Article made of binary Ti-Hf or Zr-Hf alloy and comprising dark oxide layer

Abstract

The invention discloses an article made of a binary Ti-Hf alloy containing 25 at.% to 99 at.% Hf, the remainder being Ti, and containing inevitable impurities in an amount of not more than 0.3 at.% for the final composition, or a binary Zr-Hf alloy containing 10 at.% to 99 at.% Hf, the remainder being Zr, and containing inevitable impurities in an amount of not more than 0.3 at.% for the final composition, the article has a dark surface layer on one or more surfaces. The invention also discloses a method for obtaining the product through oxidation heat treatment, the use of the binary Ti-Hf or Zr-Hf alloy as a material of a watch external part or a watch movement part, and the watch external part or the watch movement part obtained through the method.

Description

Technical Field

[0001] The present invention relates to an article made of a binary Ti—Hf alloy or Zr—Hf alloy, the article having a dark surface layer on one or more surfaces, and a method for obtaining the article, preferably a watch exterior or a watch movement component. Furthermore, the present invention relates to the use of the binary Ti—Hf or Zr—Hf alloy as a material for a watch exterior or a watch movement component. Background Art

[0002] Hafnium alloy

[0003] JPS59208080A (Toshiba Corporation - 1983) discloses heat treatment of hafnium at 370-500°C under water vapor pressure to form a decorative layer having corrosion resistance and wear resistance (hardness) and being blue, gold or black.

[0004] Zirconium alloy

[0005] US5037438A (Richards Medical Company - 1989) and EP0410711A1 (Smith & Nephew Inc. - 1989) relate to the thermal oxidation treatment of pure zirconium or zirconium alloys (containing niobium, tantalum, or titanium, yttrium, and in addition hafnium, with a zirconium content greater than about 80 wt.%). US5037438A discloses commercially available zirconium alloys such as Zircadyne 702 and 705, as well as zirconium-tin alloys, which are suitable for the claimed oxidation treatment. The maximum hafnium content in Zircadyne alloys is 4.5 wt.%. Zirconium-tin alloys are zirconium alloys with a zirconium content exceeding 90% and may contain other metals such as tin (about 1.5%) as well as iron, chromium, nickel, or niobium. This oxidized alloy is sold by Smith & Nephew under the trade name Oxinium for orthopedic applications (particularly knee and hip prostheses). The treatment involves oxidation in air, steam, water or a salt bath, for example heat treatment in air at 370-595° C. for 6 hours. This treatment produces a bluish-black zirconium oxide layer that has good adhesion to the substrate.

[0006] Titanium-Hafnium binary alloy

[0007] Ti-Hf binary alloys are known, but such binary alloys with a hard black oxide layer have not been reported.

[0008] Ti-Hf binary alloys are primarily discussed in the non-patent biomedical literature, which focuses on their applications in prosthetics. A comprehensive range of compositions was analyzed for biocompatibility, specifically with regard to: a) corrosion resistance to body fluids; and b) mechanical properties designed to approximate those of bone tissue.

[0009] MIT's WO2013137857A2, filed on March 12, 2012, discloses principles for identifying binary alloys with stable nanocrystalline structures based on thermodynamic parameters. Among the numerous alloys listed, the Ti-Hf system is specifically mentioned. However, no experimental preparation methods for Ti-Hf nanocrystalline alloys are provided. Compositional details are not disclosed, and no mention is made of oxidation treatment.

[0010] In "Development of hafnium metal and titanium-hafnium alloys having apatite-forming ability by chemical surface modification," J. Biomed. Mat. Res.: Part B-Appl. Biomat. 106, pp. 2519-2523, 2018, T. Myazaki et al. investigated the osseointegration ability of Ti-xHf alloys (x = 20%, 40%, 60%, 80%, and 100 at.%). The pure metals and alloys were treated with sodium hydroxide and then heated. The study found that, depending on the hafnium content, the alloy surfaces formed anatase, sodium titanate, hafnium titanate, apatite, or hafnium oxide.

[0011] In "Multiple-species ion beams from titanium-halfnium alloy cathodes invacuum arc plasmas," J. Appl. Phys. 73, 7184-7187, 1993, J. Sasaki et al. investigated the Ti-Hf system as a cathode sputtering target in detail, covering a full range of compositions. Specifically, they studied plasmas generated by a metal vapor vacuum arc using a TiHf solid solution with a variable chemical composition as the cathode.

[0012] In "Sputtered Hf–Ti nanostructures: A segregation and high-temperature stability study," Acta Materialia 108, pp. 8–16, 2016, M. Polyakov et al. investigated the preparation of Hf-Ti nanostructured alloys by sputtering. They found that after annealing an Hf-23Ti (at.%) alloy at 800°C for 96 hours, segregation of hafnium and titanium occurred at the nanoscale, despite the bulk Hf-Ti phase diagram predicting a homogeneous solid solution.

[0013] Zirconium-Hafnium binary alloy

[0014] Generally speaking, zirconium-hafnium binary alloys are also known, but zirconium-hafnium binary alloys having a hard black oxide layer have not been reported.

[0015] US3373017A discloses coins made of Zr-xHf (x=8.5-20 wt.%) alloys, which can replace silver and serve as an anti-counterfeiting measure.

[0016] JPH04318137A discloses a corrosion-resistant binary alloy for nuclear material processing, the alloy comprising 1-50 wt. % of hafnium, with the balance comprising one or more metals selected from titanium, zirconium, niobium and tantalum.

[0017] EP0570308A1 discloses a method for preparing Nb-Ti and Hf-Zr ingots with a specific crystal structure by co-electrodepositing the metals in a molten salt bath, and provides an example of the preparation of Zr-66.2Hf (wt.%).

[0018] JP2013054037A discloses a nuclear reactor component made of a zirconium-hafnium alloy. The patent claims the use of a binary Zr-Hf alloy, but does not describe its composition in detail. Instead, it mentions alloys with additions of niobium, iron, chromium, tin, and nickel.

[0019] CN112195369A discloses a corrosion-resistant and neutron-resistant Zr-xHf (x=49-51 wt.%) binary alloy and a method for manufacturing components therefrom.

[0020] In "Interdiffusion behaviors and mechanical properties of Zr-X (X = Nb, Ta, Hf) binary systems," J. Alloys Compds., 2022, p. 164910, J. Wang et al. investigated the diffusion behavior of Zr-Hf alloys in several binary zirconium alloys for nuclear, aerospace, and prosthetic applications. They also measured the hardness of these alloys using nanoindentation and found that the hardness was approximately 4 GPa for hafnium content up to approximately 20 at.%, then gradually increased to approximately 6 GPa at 28 at.%.

[0021] In "The Hf-Zr (Hafnium-Zirconium) System," Bull. Alloy Phase Diagrams, p. 29, 1982, Abriata et al. proposed a phase diagram for the Hf-Zr system based on a literature review. They pointed out that the two elements are completely miscible. Summary of the Invention

[0022] The inventors previously developed a ternary Ti-Zr-Hf alloy, articles made from this alloy with a dark oxide layer, and methods for obtaining such articles, and the applicant disclosed this teaching in European patent application EP23171553.3. This application claims priority from this prior European application. The ternary alloy comprises 18.4 to 80 at.% zirconium and 2 to 40 at.% hafnium, with the remainder being titanium. The articles have a dark oxide layer and, due to their excellent mechanical properties, are particularly suitable for watch exteriors and watch movement components.

[0023] On one hand, the inventors aimed to develop an alloy with variable density, paramagnetic properties, and excellent mechanical properties. This alloy could form a dark, hard layer on its surface, resulting in a durable, dark appearance. Specifically, the inventors sought to develop an alloy with a dark surface layer. Furthermore, the material needed to be paramagnetic.

[0024] The inventors studied the process they developed for the above-mentioned ternary alloys and extended its application to binary alloys, thereby completing the present invention.

[0025] Therefore, the problem to be solved by the present invention is to develop a binary alloy that meets the above requirements. To this end, the inventors have developed the binary Ti-Hf and Zr-Hf alloys of the present invention to solve this problem.

[0026] This problem is solved by an article made of a binary Ti—Hf or Zr—Hf alloy with a dark surface layer as claimed in claim 1. Preferred embodiments, watch parts and / or watch movement parts, a method for obtaining the article, and the use of the alloy are also specified in the claims.

[0027] Specifically, the present invention provides the following embodiments:

[0028] In the binary Ti-Hf or Zr-Hf alloy of the present invention, the density of the alloy is 7-12 g / cm 3 The density of the alloy can be adjusted as needed by appropriately selecting the hafnium content in the binary alloy.

[0029] The binary alloy of the present invention has paramagnetic properties.

[0030] The present invention provides an article made from the binary alloy. The article has a dark oxide layer on one or more surfaces. For example, the article is a watch exterior component or a watch movement component.

[0031] The Vickers hardness HV1 (ISO 6507, HV1) of the Ti-Hf alloy of the present invention increases with increasing hafnium content, with a maximum hardness of HV1 = 323, corresponding to a Ti-50Hf alloy. The HV1 hardness of the Zr-Hf alloy of the present invention increases with increasing hafnium content, with a maximum hardness of HV1 = 223, corresponding to a Zr-50Hf alloy.

[0032] The thickness of the dark oxide layer is at least 5 μm, preferably at least 10 μm, typically 5-25 μm, preferably 7-20 μm, and more preferably about 15 μm. The thickness of the oxide layer depends on the time of heat treatment in air. It is obvious to those skilled in the art that the heat treatment of more than 1 hour mentioned as an example of the present invention will make the oxide layer thicker, but the relationship between the oxide layer thickness and time is not linear. In fact, the oxide layer thickness is related to (D*t) 1 / 2 is proportional to, where t is time and D is the diffusion coefficient of oxygen species.

[0033] The hardness of the dark oxide layer measured by nanoindentation is at least 10 GPa H according to ISO 14577-1, 1st edition, 2002, Metallic materials—Instrumented indentation test for hardness and materials parameters—Part 1: Test method IT , more preferably even higher than 13 GPa H IT .

[0034] Optionally, the surface of the dark oxide layer may be partially or fully polished. Polishing may be achieved by conventional techniques. Alternatively, the surface may be partially or fully finished by other commonly used finishing techniques (e.g., sandblasting, brushing, or satin finishing).

[0035] The present invention also provides a method for obtaining a product having the above-mentioned dark oxide layer made of the binary Ti-Hf and Zr-Hf alloy, the method comprising the following steps:

[0036] 1) preparing a binary alloy containing the desired amounts of Ti and Hf, or Zr and Hf, and unavoidable impurities by a conventional melting process (which may include several melting and cooling steps);

[0037] 2) forming the alloy into a desired product shape;

[0038] 3) optionally grinding, finishing, sandblasting, brushing, and / or polishing one or more surfaces of the article;

[0039] 4) oxidizing the surface of the article by heat treatment at a temperature of 400-750° C. (preferably 400-700° C., more preferably 400-650° C.) in an oxygen-containing atmosphere for an appropriate period of time; and,

[0040] 5) Optionally sandblast, brush, satin finish or polish the oxidized surface.

[0041] The temperature range of the heat treatment is selected according to the characteristics of the hardening process by oxide layer conversion in order to optimize process parameters, such as process duration. The lower limit temperature is determined by the oxidation reaction. If the reaction temperature is too low, it will slow down to an unacceptable level due to lack of reactivity. The upper limit temperature is determined by the slower oxygen diffusion in the β phase and the risk of oxide layer delamination caused by the β→α phase transformation during cooling. The β phase transformation point of the alloy can be determined by DSC (differential scanning calorimetry) before heat treatment. In addition, it is preferred to find the best solution to use the highest possible temperature to accelerate conversion hardening, while using the lowest possible temperature to reduce the cost of heating equipment, its operating costs and thermal deformation of the parts to be treated.

[0042] In one embodiment, the oxidative heat treatment is carried out at 400-750°C, preferably 450-700°C, and more preferably 450-650°C. The oxidative heat treatment time of the article is preferably 1-420 minutes, preferably 60-400 minutes, more preferably 100-400 minutes, and most preferably about 180-360 minutes. The oxygen-containing atmosphere is air, pure oxygen, or an oxygen-containing environment, such as a mixture of oxygen and an inert gas such as argon. Most preferably, the oxidative heat treatment is carried out by heating in an oven.

[0043] The present invention provides the use of the above binary Ti—Hf or Zr—Hf alloy as a material for watch exterior parts and / or watch movement parts having a dark surface layer on one or more surfaces.

[0044] The watch exterior part or watch movement part made of the binary Ti-Hf or Zr-Hf alloy of the present invention has the above-mentioned dark surface layer. The watch exterior part or watch movement part of the present invention can be obtained by the method of the present invention. Detailed Description of the Invention

[0046] The inventors developed the Ti-Hf and Zr-Hf alloys of the present invention to obtain a material that can form a hard, thick, adherent, dark layer on the surface of watch components. The alloy has application value in movement (such as shafts, spindles or pinions) as well as watch cases, watch straps and watch strap pins. The alloy can also adjust the density (especially in the range of 7-12 g / cm 3 This may provide greater freedom in the design and conception of watch components.

[0047] The binary Ti-Hf and Zr-Hf alloys of the present invention are particularly interesting in this regard because they are easy to alloy, can be used to manufacture watch parts, are easy to finish, and readily form an oxide layer. The oxide layer formed by heat treatment conversion is adherent, thick, hard, and dark in color. Surprisingly, Ti and Hf, and Zr and Hf, form perfect solid solutions in the high-temperature β phase and the low-temperature α phase, respectively, which facilitates alloying and enables the production of watch exterior parts with a satisfactory finish.

[0048] The binary alloy of the present invention comprises at least 25 to 99 at.% Hf, with the remainder being Ti; or at least 10 to 99 at.% Hf, with the remainder being Zr, with unavoidable impurities comprising up to 0.3 at.% of the final composition. The upper limit of hafnium content in the Ti-Hf alloy and the Zr-Hf alloy is preferably 95 at.%, and more preferably 80 at.%, respectively. Preferred contents of the Ti-Hf and Zr-Hf alloy components are defined in the appended claims.

[0049] Specific examples of the alloys of the present invention are as follows:

[0050] Binary Ti-Hf alloy comprising:

[0051] 25at.%Hf, or

[0052] 27at.%Hf, or

[0053] 30at.%Hf, or

[0054] 50at.%Hf, or

[0055] 60at.%Hf, or

[0056] 75at.%Hf, or

[0057] 80at.%Hf,

[0058] In each case, the balance is Ti and unavoidable impurities.

[0059] Binary Zr-Hf alloy, comprising:

[0060] 10at.%Hf, or

[0061] 20at.%Hf, or

[0062] 35at.%Hf, or

[0063] 50at.%Hf, or

[0064] 75at.%Hf,

[0065] The balance in each case is titanium and unavoidable impurities.

[0066] The alloy of the present invention is preferably composed of Ti and Hf, or Zr and Hf. However, the content of unavoidable impurities can account for up to about 0.3 at.% of the final composition. The impurities mainly come from the production of the alloy metal, for example, Fe, N, O, C and / or H.

[0067] In the present invention, the content of metals in the alloy is expressed in at.%. The total amount of all alloying elements is 100 at.%. Ti-Hf alloys can be represented by the following abbreviations: "Ti-yHf", which means y at.% Hf, the remainder being Ti, unless otherwise specified, the total amount being 100 at.%. Similarly, Zr-Hf alloys can be represented by the following abbreviations: "Zr-yHf", which means y at.% Hf, the remainder being Zr, unless otherwise specified, the total amount being 100 at.%. The parameter y is selected based on the Hf content defined in the claims.

[0068] The pure elements are weighed to ensure the correct relative atomic composition of the resulting alloy. Alternatively, the composition of the alloy can be determined in the alloy by conventional metal analysis methods known in the art, with X-ray fluorescence (EDXRF-energy dispersive X-ray fluorescence, WDXRF-wavelength dispersive X-ray fluorescence) and emission spectrometry (spark-OES, ICP-OES / MS-inductively coupled plasma optical emission spectrometry / mass spectrometry, LIBS-laser induced breakdown spectroscopy, SEM / EDX and SEM / WDX-scanning electron microscopy combined with energy dispersive or wavelength dispersive X-ray spectroscopy) being commonly used methods.

[0069] In this specification, the terms "titanium-based alloy", "zirconium-based alloy" or "hafnium-based alloy" can be used interchangeably, and the expressions "Zr-Hf" and "Hf-Zr", "Ti-Hf" and "Hf-Ti" can also be used interchangeably because their microstructures and properties are equivalent.

[0070] Ti, Zr, and Hf are all transition metals from Group IVb of the periodic table. The atomic weight of Hf is 178.5 g / mol, that of Zr is 91.2 g / mol, and that of Ti is 47.9 g / mol. The density of Hf is 13.3 g / cm 3 , Zr is 6.52g / cm 3 , Ti is 4.5g / cm 3 Therefore, Hf is a heavy, high-density element, and when the Hf content exceeds about 10 at.%, the alloy can be considered a hafnium alloy rather than a titanium alloy. However, as mentioned above, in this specification, "titanium alloy," "zirconium alloy," and "hafnium alloy" are used interchangeably.

[0071] The density of the binary alloy of the present invention is estimated based on the proportionality law. It can be determined by using the buoyancy method. The density is preferably 7-12 g / cm 3 The density can be adjusted by properly selecting the contents of Ti and Hf, or Zr and Hf. The density of stainless steel is about 8g / cm 3 Thus, the alloys of the present invention preferably have a wider density range than stainless steels (e.g., the austenitic 904L or 316L steel series commonly used for watch parts), allowing for the manufacture of lightweight watch parts and watches, or watches and watch parts having a desired weight.

[0072] The binary alloys of the present invention are obtained by a known melting process of the metal components of the starting alloy as described below. This process may comprise several melting and cooling steps, for example at least 5 or 10 times.

[0073] The article of the present invention is made from the above-mentioned binary alloy by conventional processes, such as cold forming or hot forming, cutting, milling, casting, drawing or any other suitable method. The article of the present invention is preferably a watch component, in particular a watch exterior component or a watch movement component. Examples of watch exterior components are watch cases, watch bands and / or components thereof (such as links, pins, clasps, connectors), crowns, bezels, hands or any other watch exterior components. Examples of watch movement components are balance wheels, barrels, splints, base plates, shafts, pinions or any other watch movement components.

[0074] The article of the present invention is paramagnetic and therefore cannot be magnetized by a magnetic field.

[0075] The article of the present invention has a dark oxide layer on one or more surfaces (preferably all surfaces) thereof. The dark oxide layer can be obtained by the method of the present invention disclosed below, which includes a thermal oxidation treatment as an essential step.

[0076] Typically, after a dark oxide layer is formed on an article, all surfaces thereof are covered with the oxide layer. However, if desired, the dark oxide layer can be removed from one or more surfaces of the article, for example, by grinding, machining, laser treatment, etc. Thus, the resulting article is covered with a dark oxide layer only on one or a few (but not all) surfaces thereof.

[0077] The thickness of the dark oxide layer is at least 3 μm, preferably at least 5 μm, more preferably at least 10 μm, usually 3-25 μm, preferably 5-20 μm, more preferably about 15 μm. The thickness of the dark oxide layer is measured by scanning electron microscopy or optical microscopy on a metallographic cross section of the article. Examples of such cross sections are as follows: Figure 3 High layer thicknesses of up to 25 microns allow for final surface finishing such as sandblasting, satin finishing, brushing and / or polishing which would not be possible with lower layer thicknesses.

[0078] The color of the product with a dark oxide layer is black or dark gray. There is no or only a very slight blue tint in the dark color. The color of the product of the present invention is in the CIELab color space L*a*b* (measured according to EN ISO 11664-4 "Colorimetry-Part 4:CIE 1976L*a*b*Colour space", 2019 edition), preferably L*<50, |a*|<5, |b*|<5, more preferably L*<40, |a*|<1, |b*|<1. In the CIELab color system, L* represents perceived brightness, and a* and b* represent the unique colors of human vision: red, green, blue and yellow. L* defines black as 0 and white as 100. The a* axis is associated with the red-green opposition, with negative values ​​tending towards green and positive values ​​tending towards red. The b* axis represents the blue-yellow opposition, with negative values ​​tending towards blue and positive values ​​tending towards yellow.

[0079] The hardness of the oxide layer measured by nanoindentation (according to ISO 14577-1, 1st edition, 2002, "Metallic materials - Instrumented indentation tests for hardness and material parameters - Part 1: Test methods") is at least 10 GPa H IT , preferably 10-13 GPa H IT , more preferably higher than 13 GPa H ITThat is, the surface of the article is very hard and therefore scratch-resistant, and has improved overall mechanical resistance. It should be noted that measuring the standard hardness of the oxide layer by indentation (e.g., Vickers hardness according to ISO 6507, 2nd edition, 1997 cited above) is very difficult because the optical contrast of the indentation on the dark oxide surface is very low. To overcome this problem, the nanoindentation method of ISO 14577-1, 1st edition, 2002 cited above can be used in the present invention to measure the hardness of the oxide layer because this technique does not require an optical microscope to analyze the shape of the indentation and measure its size.

[0080] A distinction should be made between the hardness of the base alloy and the hardness of the surface conversion oxide layer. The conversion oxide layer is much harder than the unoxidized base alloy.

[0081] The concept of "conversion" stems from the gradual formation of the oxide layer from the surface inward. An oxygen concentration gradient exists between the oxide layer and the bulk material, where the oxygen concentration in the bulk material drops to almost zero, and the hardness decreases from the hardness of the oxide layer to the hardness of the bulk alloy. This gradient, typically measured by GDOES (glow discharge spectroscopy), typically occurs in the 100-1000 nm range and, due to the relatively narrow region, is not always resolvable in metallographic cross-sections using optical or electron microscopy (due to lack of contrast or resolution).

[0082] The dark surface oxide layer has very strong adhesion to the alloy core, i.e., it will not flake off. The peel test according to ISO 2409, 4th edition, 2013, using a type 1a cutting tool, the test result is rated 1 on a scale of 0 to 5. Its surface is dense, with virtually no pinholes or surface defects. This is determined by the oxide formation process, when the oxide layer grows inward between the initial native oxide layer of a few nanometers thick and the bulk metal alloy, allowing the oxide layer to grow uniformly at the oxide-metal interface, such as Figure 2 As shown in the figure on the right (O2- migration mechanism).

[0083] The present invention provides a method for obtaining the product having a dark oxide layer.

[0084] In a first step, an alloy of the desired composition is formed by conventional melting processes.

[0085] The amount of starting metals is selected and weighed according to the desired alloy composition.

[0086] Optionally, the starting material (eg metal chips, sponges, chunks or ingots of the corresponding alloy metal) is cleaned before melting, for example by ultrasonic cleaning.

[0087] The elements are melted in an inert atmosphere or vacuum using any alloy ingot manufacturing method based on melting and solidification, such as vacuum induction melting (VIM) or vacuum arc melting (VAR). Typically, the method includes several melting and cooling steps to produce an alloy ingot, which ensures homogeneity, and then solidifies in an inert chamber and cools to room temperature.

[0088] Then, in an optional step, the alloy is annealed and quenched to adjust the mechanical properties of the material.

[0089] The resulting alloy is formed into the desired article shape by conventional processes known in the art (such as hot forming and / or cold forming, cutting, milling, casting, etc.).

[0090] One or more surfaces of the thus obtained article may optionally be subjected to surface treatment, such as grinding, fine machining, sandblasting and / or polishing, as desired.

[0091] Then, in the next step, the surface of the article is oxidized to obtain the desired dark surface layer. In one embodiment, the surface oxidation can be carried out by heat treatment at a temperature of 400-750°C (preferably 450-700°C, more preferably 450-650°C) in an oxygen-containing atmosphere for an appropriate time, preferably 1-420 minutes, more preferably 30-300 minutes, further preferably 40-240 minutes, and most preferably 50-70 minutes. In another embodiment, the oxidation treatment time is 1-30 minutes, preferably 5-20 minutes, and typically about 10 minutes.

[0092] The temperature is kept below the alpha to beta phase transition temperature.

[0093] Experiments have shown that the oxidation time and adhesion of the black surface layer on Ti-Hf and Zr-Hf alloys are related to the surface finish. In practice, grit-blasted samples of these alloys require oxidation at maximum temperature for 1-10 minutes, followed by cooling, to obtain a delamination-free oxide layer approximately 5 μm thick. Longer oxidation times lead to delamination of the formed oxide layer.

[0094] In contrast, polished or finely ground samples required oxidation for an hour or more to achieve the same oxide layer thickness. No delamination was observed for the polished or finely ground samples.

[0095] The oxidative heat treatment is preferably carried out by heating in an oven, preferably an electrically heated oven.

[0096] Alternatively, the oxidation treatment may be performed using plasma electrolytic oxidation.

[0097] The oxygen-containing atmosphere may be air, pure oxygen, or an oxygen-containing environment, such as a mixture of oxygen and an inert gas (such as argon).

[0098] If necessary, the surface of the surface-oxidized article obtained as described above may be subjected to conventional finishing treatments such as sandblasting, brushing, satin finishing or polishing.

[0099] The present invention provides the use of the aforementioned binary Ti-Hf or Zr-Hf alloy as a material for the aforementioned watch exterior components and / or watch movement components. Preferably and typically, the alloy is formed into a watch component and then surface-oxidized as described above. Preferred watch components are watch cases and watch straps, mechanical watch components (such as watch strap pins), or watch movement shafts (such as balance shafts or pinion shafts).

[0100] Finally, the present invention provides a watch component having a dark surface layer. In addition to being used for watch exterior components, the alloy of the present invention can also be applied to watch movement components, such as watch band pins and movement pins (e.g., balance shafts or pinion shafts). The hard oxide layer provides excellent wear resistance. Furthermore, the component is paramagnetic. BRIEF DESCRIPTION OF THE DRAWINGS

[0101] Figure 1 Binary phase diagrams for Hf-Ti (top) and Zr-Hf (bottom). "rt" and "ht" denote "room temperature" (α phase) and "high temperature" (β phase), respectively.

[0102] Figure 2 These are two different oxidation mechanisms of Ti-Hf alloys with different hafnium contents.

[0103] Figure 3 These are photos of two Zr-35Hf alloy samples: A on the left is the unoxidized (polished) sample; B on the right is the sample that was oxidized in air and then polished.

[0104] Figure 4 Scanning electron microscope micrographs showing the conversion layer on a Zr-75Hf alloy sample oxidized at 750°C in air for 1 hour.

[0105] Crystal structure, properties

[0106] To ensure good adhesion of the oxide layer to the metal or metal alloy surface, it is preferable to avoid oxidation at temperatures where a crystal structure transition can occur. This can create undesirable stresses in the oxide layer and increase the risk of spalling. All three alloying elements, Ti, Zr, and Hf, have a known phase transition at a temperature above 800°C, from the α phase with a hexagonal close-packed (hcp) structure to a body-centered cubic (bcc) structure known as the β phase.

[0107] Oxidation mechanism

[0108] Figure 2 Schematic illustration of the oxidation mechanism of Ti-Hf alloys with different hafnium contents.

[0109] For titanium alloys with little or no hafnium content, during oxidation treatment in air, Ti 4+ Cations diffuse at the atmosphere / oxide interface, causing the growth of a porous TiO2 layer. This layer has poor adhesion and weak mechanical properties. In contrast, in Ti-Hf alloys containing a "sufficient" amount of hafnium (approximately >55.4 wt.%, >25 at.%, according to the inventors' experiments), oxygen anions diffuse through the oxide layer, transforming the surface into a dense, adherent, and hard oxide layer. The claimed hafnium content ensures that the alloy contains sufficient hafnium to enable the oxidation mechanism of oxygen diffusion into the matrix, and the hafnium's hardening effect imparts good mechanical properties to the alloy.

[0110] The oxidation mechanism of titanium alloys is described in C. Leyens, “Oxidation and Protection of Titanium Alloys and Titanium Aluminides,” pp. 187-230, in: C. Leyens and M. Peters, “Titanium and titanium alloys: fundamentals and applications,” John Wiley & Sons, 2003. However, this document does not disclose any criteria for defining the oxidation mechanism of titanium alloys based on the hafnium content.

[0111] The inventors have found that for binary Zr or Ti alloys, the minimum hafnium content should be between 10 at.% and 25 at.% in order to form a dark and adherent oxide layer.

[0112] In the method of the present invention, the oxidation heat treatment temperature must be kept below the α→β transition temperature for two reasons:

[0113] Oxygen diffuses faster in the α phase than in the β phase, thus promoting the growth of the oxide layer;

[0114] Even partial transformation of the α phase into the β phase during thermal oxidation treatment can result in microstructural changes (such as grain growth) that can manifest themselves on the surface of the oxide layer, forming a morphology inconsistent with a uniform appearance (orange peel effect).

[0115] In summary, for the binary Ti—Hf and Zr—Hf alloys of the present invention, the oxidative heat treatment can be performed within the claimed composition range at a temperature below the α→β transformation temperature.

[0116] Ti-Hf alloy

[0117] Different Ti-Hf alloy compositions were prepared and subjected to HV1 hardness measurements before oxidation. The measurements were performed according to ISO 6507-1, 2nd edition, 1997, using a Knoop / Vickers hardness tester with a load of 1 kgf. These tests showed (Table 1):

[0118] The hardness of the alloy varies with the hafnium content and reaches its maximum value in the equiatomic composition of Ti-50Hf alloy;

[0119] ●Imgram pointed out that hafnium has a hardening effect on Ti-(5-40wt.%)Hf binary alloys (A.Imgram, D.

[0120] Williams,and H.Ogden,"Tensile properties of binary titanium-zirconiumand titanium-hafnium alloys,"Journal of the Less Common Metals,vol.4,pp.

[0121] 217-225,1962.).

[0122] Table 1: Composition and bulk hardness of Ti-Hf alloy

[0123] Alloy (atomic composition) Hardness HV1 Standard Deviation Pure Hf* 166.4 8.0 Ti-20Hf 287.4 13.2 Ti-25Hf 285 5.2 Ti-27Hf 283.2 9.7 Ti-30Hf 297.2 6.5 Ti-50Hf 323.2 13.7

[0124] *Not the present invention

[0125] Zr-Hf alloy

[0126] Several Zr-Hf binary alloys with different compositions were prepared and characterized by their HV1 hardness as described above (Table 2). As shown in Table 2, the hardness of the Zr-Hf alloy increases with increasing hafnium content, with the maximum hardness HV1 = 223 corresponding to Zr-50Hf.

[0127] Table 2: Composition and bulk hardness of Zr-Hf alloy

[0128] Alloy (atomic composition) Hardness HV1 Standard Deviation Zr-10Hf 197.4 12.6 Zr-20Hf 203.2 12.3 Zr-50Hf 223.4 17.8

[0129] Oxidative heat treatment in air

[0130] Subsequently, the Zr-Hf and Ti-Hf alloy samples were subjected to oxidation heat treatment in air for 60 minutes under different temperature conditions. The results are shown in Table 3.

[0131] Table 3: Summary of heat treatment conditions of Ti-Hf and Zr-Hf binary alloys in air

[0132]

[0133] *Not the present invention

[0134] X means no oxidation; O means oxidation forms a thick dark oxide layer; -- means not tested; NA means not measured

[0135] Table 3 shows the oxide layer thickness of different samples.

[0136] Maximum zirconium content

[0137] The inventors conducted thermal oxidation tests in air on 90Zr-10Hf, 80Zr-20Hf, and pure Zr (which is not an alloy and is outside the scope of the present invention), all of which demonstrated the ability to obtain a dense, dark layer on the surface. These tests (already disclosed in the prior application) confirmed the high zirconium content. Example

[0138] The present invention is illustrated below by way of examples. All alloy samples in Table 3 were prepared and characterized in the same manner.

[0139] Example 1

[0140] An arc melting furnace equipped with a non-consumable tungsten electrode was used to prepare a molten copper crucible with an inert atmosphere. 3 Alloy samples (button ingots) with the compositions shown in Table 3 were obtained.

[0141] Pure elemental titanium, zirconium, and hafnium were purchased from commercial suppliers in the form of billets, sponges, or chips. The pure elements were weighed and ultrasonically treated. The metals were mixed and melted in an arc melting furnace. To homogenize the chemical composition of the button ingots, the ingots were flipped and remelted 10 times. After cooling, the ingots were then cut into 2-4 mm thick slices. The slices were manually polished to a flat surface using P320 sandpaper.

[0142] After these steps of preparing the alloy samples, the samples were subjected to an oxidative heat treatment in air at the temperature shown in Table 3 for 60 minutes using an oven. For the Zr-75Hf alloy, this treatment was performed by oxidative heat treatment at 750°C, forming a dark layer about 1.7 μm thick. The metallographic cross section of the oxidized sample is shown in FIG. Figure 4 shown.

[0143] Figure 3 Two Zr-35Hf samples are shown: unoxidized (A) and oxidized in air at 550°C for 1 hour (B).

[0144] After heat treatment, the color values ​​obtained according to the above CIE L*a*b* system are shown in Table 4.

[0145] Table 4: CIE L*a*b* values ​​after 1 hour of oxidative heat treatment in air according to Table 3

[0146] Alloy (atomic composition) L* a* b* Ti-25Hf 46.99 -0.51 -9.01 Ti-27Hf 44.54 0.14 -7.39 Ti-30Hf 44.19 0.18 -5.01 Ti-50Hf 46.29 0.04 -1.9 Zr-20Hf 45.09 0.33 0.27 Zr-50Hf 48.27 0.43 2.54

[0147] The values ​​are the average values ​​of measurements at three different locations for each sample.

Claims

1. An article made of a binary Ti-Hf alloy, made from the following alloy: A binary Ti-Hf alloy comprising 25 at.% to 99 at.% Hf, the remainder being Ti, and unavoidable impurities in an amount not exceeding 0.3 at.% of the final composition; or A binary Zr-Hf alloy comprising: 10 at.% to 99 at.% Hf, the remainder being Zr, and unavoidable impurities in an amount not exceeding 0.3 at.% of the final composition; The article has a dark surface layer on one or more surfaces.

2. The article made of a binary Ti-Hf alloy according to claim 1, wherein the hafnium content is 95 at.% or less, 80 at.% or less, more preferably 75 at.% or less, more preferably 60 at.% or less, even more preferably 50 at.% or less, or 30 at.% or less.

3. The article made of a binary Ti-Hf alloy according to claim 1 or 2, wherein the hafnium content is 25 at.% or more, preferably 27 at.% or more, more preferably 30 at.% or more, more preferably 50 at.% or more, 60 at.% or more, 75 at.% or 80 at.%.

4. The article made of a binary Ti-Hf alloy according to any one of claims 1 to 3, wherein the hafnium content is 20-80 at.%, more preferably 23-75 at.%, even more preferably 23-50 at.%, and most preferably 25-30 at.%.

5. The article of claim 1 , wherein the hafnium content is 99 at.% or less, 95 at.% or less, 80 at.% or less, 70 at.% or less, or 50 at.% or less.

6. The article made of a binary Zr-Hf alloy according to claim 1 or 5, wherein the hafnium content is 20 at.% or more, 30 at.% or more, or 50 at.% or more.

7. An article made of a binary Zr-Hf alloy according to claim 1, 5 or 6, wherein the hafnium content is 10-60 at.%, more preferably 10-50 at.%.

8. An article made of a binary Ti-Hf or Zr-Hf alloy according to any one of claims 1 to 7, wherein the alloy has a density of 7 to 12 g / cm3.

9. A method for obtaining an article made of a binary Ti-Hf alloy or a Zr-Hf alloy according to any one of claims 1 to 8, wherein the article has a dark oxide layer on at least one surface, the method comprising the following steps: (1) preparing an alloy containing the required amounts of Ti and Hf, or Zr and Hf and unavoidable impurities by a conventional melting process; (2) forming the alloy into the desired product shape; (3) optionally grinding, finishing, sandblasting, brushing, and / or polishing one or more surfaces of the article; (4) oxidizing the surface of the article by heat treatment in an oxygen-containing atmosphere at a temperature of 400-750° C., preferably 400-700° C., more preferably 400-650° C. for an appropriate time; and, (5) Optionally, the oxidized surface may be sandblasted, brushed, satin-finished, or polished.

10. The method according to claim 9, wherein the oxidative heat treatment time of the product is 1-420 minutes, preferably 60-400 minutes, more preferably 100-400 minutes, and most preferably 180-360 minutes.

11. The method according to claim 9 or 10, wherein the oxygen-containing atmosphere is air, pure oxygen or an oxygen-containing environment.

12. The method according to any one of claims 9 to 11, wherein the oxidative heat treatment is performed by oven heating or plasma electrolytic oxidation.

13. Use of the binary Ti-Hf or Zr-Hf alloy according to any one of claims 1 to 8 as a material for watch exterior parts and / or watch movement parts.

14. The article according to any one of claims 1 to 8 or the article obtained by the method according to any one of claims 9 to 12, which is a watch exterior part or a watch movement part.

15. The article according to any one of claims 1 to 8 or the article obtained by the method according to any one of claims 9 to 12, wherein the oxide layer has a thickness of 5 to 25 μm, preferably 7 to 20 μm.

16. The article according to any one of claims 1 to 8, 14 or 15, or obtained by the method according to any one of claims 9 to 12, wherein the hardness of the oxide layer measured by nanoindentation according to ISO 14577-1, 1st edition, 2002, "Metallic materials - Instrumented indentation testing of hardness and material parameters - Part 1: Test methods" is at least 10 GPa, preferably 10-13 GPa, more preferably higher than 13 GPa.

17. Watch exterior component or watch movement component obtainable by the method according to any one of claims 9 to 12.

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