CN113881863B - A kind of preparation method of NiTi-Al based alloy - Google Patents

Abstract

The invention relates to a preparation method of NiTi-Al-based alloy. The method uses a vacuum non-consumable electric arc furnace to prepare NiTi-Al-based as-cast alloy, cuts out a bar from the as-cast alloy, and removes the oxide scale on the surface of the bar. After grinding, put it into an alumina ceramic tube with a Y ₂ O ₃ sintered coating on the inner wall. The ceramic tube containing the NiTi-Al-based as-cast alloy bars was put into a Bridgman-type directional solidification furnace cooled by liquid metal, and after being evacuated to 3-5 × 10-3Pa, filled with high-purity argon gas to make the vacuum degree reach 0.05MPa, heated to 1450℃～1650℃ and kept for 20～40min, pulled at a rate of 0.05～0.2mm/min and then quenched into liquid metal, and finally carried out solution and aging treatment to prepare NiTi-Al based alloy. The room temperature plasticity and high temperature strength of the NiTi-Al-based alloy prepared by the above process have been improved.

Description

A kind of preparation method of NiTi-Al based alloy

technical field

The invention relates to a preparation method of NiTi-Al base alloy, which belongs to the technical field of preparation of light-weight and high-strength NiTi-Al intermetallic compounds.

Background technique

Since the experimental study by Dr. WJ Buehler's research group of the US Naval Weapons Laboratory in 1963 found that NiTi alloys with nearly equal atomic ratios have good shape memory effects, after decades of research, NiTi-based alloys, as a shape memory material, have Excellent mechanical properties, corrosion resistance and biocompatibility have been widely used in aviation, aerospace, energy, medical and construction fields. At the same time, as an intermetallic compound, it has attracted extensive attention of scientific researchers because of its good plasticity at room temperature and high strength after certain alloying. In 1997, Y. Koizumi et al reported NiTi-Al based on NiTi for the first time in "NiTi-base intermetallic alloys strengthened by Al substitution" Materials Science and Engineering A, 1997, 223(1-2):36-41. Based on ternary alloy, the results show that adding Al element in NiTi alloy will form a strengthening phase-Ni ₂ TiAl phase on the NiTi matrix phase after a certain heat treatment. The Ni ₂ TiAl phase has a higher melting point (1513°C), and its yield strength above 750°C is 2-3 times higher than that of NiAl and TiAl. In addition, the creep resistance of Ni ₂ TiAl is higher than that of the B2 structure with high symmetry The strength of NiAl at room temperature exceeds that of Ni-based superalloy Rene95, and the strength at 1000 °C is comparable to that of superalloys currently used in the medium temperature range;

However, in NiTi-Al-based alloys, in addition to the strengthening phase Ni ₂ TiAl phase, there is another precipitation phase—Ti ₂ Ni phase, and the volume fraction is generally above 2% (atomic percentage, the same below), such as In the Ni-43Ti-7Al alloy, the volume fraction of the Ti ₂ Ni phase is about 2.5%. Kollerov found in the research of "Structural aspects of the manufacture of semiproducts made from titanium nickelide-based alloys" Russian Metallurgy (Metal), 2007, 2007(5): 408-414: The microhardness of Ti ₂ Ni phase reaches 10Gpa, which is 4 to 5 times the B2 phase, so the strength of NiTi-Al based alloys can be significantly improved. However, the research results of Mentz et al. in "Improvement of Mechanical Properties of Powder Metallurgical NiTi Shape Memory" Advanced Engineering Materials, 2006, 8(4): 247-252 show that the Ti ₂ Ni phase is a typical brittle phase, and the size is often smaller Large (micron scale), showing a strip-like or droplet-like morphology, it often becomes the origin of cracks when the alloy is fractured, thus adversely affecting the room temperature plasticity of the alloy. At the same time, since the melting point of the Ti ₂ Ni phase is only about 984°C, at a high temperature (such as 800°C), the Ti ₂ Ni strengthened phase has already begun to soften, and is basically in the stage of dynamic recovery and recrystallization. Melting elements also cannot greatly increase their high temperature strength.

Therefore, how to eliminate or reduce the Ti ₂ Ni phase is the key to improving the mechanical properties of NiTi-Ni ₂ TiAl dual-phase NiTi-Al based alloys. However, FCYen et al. found in "Shape memory characteristics and mechanicalproperties of high-density powder metal TiNi with post-sintering heattreatment" Materials Science and Engineering A, 2011, 528(15): 5296-5305: even in excess of Ti ₂ Ni Annealing at the temperature (1004°C) of the melting point of the phase (984°C) could not eliminate the Ti ₂ Ni phase. Pan Liwen et al. in "Directional Solidification Ultra-High Strength NiTi Matrix In-situ Composites" Journal of Composite Materials, 2013, 1(30): 141-146 found that: Ni-43Ti-4A-2Nb-2Hf alloy after 950 ℃ After the vacuum homogenization heat treatment, not only the Ti ₂ Ni phase was not eliminated, but also the volume fraction of the Ti ₂ Ni phase increased with the extension of the homogenization heat treatment time. It can be seen that once the Ti ₂ Ni phase is formed in the NiTi-Al based alloy, it is difficult to be eliminated by the subsequent heat treatment process. In addition, XiaoyunSong et al. also found in "Microstructure and mechanical properties of Nb-and Mo-modified NiTi–Al-based intermetallics processed by isothermal forging" Materials Science and Engineering A, 2014, 594(15): 229-234, even if The large-sized Ti ₂ Ni phase is broken by isothermal forging, but the Ti ₂ Ni phase is still distributed at the grain boundaries; although the elongation after fracture at room temperature is improved compared with the as-cast alloy, it is still low, only less than 2%, which limits the wide application of NiTi-Al based alloys.

SUMMARY OF THE INVENTION

The present invention is designed to provide a preparation method of NiTi-Al-based alloy just in view of the above-mentioned existing technical situation, and its purpose is to solve the problems of low room temperature plasticity and insufficient high-temperature strength of NiTi-Al-based alloy faced by conventional preparation technology, and obtain The NiTi-Al based alloy has excellent room temperature plasticity and high temperature strength properties.

For implementing the above object, the technical solution of the present invention is as follows:

The steps of the preparation method of this NiTi-Al based alloy are as follows:

Step 1, using a vacuum non-consumable electric arc furnace to prepare a NiTi-Al based as-cast alloy;

Step 2, cut out the bar stock from the as-cast alloy prepared in Step 1, and put it into an alumina ceramic tube with a Y ₂ O ₃ sintered coating on the inner wall;

Step 3. Put the alumina ceramic tube with NiTi-Al-based as-cast alloy bars into the Bridgman type directional solidification furnace cooled by liquid metal, and after vacuuming to 3～5×10 ^-3 Pa, fill with high purity Argon gas makes the vacuum degree reach 0.05MPa, heated to 1450℃～1650℃ and kept for 20～40min, pulled at a rate of 0.05～0.2mm/min and then quenched into the liquid metal, and finally subjected to solid solution and aging treatment to prepare the NiTi-Al based alloys.

In the implementation, in step 2, the wire cutting method is used to cut the bar from the as-cast alloy.

In the implementation, in step 2, after cutting the bar, the oxide scale on its surface is polished clean.

In implementation, the liquid metal quenched in step 3 is a Ga-In-Sn alloy.

In implementation, the NiTi-Al-based as-cast alloy in step 1 is a Ni-43Ti-7Al as-cast alloy. Further, the solid solution and aging treatment described in step 3 is solid solution + secondary aging treatment, and the solution treatment system is 1170°C/12h/air cooling; the first-level aging system is 700°C/15h and then heated to 800°C for secondary Aging, after holding time of 5h, air-cooled.

In implementation, the NiTi-Al-based as-cast alloy in step 1 is Ni-41Ti-7Al-1Cr-1Nb as-cast alloy. Further, the solid solution and aging treatment described in step 3 is solid solution + secondary aging treatment, and the solution treatment system is 1210°C/12h/air cooling; the primary aging system is 730°C/15h, and then the temperature is raised to 830°C for secondary Aging, after holding time of 5h, air-cooled.

Compared with the traditional NiTi-Al-based as-cast or powder alloy, the NiTi-Al-based directional solidification alloy prepared by the directional solidification preparation process in the technical scheme of the present invention not only has a small size of the Ti ₂ Ni phase, but also is dispersed and distributed at the grain boundary. And the volume fraction is greatly reduced. In addition, due to the decrease in the volume fraction of Ti ₂ Ni phase, the Ni/Ti ratio and Al content on the alloy matrix were adjusted. After solution and aging treatment, the precipitation of Ni ₂ TiAl strengthening phase was more sufficient, thereby significantly improving the alloy's performance. Room temperature plasticity and high temperature strength.

The study found that a complete unit cell of Ti ₂ Ni phase is composed of 8 twisted icosahedrons, and the presence of oxygen in the alloy melt can promote a large number of icosahedral clusters, and these The icosahedral clusters can act as the embryos of the Ti ₂ Ni phase nucleation during the nucleation of the Ti ₂ Ni phase, thereby reducing its nucleation undercooling and promoting the nucleation of the Ti ₂ Ni phase, even in the Ni-Ti binary phase. In the NiTi single-phase region in the figure, the Ti ₂ Ni phase still exists, and once the Ti ₂ Ni phase is formed, it is difficult to eliminate it through the subsequent heat treatment process.

The directional solidification process of the present invention selects a heating temperature of 1450°C to 1650°C, which can fully dissolve the NiTi-Al-based alloy, and will not cause a high degree of nucleation and undercooling due to excessive heating temperature, thereby promoting Ti ₂ Nucleation of Ni phase.

During the directional solidification process, since the oxygen content is almost insoluble in the NiTi phase matrix, the NiTi phase nucleation and growth will continuously discharge oxygen elements. The oxygen element is lighter, resulting in a lower liquid phase density between the cells, so that the lower density melt continuously floats up to the solid-liquid interface. The directional solidification process of the present invention selects a pulling rate of 0.05-0.2 mm/min, which ensures that the oxygen element can be fully floated, and reduces the oxygen element content in the steady-state stage of directional solidification (from 400ppm or more of the as-cast alloy). to less than 80ppm), thereby greatly reducing the volume fraction of Ti ₂ Ni phase (from more than 2.5% of as-cast alloy to less than 0.5%); it can also inhibit the formation trend of NiTi+Ti ₂ Ni phase irregular eutectic structure , so that it is distributed at the grain boundary in the form of fine dots, which further reduces the harmfulness of the Ti ₂ Ni phase.

The NiTi-Al based alloy prepared by the above directional solidification process greatly reduces the volume fraction of Ti ₂ Ni phase by inhibiting the growth of Ti ₂ Ni phase, and adjusts the Ni/Ti ratio and Al content on the alloy matrix. After the subsequent solid solution and aging treatment, the Ni ₂ TiAl phase is sufficiently precipitated, which improves the room temperature plasticity and high temperature strength of the alloy. The elongation at break at room temperature is greater than 4%; the yield strength at 800°C is greater than 400MPa.

The beneficial effects of the present invention are:

(1) The NiTi-Al-based alloy prepared by the above-mentioned directional solidification process not only greatly reduces the volume fraction of the Ti ₂ Ni phase (from more than 2.5% of the as-cast alloy to less than 0.5%), but also can adjust the alloy matrix The Ni ₂ TiAl phase precipitation is more sufficient after the subsequent solid solution and aging treatment.

(2) The NiTi-Al-based directionally solidified alloy prepared by the above-mentioned directional solidification process has an elongation after fracture at room temperature greater than 4%, and a yield strength at 800° C. greater than 400 MPa.

Description of drawings

FIG. 1 is a microstructure photograph of the as-cast alloy in Example 1 of the present invention.

FIG. 2 is a photomicrograph of the cross section of the directionally solidified alloy of Example 1 of the present invention.

FIG. 3 is a microstructure photograph of the as-cast alloy in Example 2 of the present invention.

4 is a microstructure photograph of a cross section of the directionally solidified alloy of Example 2 of the present invention.

5 is a transmission electron microscope photograph of the fine Ti ₂ Ni phase in Example 2 of the present invention.

轴选区电子衍射斑点。Fig. 6 is the fine Ti ₂ Ni phase in Example 2 of the present invention

Axial Selected Area Electron Diffraction Spots.

Detailed ways

The technical scheme of the present invention will be described in further detail below in conjunction with the accompanying drawings and embodiments:

Example 1

A step of preparing NiTi-Al based alloy by the method of the present invention is as follows:

Step 1. Prepare Ni-43Ti-7Al as-cast alloy by using a vacuum non-consumable electric arc furnace, the microstructure photo of which is shown in Figure 1, the content of oxygen element is 415ppm, and the volume fraction of Ti ₂ Ni phase is 2.5%;

Step 2: Cut out the bar from the Ni-43Ti-7Al as-cast alloy, and after polishing the oxide scale on the surface of the bar, put it into an alumina ceramic tube with a Y ₂ O ₃ sintered coating on the inner wall;

Step 3. Put the ceramic tube containing the Ni-43Ti-7Al as-cast alloy bar into the Bridgman type directional solidification furnace cooled by liquid metal, evacuate to 3～5×10-3Pa, and then fill with high-purity argon gas Make the degree of vacuum reach 0.05MPa, heat to 1550℃ and keep for 20min, pull it at a rate of 0.12mm/min and then quench it into the liquid metal (Ga-In-Sn alloy);

Step 4. Perform solution + secondary aging treatment on the Ni-43Ti-7Al directionally solidified alloy. The solution treatment system is 1170°C/12h/air cooling; the primary aging system is 700°C/15h, and then the temperature is raised to 800°C for secondary aging. , After the holding time of 5h, the oven is air-cooled.

Compared with the Ni-43Ti-7Al as-cast alloy, the Ni-43Ti-7Al directionally solidified alloy prepared by the above process has an oxygen content of only 70 ppm, and the Ti ₂ Ni phase in the alloy is small and its volume fraction is only 0.3%. as shown in picture 2.

The Ni-43Ti-7Al based alloy prepared by the above process has room temperature properties: tensile strength 1956MPa, yield strength 1634MPa, elongation after fracture 4.4%; performance at 800 degrees: tensile strength 505MPa, yield strength 413MPa, elongation after fracture 25.6 %.

Example 2

A step of preparing NiTi-Al based alloy by the method of the present invention is as follows:

Step 1. Ni-41Ti-7Al-1Cr-1Nb as-cast alloy is prepared by vacuum non-consumable electric arc furnace. The microstructure photo is shown in Figure 3. The oxygen element content is 478ppm and the volume fraction of Ti2Ni phase is 3.0% ;

Step 2: Cut out the bar from the Ni-41Ti-7Al-1Cr-1Nb as-cast alloy, and after polishing the oxide scale on the surface of the bar, put it into an alumina ceramic tube with a Y2O3 sintered coating on the inner wall;

Step 3. Put the ceramic tube containing the Ni-41Ti-7Al-1Cr-1Nb as-cast alloy bar into the Bridgman-type directional solidification furnace cooled by liquid metal, and after vacuuming to 3~5×10-3Pa, fill it with High-purity argon gas makes the vacuum degree reach 0.05MPa, and heats it to 1650℃. Due to the addition of Cr and Nb refractory elements, the directional solidification heating temperature in this example is improved compared with Example 1. After being pulled at a high rate, it is quenched into the liquid metal (Ga-In-Sn alloy), and the viscosity of the alloy melt increases due to the addition of Cr and Nb refractory elements. reduce;

Step 4. Perform solid solution + secondary aging treatment on the Ni-41Ti-7Al-1Cr-1Nb powder alloy. The solution treatment system is 1210°C/12h/air cooling; Second-level aging, after holding time of 5h, air-cooled.

The Ni-41Ti-7Al-1Cr-1Nb-based alloy prepared by the above process has an oxygen content of only 78 ppm, a small Ti2Ni phase, and a volume fraction of only 0.4%. As shown in Figures 4, 5 and 6.

The Ni-41Ti-7Al-1Cr-1Nb-based alloy, room temperature properties: tensile strength 2031MPa, yield strength 1789MPa, elongation after fracture 4.1%; 800 degree performance: tensile strength 524MPa, yield strength 427MPa, elongation after fracture rate 27.3%.

Claims (7)

1. A preparation method of NiTi-Al-based alloy is characterized by comprising the following steps: the preparation method comprises the following steps:

step one, preparing a NiTi-Al-based as-cast alloy by using a vacuum non-consumable electric arc furnace;

step two, cutting a bar stock from the as-cast alloy prepared in the step one, and placing the bar stock into a die with Y on the inner wall₂O₃Sintering the coated alumina ceramic tube;

step three, putting the alumina ceramic tube filled with the NiTi-Al-based as-cast alloy bar material into a Bridgman type directional solidification furnace cooled by liquid metal, and vacuumizing to 3-5 multiplied by 10^-3After Pa, filling high-purity argon to ensure that the vacuum degree reaches 0.05MPa, heating to 1450-1650 ℃, preserving heat for 20-40 min, drawing at the speed of 0.05-0.2 mm/min, quenching into liquid metal, wherein the liquid metal is Ga-In-Sn alloy, and finally performing solid solution and aging treatment, wherein the solid solution and aging treatment is solid solution and secondary aging treatment, the solid solution treatment system is (1170-1210 ℃)/12 h/air cooling, and the secondary aging treatment system is: heating to 800-830 ℃ for secondary aging after primary aging (700-730 ℃) and 15h, taking out of the furnace for air cooling after the heat preservation time is 5h, and preparing the NiTi-Al-based alloy.

2. The method for producing a NiTi-Al-based alloy according to claim 1, characterized in that: and step two, cutting the bar stock from the as-cast alloy by adopting a wire cutting method.

3. The method for producing a NiTi-Al-based alloy according to claim 1, characterized in that: and step two, after the bar stock is cut, polishing the oxide skin on the surface of the bar stock.

4. The method for producing a NiTi-Al-based alloy according to claim 1, characterized in that: the NiTi-Al-based as-cast alloy in the step one is Ni-43Ti-7Al as-cast alloy.

5. The method for producing an NiTi-Al-based alloy according to claim 4, characterized in that: the solid solution and aging treatment in the third step is solid solution and secondary aging treatment, and the solid solution treatment system is 1170 ℃/12 h/air cooling; the primary aging system is 700 ℃/15h, then the temperature is raised to 800 ℃ for secondary aging, the heat preservation time is 5h, and the product is discharged from the furnace and cooled in air.

6. The method for producing a NiTi-Al-based alloy according to claim 1, characterized in that: the NiTi-Al-based as-cast alloy in the first step is Ni-41Ti-7Al-1Cr-1Nb as-cast alloy.

7. The method for producing an NiTi-Al-based alloy according to claim 6, characterized in that: the solid solution and aging treatment in the third step is solid solution and secondary aging treatment, and the solid solution treatment system is 1210 ℃/12 h/air cooling; the first-stage aging system is that the temperature is increased to 830 ℃ after 730 ℃/15h for second-stage aging, and the temperature is kept for 5h, and then the product is discharged from the furnace and cooled in air.

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