A method for grain boundary purification of boron-doped molybdenum-tungsten refractory metal

Abstract

The application provides a boron-doped molybdenum-tungsten refractory metal grain boundary purification method, and belongs to the technical field of powder metallurgy engineering. Pure boron, boric acid, molybdate and tungstate, rare earth nitrate and carbide are put into deionized water to prepare a solution or a suspension; then, the solution or the suspension is pre-frozen by liquid nitrogen and freeze-dried in a freeze dryer; the precursor powder is subjected to two-step calcination and two-step reduction; cold isostatic pressing is performed after ball milling; and finally, the boron-doped molybdenum-tungsten alloy is obtained through multi-step sintering in a reducing atmosphere. The composite powder obtained by the application has fine and uniform grains, the molybdenum-tungsten alloy sintered by using the composite powder has fine grain structure, high sintering relative density, and fine and dispersed carbide / oxide, and more importantly, the grain boundary of the boron-doped molybdenum-tungsten alloy is modified, the fracture morphology is changed from intergranular fracture to quasi-cleavage fracture, which helps to improve the low-temperature brittleness of the molybdenum-tungsten alloy and significantly optimize the room temperature mechanical properties of the molybdenum-tungsten alloy.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of powder metallurgy engineering, and particularly relates to a boron-doped molybdenum-tungsten refractory metal grain boundary purification method. BACKGROUND

[0002] Molybdenum alloy and tungsten alloy have excellent softening resistance, thermal stability, heat corrosion resistance and superior mechanical properties at high temperature, and are widely used in nuclear fusion materials, armor-piercing bullets, engines, ship boilers and other fields. However, as the use requirements of molybdenum alloy and tungsten alloy become higher and higher, BCC Mo / W metal often faces problems such as coarse grain size, insufficient strength, low-temperature brittleness and the like. At present, the addition of second phase particles such as oxides (Y2O3, La2O3, CeO2, etc.) and carbides (ZrC, TiC, HfC, etc.) in Mo / W alloy has been widely used to improve the above problems. The intragranular second phase particles effectively improve the alloy strength by pinning dislocations, and the intergranular particles refine the metal grains and improve the alloy thermal stability. The second phase dispersion strengthening and fine grain strengthening complement each other, and are the most important strengthening method for molybdenum alloy and tungsten alloy at present.

[0003] In contrast, the grain boundary strengthening of molybdenum alloy and tungsten alloy is easily ignored. The current research work related to the grain boundary of tungsten molybdenum alloy is usually limited to how to increase the number of grain boundaries by improving the sintering process to reduce the sintering temperature, or selecting a more suitable second phase to refine the grains. However, there are few studies on the modification of the grain boundary itself to improve the strength, and the optimization space is huge. On the other hand, with the grain refinement of tungsten molybdenum alloy, the O impurity content at the grain boundary also increases, which easily leads to grain boundary debonding in the use process. Grain boundary debonding is considered to be the root cause of the brittleness of many alloys, leading to intergranular cracking. This brittle intergranular fracture is a disastrous rapid failure mode, which has a very adverse effect on the ductility of the alloy. More importantly, the low-temperature brittleness of BCC Mo / W alloy will bring difficulties to the subsequent room temperature processing such as forging and rolling, which also greatly limits the room temperature preparation and processability of alloy parts. Therefore, how to modify the grain boundary of tungsten molybdenum alloy to improve the above problems is very important. SUMMARY

[0004] The present application provides a boron-doped molybdenum-tungsten refractory metal grain boundary purification method to solve the technical problems in the prior art. The novel freeze-drying method is used to dope boron components, realizing the modification of the grain boundary of tungsten molybdenum alloy and the improvement of the room temperature mechanical properties. The composite powder obtained by the present application has small grain size, and the sintered alloy has small grain size, high relative density and small carbide / oxide dispersion. More importantly, the grain boundary of the boron-doped molybdenum-tungsten alloy is modified, and the fracture morphology changes from intergranular fracture to quasi-cleavage fracture.

[0005] The freeze-drying method has the advantages of simple operation process, good controllability of powder, high quality, fine particle size and extremely narrow distribution, and is capable of preparing ultrafine boron-doped molybdenum-tungsten composite powder with high sintering activity, and dispersing the second phase particles and B element.

[0006] The technical scheme adopted by the present application is as follows: a boron-doped molybdenum-tungsten refractory metal grain boundary purification method, comprising the following steps:

[0007] Step 1: one or both of pure boron and boric acid, one or more of molybdate and tungstate, one or more of rare earth nitrate and carbide are put into deionized water to prepare a solution or suspension under ultrasonic treatment and magnetic stirring;

[0008] Step 2: the solution or suspension is sprayed into liquid nitrogen for pre-freezing, and then put into a freeze dryer for freeze-drying for 50-100 hours under a vacuum degree of 30 Pa or less to obtain a freeze-dried precursor powder;

[0009] Step 3: the freeze-dried precursor powder is subjected to two-step calcination and two-step reduction in a reducing atmosphere to obtain boron-doped molybdenum-tungsten composite powder;

[0010] Step 4: the boron-doped molybdenum-tungsten composite powder is ball milled, and then cold isostatic pressed to obtain a pre-formed powder;

[0011] Step 5: the pre-formed powder is subjected to multi-step sintering in a reducing atmosphere and cooled to room temperature in a reducing atmosphere to finally obtain boron-doped molybdenum-tungsten alloy.

[0012] Further, in step 1, the molybdate includes ammonium molybdate, ammonium molybdate tetrahydrate and ammonium molybdate heptahydrate; the tungstate includes ammonium paratungstate and ammonium metatungstate; the rare earth nitrate includes yttrium nitrate, lanthanum nitrate, zirconium nitrate, cerium nitrate, erbium nitrate, hafnium nitrate and ytterbium nitrate; and the carbide particles include titanium carbide, zirconium carbide and hafnium carbide.

[0013] Further, in step 1, the concentration of the molybdate in the solution is 0.01-0.4 g / mL; and the concentration of the tungstate in the solution is 0.01-0.2 g / mL.

[0014] Further, in step 1, the total mass of the pure boron and boric acid is 0.05-2% of the total mass of the molybdate and tungstate; and the total mass of the rare earth nitrate and carbide is 0.1-3% of the total mass of the molybdate and tungstate.

[0015] Furthermore, in step 1, the ultrasonic treatment power is 100-300W, the ultrasonic treatment time is 0.5-3h, and the magnetic stirring is 0.5-300r / min.

[0016] The various concentration parameters, proportioning parameters, ultrasonic and stirring parameters involved in step 1 are all set for specific pure boron or boric acid doping. Within this range, pure boron or boric acid will be dispersed as much as possible, without sedimentation, polymerization, or other phenomena.

[0017] Furthermore, in step 2, the freeze-drying temperature in the freeze dryer is below -40°C.

[0018] Furthermore, in step 3, in a flowing air atmosphere, the sample is first calcined at 120–150°C in a constant temperature furnace for 0.5–1 h, and then transferred to another constant temperature furnace for calcination at 400–500°C for 0.5–2 h; then reduced with pure hydrogen at 600–750°C and 800–900°C for 1.5–3 h respectively.

[0019] Step 3 involves two calcination and reduction parameters set specifically for pure boron or boric acid doping. The purpose of the first step, low-temperature calcination, is to remove as much water of crystallization as possible while preventing the boric acid from melting. The purpose of the second step, rapid furnace-change calcination, is to decompose the boric acid as quickly as possible before it melts, yielding boron oxide products. Boron doping also affects the concentration. The purpose of the first step, low-temperature reduction, is to cause the oxide obtained from calcination to crack first, turning large particles into smaller particles, which is more conducive to subsequent reduction by reducing gas. The purpose of the second step, high-temperature reduction, is to reduce the composite oxide as much as possible into boron-doped molybdenum-tungsten composite powder.

[0020] Furthermore, in step 4, the ball milling parameters are 200-400 r / min for 1-20 h, and the ball-to-material mass ratio is 3:1-10:1; the cold isostatic pressing parameters are 150-300 MPa for 0.25-2 h.

[0021] Furthermore, in step 4, the boron-doped molybdenum-tungsten composite powder is ball-milled in an argon protective atmosphere.

[0022] The ball milling and cold isostatic pressing parameters involved in step 4 are set for boron-doped molybdenum-tungsten composite powders. Ball milling aims to break down some large particles and increase the sintering activity of the powder; cold isostatic pressing is to increase the relative density as much as possible during the preforming process.

[0023] Furthermore, in step 5, the metal is sintered with pure hydrogen at 1100–1200℃, 1250–1350℃, and 1400–1500℃ for 1–2 hours respectively, and finally sintered at 1600–1800℃ for 2–8 hours.

[0024] The multi-step sintering parameters involved in step 5 are set for the boron-doped molybdenum-tungsten composite powder. The multi-step sintering can increase the relative density of the boron-doped molybdenum-tungsten alloy, reduce the grain size, and reduce the generation of cracks.

[0025] Compared with the prior art, the present application has the beneficial effects that:

[0026] 1. The present application adopts the freeze-drying method, which can realize good regulation and control of the size, shape and distribution of powder grains on the basis of ensuring purity and uniformity of doping.

[0027] 2. The present application adopts freeze-drying, which does not need to use expensive alcohol, and the freeze-drying effectively improves particle agglomeration. When water is frozen into ice, the volume expands and becomes larger, which properly separates the particles close to each other, and the formation of solid prevents the re-aggregation of solutes, so the agglomeration is small, which is very suitable for boron doping.

[0028] 3. After low-temperature sintering of the boron-doped molybdenum-tungsten composite powder of the present application, the grain of the sintered alloy is small, the relative density is high, and the carbide / oxide is small and dispersed.

[0029] 4. The grain boundary of the boron-doped molybdenum-tungsten alloy of the present application is modified, which solves the low-temperature brittleness problem of the traditional molybdenum-tungsten alloy.

[0030] 5. The fracture of the boron-doped molybdenum-tungsten alloy of the present application is no longer intergranular fracture, but quasi-cleavage fracture, which shows that the lowest energy interface of the molybdenum-tungsten alloy is no longer the grain boundary but the intracrystalline, thereby improving the threshold of the entire alloy system, thereby being beneficial to improving the performance.

[0031] 6. The present application not only can realize the preparation of high-performance ultra-fine molybdenum-tungsten alloy, but also is very suitable for the preparation of a large number of high-performance molybdenum-tungsten alloy in a single batch. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 SEM image of Mo-W-Y2O3-B composite powder prepared in Example 1 of the present application;

[0033] Figure 2 SEM image of the fracture of Mo-W-Y2O3-B alloy prepared in Example 1 of the present application;

[0034] Figure 3 SEM image of the fracture of Mo-W-Y2O3 alloy prepared in Example 1 of the present application;

[0035] Figure 4 Comparison chart of room temperature compression performance of Mo-W-Y2O3 alloy and Mo-W-Y2O3-B alloy prepared in Example 1 of the present application;

[0036] Figure 5The fracture SEM image of the Mo-W-La2O3-B alloy prepared in Example 2 of the present application. DETAILED DESCRIPTION

[0037] In order to make the skilled in the art better understand the technical solutions of the present application, the present application will be described in detail below in combination with the drawings and specific examples.

[0038] Example 1

[0039] The embodiments of the present application provide a method for purifying the grain boundary of boron-doped molybdenum-tungsten refractory metal, which comprises the following steps:

[0040] Step 1: 0.4 g of boric acid, 100 g of ammonium molybdate tetrahydrate and 10 g of ammonium metatungstate, 0.5 g of yttrium nitrate hexahydrate are put into 1000 mL of deionized water, and ultrasonic treatment is carried out at a power of 200 W for 1 h, and magnetic stirring is carried out during the ultrasonic treatment at a rotating speed of 50 r / min, to prepare a solution;

[0041] Step 2: The solution is sprayed into liquid nitrogen for pre-freezing, and then put into a freeze-drying machine, and freeze-drying is carried out at a freeze-drying temperature of-60℃ and a vacuum degree of 30 Pa or less for 50 h, to prepare a loose freeze-dried precursor powder;

[0042] Step 3: The freeze-dried precursor powder is first calcined at 150℃ for 1 h in a constant-temperature furnace, and then transferred to another constant-temperature furnace for calcination at 500℃ for 1 h; and then reduced at 600℃ for 1.5 h and at 800℃ for 2 h in pure hydrogen;

[0043] Step 4: The reduced boron-doped molybdenum-tungsten composite powder is ball milled at 200 r / min for 3 h in an argon protective atmosphere, with a ball-to-material mass ratio of 10:1, to break large particles in the powder, and then cold isostatic pressing is carried out at a pressure of 250 MPa for 0.5 h to pre-form the powder;

[0044] Step 5: The pre-formed powder is sintered at 1100℃, 1250℃ and 1400℃ respectively for 1 h in a pure hydrogen atmosphere, and then sintered at 1650℃ for 6 h, and finally cooled to room temperature in a reducing atmosphere (pure hydrogen), to finally obtain a boron-doped molybdenum-tungsten alloy.

[0045] The average grain size of the Mo-W-Y2O3-B powder prepared in Step 2 of Example 1 is only 90 nm, the grain is extremely small and the particle size scale is extremely narrow, and the surface morphology is as shown in Figure 1 . After low-temperature sintering, the fracture morphology of the final product is as shown in Figure 2 , the grain is fine and only about 1 μm, and the fracture presents a quasi-cleavage fracture morphology.

[0046] Comparative Example 1

[0047] As a comparison, a Mo-W-Y2O3 alloy was prepared, and the remaining steps were the same as Example 1 except that boric acid was not added. It was found that the Mo-W-Y2O3 grain was also about 1 μm, but the fracture morphology was a traditional intergranular fracture morphology, and the surface morphology was as shown in Figure 3 .

[0048] The room temperature compression performance of the Mo-W-Y2O3 alloy and the Mo-W-Y2O3-B alloy is as shown in Figure 4 , and it can be found that after B doping, the compression strength and compression deformation are sharply improved, the strength is increased by more than 4 times, and the deformation is also increased by more than 4 times. In summary, the addition of boron in the tungsten-molybdenum alloy does not affect the grain size, but purifies the grain boundary, which helps to improve the low-temperature brittleness of the molybdenum-tungsten alloy and is beneficial to improve the mechanical properties of the molybdenum-tungsten alloy.

[0049] Example 2

[0050] The embodiment of the present application provides a boron-doped molybdenum-tungsten refractory metal grain boundary purification method, which comprises the following steps:

[0051] Step 1: 0.85g boric acid, 150g ammonium molybdate and 20g ammonium metatungstate, 0.17g lanthanum nitrate are put into 1000ml deionized water together, ultrasonic treatment is carried out under 100w power for 3h, and magnetic stirring is carried out during ultrasonic treatment, and the rotating speed is 180r / min, to prepare a solution;

[0052] Step 2: the solution is sprayed into liquid nitrogen for pre-freezing, and then put into a freeze-drying machine, and the freeze-drying temperature is-50℃, the vacuum degree is below 20Pa, and the freeze-drying time is 60h, to prepare loose freeze-dried precursor powder;

[0053] Step 3: the freeze-dried precursor powder is first calcined at 120℃ for 1h in a constant-temperature furnace, and then transferred to another constant-temperature furnace for calcination at 400℃ for 2h; and then reduced at 700℃ for 2h and at 850℃ for 1.5h in pure hydrogen;

[0054] Step 4: the boron-doped molybdenum-tungsten composite powder after reduction is ball milled in an argon protective atmosphere at 200r / min for 5h, the ball-to-material mass ratio is 8:1, to break large particles in the powder, and then cold isostatic pressing is carried out on the powder at a pressure of 300MPa for 0.25h, to pre-form the powder;

[0055] Step 5: the pre-formed powder is sintered at 1200℃, 1350℃ and 1500℃ respectively for 1.6h in a pure hydrogen atmosphere, and then sintered at 1800℃ for 4h, and finally cooled to room temperature in a reducing atmosphere, to finally obtain a boron-doped molybdenum-tungsten alloy.

[0056] The Mo-W-La2O3-B alloy prepared in Example 2 has a grain size of about 10 μm, and the fracture morphology is shown in Fig. 2, which presents quasi-cleavage fracture morphology after grain boundary modification. Figure 5 As can be seen, the doped B can purify the grain boundary, which helps to improve the low-temperature brittleness of the molybdenum-tungsten alloy and is conducive to improving the mechanical properties of the molybdenum-tungsten alloy.

[0057] Example 3

[0058] The embodiment of the present application provides a grain boundary purification method of boron-doped molybdenum-tungsten refractory metal, which comprises the following steps:

[0059] Step 1: 3g of boric acid, 200g of ammonium molybdate and 100g of ammonium metatungstate, 9g of zirconium nitrate are put into 1000mL of deionized water, ultrasonic treatment is carried out at a power of 200W for 2h, magnetic stirring is carried out during ultrasonic treatment, and the rotating speed is 30r / min, so as to prepare a solution;

[0060] Step 2: the solution is sprayed into liquid nitrogen for pre-freezing, and then is put into a freeze dryer, the freeze-drying temperature is-55℃, the vacuum degree is below 25Pa, and the freeze-drying is carried out for 100h, so as to prepare loose freeze-dried precursor powder;

[0061] Step 3: the freeze-dried precursor powder is first calcined at 140℃ for 0.5h in a constant-temperature furnace, then is transferred to another constant-temperature furnace and calcined at 450℃ for 1.5h, and then is reduced at 750℃ for 1.8h and at 900℃ for 1.5h in pure hydrogen;

[0062] Step 4: the reduced boron-doped molybdenum-tungsten composite powder is ball milled at 300r / min for 6h in an argon protection atmosphere, the ball-to-material mass ratio is 6:1, so as to break large particles in the powder, and then the powder is preformed by cold isostatic pressing at a pressure of 150MPa for 2h;

[0063] Step 5: the preformed powder is sintered at 1150℃, 1300℃ and 1450℃ respectively for 1.8h in a pure hydrogen atmosphere, and then is sintered at 1600℃ for 8h, finally is cooled to room temperature in a reducing atmosphere, and finally boron-doped molybdenum-tungsten alloy is obtained.

[0064] The Mo-W-ZrO2-B alloy prepared in Example 3 has a fine grain size of about 500nm, and presents quasi-cleavage fracture morphology after grain boundary modification. As can be seen, the doped B can purify the grain boundary, which helps to improve the low-temperature brittleness of the molybdenum-tungsten alloy and is conducive to improving the mechanical properties of the molybdenum-tungsten alloy.

[0065] Example 4

[0066] The embodiment of the present application provides a grain boundary purification method of boron-doped molybdenum-tungsten refractory metal, which comprises the following steps:

[0067] Step 1: Put 10 g of boric acid, 300 g of ammonium molybdate and 200 g of ammonium tungstate, 0.5 g of erbium nitrate into 1000 mL of deionized water, ultrasonic for 2.5 h under the power of 150 W, magnetic stirring during ultrasonic treatment, the speed is 100 r / min, to prepare a solution;

[0068] Step 2: The solution is sprayed into liquid nitrogen for pre-freezing, and then put into a freeze dryer, freeze-dried at a freeze-drying temperature of-65℃ and a vacuum degree of 15 Pa or less for 50 h to obtain a loose freeze-dried precursor powder;

[0069] Step 3: The freeze-dried precursor powder is first calcined at 120℃ for 0.9 h in a constant temperature furnace, then transferred to another constant temperature furnace and calcined at 480℃ for 0.5 h, and then reduced at 750℃ for 1.5 h and at 820℃ for 1.8 h in pure hydrogen atmosphere;

[0070] Step 4: The reduced boron-doped molybdenum-tungsten composite powder is ball milled at 250 r / min for 10 h in an argon protective atmosphere, the ball-to-material mass ratio is 5:1, to break the large particles in the powder, and then preformed by cold isostatic pressing at a pressure of 180 MPa for 0.4 h;

[0071] Step 5: Finally, the preformed powder is sintered at 1120℃, 1270℃ and 1420℃ respectively for 1.2 h, and then sintered at 1650℃ for 6 h in a pure hydrogen atmosphere, and finally cooled to room temperature in a reducing atmosphere, to obtain a boron-doped molybdenum-tungsten alloy.

[0072] The Mo-W-Er2O3-B alloy prepared in Example 4 has a fine grain size of only about 1.5 μm, and exhibits a quasi-cleavage fracture morphology after grain boundary modification. It can be seen that the doped B can purify the grain boundary, which helps to improve the low-temperature brittleness of the molybdenum-tungsten alloy and improve the mechanical properties of the molybdenum-tungsten alloy.

[0073] Example 5

[0074] The embodiment of the present application provides a grain boundary purification method of boron-doped molybdenum-tungsten refractory metal, which comprises the following steps:

[0075] Step 1: Put 0.225 g of pure boron, 400 g of ammonium molybdate, 50 g of ammonium paratungstate, 1 g of lanthanum nitrate and 1.25 g of titanium carbide into 1000 mL of deionized water, ultrasonic for 1.5 h under the power of 250 W, magnetic stirring during ultrasonic treatment, the speed is 60 r / min, to prepare a solution;

[0076] Step 2: The solution is pre-frozen by spraying into liquid nitrogen, and then is placed into a freeze dryer to be freeze-dried at a freeze-drying temperature of -40℃ and a vacuum degree of 30Pa or less for 80h to obtain loose freeze-dried precursor powder;

[0077] Step 3: The freeze-dried precursor powder is calcined at 130℃ for 0.6h in a constant-temperature furnace, and then is transferred to another constant-temperature furnace to be calcined at 470℃ for 0.8h, and then is reduced at 640℃ for 1.5h and at 860℃ for 2.1h in pure hydrogen;

[0078] Step 4: The reduced boron-doped molybdenum-tungsten composite powder is broken into large particles by ball milling at 400r / min for 1h in an argon protective atmosphere with a ball-to-material mass ratio of 4:1, and then is pre-formed by cold isostatic pressing at a pressure of 200MPa for 0.6h;

[0079] Step 5: Finally, the pre-formed powder is sintered at 1140℃, 1290℃ and 1440℃ for 1.5h respectively, and then is sintered at 1750℃ for 3h in a pure hydrogen atmosphere, and finally is cooled to room temperature in a reducing atmosphere to obtain a boron-doped molybdenum-tungsten alloy.

[0080] The Mo-W-TiC-B alloy prepared in the embodiment 5 has fine grains of about 6μm and presents quasi-cleavage fracture morphology after grain boundary modification.

[0081] Embodiment 6

[0082] The embodiment of the present application provides a boron-doped molybdenum-tungsten refractory metal grain boundary purification method, which comprises the following steps:

[0083] Step 1: 1g of boric acid, 1g of pure boron, 50g of ammonium molybdate and 150g of ammonium paratungstate, and 1.5g of zirconium carbide are placed into 1000mL of deionized water, and ultrasonic treatment is carried out at a power of 180W for 2.4h, and magnetic stirring is carried out at a rotating speed of 90r / min during the ultrasonic treatment to prepare a solution;

[0084] Step 2: The solution is pre-frozen by spraying into liquid nitrogen, and then is placed into a freeze dryer to be freeze-dried at a freeze-drying temperature of -70℃ and a vacuum degree of 18Pa or less for 100h to obtain loose freeze-dried precursor powder;

[0085] Step 3: The freeze-dried precursor powder is calcined at 140℃ for 0.7h in a constant-temperature furnace, and then is transferred to another constant-temperature furnace to be calcined at 420℃ for 1.8h, and then is reduced at 670℃ for 3h and at 870℃ for 2.6h in pure hydrogen;

[0086] Step 4: The reduced boron-doped molybdenum-tungsten composite powder is ball milled in an argon protective atmosphere at 350 r / min for 1.5 h, with a ball-to-powder mass ratio of 3:1, to break up large particles in the powder, and then cold isostatic pressing is performed on the powder at a pressure of 220 MPa for 1 h to pre-form the powder;

[0087] Step 5: Finally, the pre-formed powder is sintered in a pure hydrogen atmosphere at 1160℃, 1310℃, and 1460℃, respectively, for 2 h, and then sintered at 1800℃ for 2 h, and finally cooled to room temperature in a reducing atmosphere, to obtain a boron-doped molybdenum-tungsten alloy.

[0088] The Mo-W-ZrC-B alloy prepared in Example 6 has a fine grain size of only about 8 μm and exhibits a quasi-cleavage fracture morphology after grain boundary modification. It can be seen that the doped B can purify the grain boundary, which helps to improve the low-temperature brittleness of the molybdenum-tungsten alloy and is conducive to improving the mechanical properties of the molybdenum-tungsten alloy.

[0089] Example 7

[0090] The embodiment of the present application provides a boron-doped molybdenum-tungsten refractory metal grain boundary purification method, which comprises the following steps:

[0091] Step 1: 0.5 g of boric acid, 0.5 g of pure boron, 10 g of ammonium molybdate tetrahydrate, and 90 g of ammonium paratungstate, 0.5 g of erbium nitrate, and 0.5 g of hafnium carbide are placed in 1000 mL of deionized water, and ultrasonic treatment is performed at a power of 220 W for 2.8 h, with magnetic stirring during the ultrasonic treatment at a speed of 120 r / min, to prepare a solution;

[0092] Step 2: The solution is sprayed into liquid nitrogen for pre-freezing, and then placed in a freeze dryer, and freeze-dried at a freeze-drying temperature of -75℃ and a vacuum degree of 13 Pa or less for 90 h, to obtain a loose freeze-dried precursor powder;

[0093] Step 3: The freeze-dried precursor powder is first calcined at 145℃ for 1 h in a constant-temperature furnace, then transferred to another constant-temperature furnace and calcined at 475℃ for 0.9 h, and then reduced in pure hydrogen at 690℃ for 2.5 h and at 890℃ for 2.9 h;

[0094] Step 4: The reduced boron-doped molybdenum-tungsten composite powder is ball milled in an argon protective atmosphere at 230 r / min for 7 h, with a ball-to-powder mass ratio of 9:1, to break up large particles in the powder, and then cold isostatic pressing is performed on the powder at a pressure of 260 MPa for 1.5 h to pre-form the powder;

[0095] Step 5: Finally, the pre-formed powder is sintered at 1180℃, 1330℃, 1480℃ respectively for 1.5h, and then sintered at 1690℃ for 8h in pure hydrogen atmosphere, and finally cooled to room temperature in reducing atmosphere, to obtain the boron-doped molybdenum-tungsten alloy.

[0096] The Mo-W-HfC-B alloy prepared in Example 7 has fine grains of about 2μm, and shows quasi-cleavage fracture morphology after grain boundary modification. It can be seen that the doped B can purify the grain boundary, which helps to improve the low-temperature brittleness of the molybdenum-tungsten alloy, and is beneficial to improve the mechanical properties of the molybdenum-tungsten alloy.

[0097] The above has been described in detail by way of examples, but the content described is only exemplary embodiments of the present application, and cannot be considered to limit the scope of the embodiments of the present application. The scope of protection of the present application is defined by the claims. Any similar technical solution that utilizes the technical solutions described in the present application, or that is inspired by the technical solutions of the present application within the spirit and protection scope of the present application, and that achieves the above technical effects, or any equivalent changes and improvements to the scope of the application, shall still fall within the scope of the patent protection of the present application.

Claims

1. A method of grain boundary purification of boron-doped molybdenum-tungsten refractory metal, characterized by, The method comprises the following steps: Step 1: pure boron and boric acid, one or more of molybdate and one or more of tungstate, one or more of rare earth nitrate and one or more of carbide are put into deionized water to prepare a solution or suspension under ultrasonic treatment and magnetic stirring; Step 2: the solution or suspension is sprayed into liquid nitrogen for pre-freezing, and then is put into a freeze dryer to be freeze-dried for 50-100 hours under a vacuum degree of 30 Pa or less to obtain a freeze-dried precursor powder; Step 3: the freeze-dried precursor powder is two-step calcined and two-step reduced in a reducing atmosphere to obtain a boron-doped molybdenum-tungsten composite powder; Step 4: the boron-doped molybdenum-tungsten composite powder is ball milled and then cold isostatic pressed to obtain a pre-formed powder; Step 5: the pre-formed powder is multi-step sintered in a reducing atmosphere and cooled to room temperature in a reducing atmosphere to finally obtain a boron-doped molybdenum-tungsten alloy; In step 1, the total mass of pure boron and boric acid is 0.05-2% of the total mass of molybdate and tungstate; the total mass of rare earth nitrate and carbide is 0.1-3% of the total mass of molybdate and tungstate; In step 5, the pure hydrogen is used for sintering at 1100-1200℃, 1250-1350℃ and 1400-1500℃ for 1-2 hours respectively, and finally sintered at 1600-1800℃ for 2-8 hours.

2. A method of grain boundary purification of boron-doped molybdenum-tungsten refractory metal as claimed in claim 1, wherein, In step 1, the molybdate includes ammonium molybdate, ammonium molybdate tetrahydrate and ammonium molybdate heptahydrate; the tungstate includes ammonium paratungstate and ammonium metatungstate; the rare earth nitrate includes yttrium nitrate, lanthanum nitrate, zirconium nitrate, cerium nitrate, erbium nitrate, hafnium nitrate and ytterbium nitrate; the carbide particles include titanium carbide, zirconium carbide and hafnium carbide.

3. A method of grain boundary purification of boron-doped molybdenum-tungsten refractory metal as claimed in claim 1, wherein, In step 1, the concentration of molybdate in the solution is 0.01-0.4 g / mL; the concentration of tungstate in the solution is 0.01-0.2 g / mL.

4. A method of grain boundary purification of boron-doped molybdenum-tungsten refractory metal as claimed in claim 1, wherein, In step 1, the ultrasonic treatment power is 100-300 W, and the ultrasonic treatment time is 0.5-3 hours; the magnetic stirring is 0.5-300 r / min.

5. A method of grain boundary purification of boron-doped molybdenum-tungsten refractory metal as claimed in claim 1, wherein, In step 2, the freeze-drying temperature in the freeze dryer is -40℃ or lower.

6. A method for grain boundary purification of boron-doped molybdenum-tungsten refractory metal as claimed in claim 1, wherein, In step 3, in the flowing air atmosphere, the first calcination is carried out at 120-150℃ in a constant temperature furnace for 0.5-1 hour, and then the second calcination is carried out at 400-500℃ in another constant temperature furnace for 0.5-2 hours; and then the first reduction is carried out at 600-750℃ for 1.5-3 hours and the second reduction is carried out at 800-900℃ for 1.5-3 hours.

7. A method for grain boundary purification of boron-doped molybdenum-tungsten refractory metal as claimed in claim 1, wherein, In step 4, the ball milling parameters are 200-400 r / min for 1-20 hours, and the ball-to-material mass ratio is 3:1-10:1; the cold isostatic pressing parameters are cold isostatic pressing at a pressure of 150-300 MPa for 0.25-2 hours.

8. A method of grain boundary purification of boron-doped molybdenum-tungsten refractory metal as claimed in claim 1 or 7, wherein, In step 4, the boron-doped molybdenum-tungsten composite powder is ball milled in an argon protective atmosphere.

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