuated atmosphere, thereby igniting said self-propagating reaction by melting at least a portion of the metallic flux, thereby forming a solution with boron, which enables the reacting S1 between tungsten and boron. Typically, it is to be expected that a melting point of the metallic flux is lowered by the presence of the elemental boron due to a diffusion of boron into the metallic flux prior to its melting. As such the melting of at least said portion of the metallic flux may occur at temperatures which is lower than a temperature which is typically associated with the melting of the element or alloy from which the metallic flux is comprised.

[0033] Preferably the heating S0b is performed at a temperature in a range of 1000-1200°C, preferably in a range of 1050-1150°C, preferably around 1100°C. In some embodiments said heating S0b may be uniform, such that the entire reaction mixture or compact is uniformly heated to ignite said reacting. However, the heating S0b may also be local, such that only a portion of the reaction mixture or compact is heated. Local heating S0b may be achieved by, for example, a focused laser. In the case of local heating, the reaction will propagate throughout the reaction mixture or compact, i.e., self-propagate, due to the self-sustaining exothermic nature of the reaction. Heating S0b may for example be achieved by Vacuum Induction Melting (VIM). In a preferred embodiment, said step of reacting S1 is achieved by heating S0b said reaction mixture or compact in an inert or evacuated atmosphere, thereby igniting said self-propagating exothermic reaction. Heating S0b in an inert or evacuated atmosphere may also, for example, be achieved by Vacuum Induction Melting (VIM).

[0034] Fig. 1 also shows that the method for preparing the cermet may further comprise a step of crushing S2 the prepared cermet into a cermet powder. Said crushing S2 may be performed by the any means known to the skilled person, including but not limited to the use of grinders, bead mills, ball mills, mortar / pestles, jet-milling, or planetary milling. The cermet may be crushed into a powder, having for example a mean particle size in the range of 10-500 µm, 10-250 µm, or 10-100 µm.

[0035] Some embodiments of the method may further comprise the step of consolidating S3 the cermet powder using sintering or hot-pressing. The consolidating S3 may be achieved by any means known to the skilled person, including but not limited to sintering, spark plasma sintering, hot pressing, or hot isostatic pressing. The step of consolidating S3 allows large, bulk cermets, for example radiation shields to be prepared. The sub-micron precipitates of the tungsten borides, including their needle-like morphology, derived from the strep of reacting S1, are preserved during the step of consolidation S3. Accordingly, engineering objects with dimensions of 10-100 cm can be manufactured, which have 10-1000 nm-sized nanostructures uniformly distributed throughout their cross-section.

[0036] Fig. 2 shows a cermet powder particle after the step of crushing S3 comprising tungsten boride precipitates in a metal binder according to the second aspect of the invention. The metal binder may be selected from the group consisting of: Fe, Sn, Cu, Cr, Ta, Hf, and Ti, or a combination thereof, in the present example Cu-Sn (bronze).

[0037] Fig. 2 further shows that a majority of the tungsten boride precipitates display a needle-like morphology when viewed in a two-dimensional plane, i.e., in a cross-sectional image. By the meaning of needle-like, it is here meant that a primary extension of a precipitate, i.e., the longest extension of the precipitate, is much longer than an associated secondary extension of the precipitate, i.e., the shortest extension of the precipitate. Hence, the word needle-like is used to emphasize the large aspect ratios between the primary and secondary extensions of the tungsten boride precipitates of the cermet. A majority of the tungsten boride precipitates may have aspect ratios between their respective primary and associated secondary extensions being in the range of 1:3-1:50, 1:6-1:50, or 1:10-1:50. The secondary extension of the majority of the precipitates may for example be in a range of 10-500 nm, or 10-1000 nm. For example, a tungsten boride precipitate may have a primary extension of 800 nm and have secondary extension of 40 nm, resulting in an aspect ratio of 1:20.

[0038] Said needle-like morphology of the tungsten boride precipitates is a direct consequence of the reaction kinetics resulting from the inventive method according to the first aspect of the invention. The needle-like morphology shown in Fig. 2 strongly influences the material properties of the cermet. For example, the sub-micron tungsten boride precipitates, i.e., the nano-structured needles of tungsten boride, vastly increases the number of grain boundaries in the material. This significantly increases the cermet's resistance to neutron induced radiation swelling.

[0039] The tungsten boride precipitates may be at least one of WB 2-x , WB 1.75 or WB 4 . Furthermore, the cermet may comprise tungsten boride precipitates in an amount of 80-99 wt%, preferably 90-99 wt%, most preferably 95-99 wt%, the rest being metal binder. Preferably, the cermet material is free of any elemental tungsten or any nitride. Accordingly, it is preferred that the respective amounts of elemental boron and tungsten provided to the reaction mixture has a stoichiometric relationship corresponding to at least one of WB 2-x , WB 1.75 or WB 4 or a combination thereof. In some embodiments of the provided cermet, the tungsten boride precipitates are comprised of boron which is isotopically enriched in 10< B. Such isotopic enrichment in 10< B improves the radiation shielding properties of the cermet.

[0040] According to the third aspect of the invention, there is provided a use of a cermet according to the second aspect as a radiation shielding material. However, the cermet according to the second aspect may have further applications of use, for example as a hard coating, a wear-resistant tool or die, and as bulletproof armour.EXAMPLESExample 1

[0041] Powders of tungsten and boron, in a ratio of 87 wt% W and 13 wt% B were mixed with a metallic flux powder, in the present example bronze (Cu-Sn) powder, according to a ratio of 90 wt% (W+B) and 10 wt% bronze powder, thereby obtaining a reaction mixture.

[0042] The reaction mixture was heated in a graphite crucible inside a VIM at 1100°C to trigger a self-propagating exothermic reaction. Fig 2 shows a micrograph of a cermet particle acquired from the step of reacting. The cermet particles comprised sub-micron structures i.e., needles, laths and particles of tungsten boride, within a bronze matrix / binder. The acquired cermet was then consolidated by hot pressing and sintering at 1100°C to form a consolidated cermet. Fig. 3 shows a cross-sectional micrograph of the consolidated cermet, which micrograph shows that the sub-micron structure of the tungsten boride was retained during the step of consolidating. The consolidated cermet material had a high hardness (>1500 HV10), good oxidation resistance at 800°C in air, good corrosion resistance in salt water, and non-magnetic properties.Example 2

[0043] Powders of tungsten and boron, in a ratio of 87 wt% W and 13 wt% B were mixed with a metallic flux powder, in the present example a Fe-Cr-C alloy powder, according to a ratio of 90 wt% (W+B) and 10 wt% Fe-Cr-C alloy powder, thereby obtaining a reaction mixture.

[0044] The reaction mixture was heated in a graphite crucible inside a VIM at 1100°C to trigger a self-propagating exothermic reaction. Cermet particles acquired from the reaction comprised sub-micron structures i.e., needles, laths and particles of tungsten boride, within a Fe-Cr-C alloy matrix / binder. The acquired cermet was then consolidated by hot pressing and sintering at 1100°C to form a consolidated cermet. The consolidated cermet material had a high hardness (>1500 HV 10 ), good oxidation resistance at 800°C in air, good corrosion resistance in salt water, and non-magnetic properties.

Examples

example 1

Example 1

[0041]Powders of tungsten and boron, in a ratio of 87 wt% W and 13 wt% B were mixed with a metallic flux powder, in the present example bronze (Cu-Sn) powder, according to a ratio of 90 wt% (W+B) and 10 wt% bronze powder, thereby obtaining a reaction mixture.

[0042]The reaction mixture was heated in a graphite crucible inside a VIM at 1100°C to trigger a self-propagating exothermic reaction. Fig 2 shows a micrograph of a cermet particle acquired from the step of reacting. The cermet particles comprised sub-micron structures i.e., needles, laths and particles of tungsten boride, within a bronze matrix / binder. The acquired cermet was then consolidated by hot pressing and sintering at 1100°C to form a consolidated cermet. Fig. 3 shows a cross-sectional micrograph of the consolidated cermet, which micrograph shows that the sub-micron structure of the tungsten boride was retained during the step of consolidating. The consolidated cermet material had a high hardness (>1500 HV10), ...

example 2

Example 2

[0043]Powders of tungsten and boron, in a ratio of 87 wt% W and 13 wt% B were mixed with a metallic flux powder, in the present example a Fe-Cr-C alloy powder, according to a ratio of 90 wt% (W+B) and 10 wt% Fe-Cr-C alloy powder, thereby obtaining a reaction mixture.

[0044]The reaction mixture was heated in a graphite crucible inside a VIM at 1100°C to trigger a self-propagating exothermic reaction. Cermet particles acquired from the reaction comprised sub-micron structures i.e., needles, laths and particles of tungsten boride, within a Fe-Cr-C alloy matrix / binder. The acquired cermet was then consolidated by hot pressing and sintering at 1100°C to form a consolidated cermet. The consolidated cermet material had a high hardness (>1500 HV 10 ), good oxidation resistance at 800°C in air, good corrosion resistance in salt water, and non-magnetic properties.

Claims

1. A method for preparing a cermet comprising precipitates of tungsten boride in a metal binder, the method comprising: - reacting elemental tungsten with elemental boron in an exothermic reaction within a melt of a metallic flux to form the precipitates of tungsten boride, wherein the metallic flux mediates a self-propagating exothermic reaction between the elemental tungsten and the elemental boron during said reacting; wherein the metallic flux is a metal selected from the group consisting of: Fe, Sn, Cu, Cr, Ta, Hf, and Ti, or a combination thereof; and wherein the metal binder of the cermet is formed from the metal of the metallic flux.

2. The method according to claim 1, further comprising a step of obtaining a reaction mixture by mixing powders of the elemental tungsten, the elemental boron, and the metallic flux.

3. The method according to claim 2, wherein the step of reacting is preceded by a step of compressing the reaction mixture into a compact, preferably the compressing is performed at a pressure in a range of 10-200 MPa.

4. The method according to claim 2 or 3, wherein the step of reacting is performed by heating said reaction mixture or compact in an inert and / or evacuated atmosphere, thereby igniting said self-propagating reaction, preferably the heating is at a temperature in a range of 1000-1200°C, preferably in a range of 1050-1150°C, preferably around 1100°C.

5. The method according to any one of claims 1 to 4, further comprising the step of crushing the prepared cermet into a cermet powder.

6. The method according to any one of claims 1 to 5, wherein the elemental boron is isotopically enriched in 10B.

7. The method according to any one of claims 5 or 6, further comprising a step of consolidating the cermet powder using sintering or hot-pressing.

8. The method according to any one of claims 1 to 7, wherein a relative amount of elemental tungsten to elemental boron is in a range of 80:20 - 95:5, preferably 85:15 - 90:10, most preferably 87:13, for tungsten and boron respectively, in wt%.

9. A cermet comprising tungsten boride precipitates in a metal binder, wherein the tungsten boride precipitates display a needle-like morphology, and wherein the metal binder is selected from the group consisting of: Fe, Sn, Cu, Cr, Ta, Hf, and Ti, or a combination thereof.

10. The cermet according to claim 9, wherein the tungsten boride precipitates have a primary extension and an associated secondary extension, and wherein a majority of the tungsten boride precipitates have aspect ratios between their respective primary and associated secondary extension being in the range of 3:1-50:1.

11. The cermet according to claim 10, wherein the secondary extensions of the majority of the precipitates are in a range of 10-1000 nm.

12. The cermet according any one of claims 9 to 11, wherein the tungsten boride precipitates are at least one of WB2-x, WB1.75 or WB4.

13. The cermet according to any one of claims 9 to 12, wherein the tungsten boride precipitates boron which is isotopically enriched in 10B.

14. The cermet according to any one of the claims 9 to 13, wherein the cermet comprises tungsten boride precipitates in an amount of 80-99 wt%, preferably 90-99 wt%, most preferably 95-99 wt%, the rest being metal binder.

15. Use of a cermet according to any one of claims 9 to 14 as a radiation shielding material.

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