Method for improving low-temperature mechanical property of TC18 titanium alloy

Abstract

The invention belongs to the technical field of TC18 titanium alloy performance, and particularly relates to a method for improving the low-temperature mechanical property of TC18 titanium alloy, which comprises the following steps of: optimizing the grain orientation of the TC18 titanium alloy, adjusting the critical shear stress ratio and generating a double-lamellar alpha-phase structure by controlling the forging deformation and a heat treatment process; therefore, the low-temperature performance is improved; according to the invention, grain orientation optimization (llt; 100 gt; through the synergistic effect of beta crystal grains), critical shear stress adjustment and double-lamellar structure design, the low-temperature toughness is remarkably improved, and meanwhile the room-temperature strength is kept. Through verification, the yield strength and toughness of the TC18 titanium alloy processed by the method disclosed by the invention are remarkably improved in an environment of-70 DEG C.

Description

Technical Field

[0001] The present invention belongs to the technical field of TC18 titanium alloy properties, and particularly relates to a method for improving the low-temperature mechanical properties of TC18 titanium alloy. Background Art

[0002] TC18 titanium alloy (Ti-5Al-5Mo-5V-1Cr-1Fe) has high strength, corrosion resistance and good welding performance, and is widely used as a structural material in the aerospace field. By adjusting the α phase (hexagonal close-packed structure) and β phase (body-centered cubic structure), the room-temperature performance is ensured to be balanced, enabling it to have high strength and high toughness, and to maintain the stability of mechanical properties for a long time; its creep resistance, fatigue resistance characteristics and actual application performance further prove its reliability as an aerospace structural material.

[0003] Although TC18 titanium alloy can maintain the stability of mechanical properties at room temperature, it will inevitably embrittle when facing a cold environment. This is because the atomic thermal motion is inhibited in a low-temperature environment, the lattice friction force increases, resulting in significant hindrance to dislocation movement, which increases the yield strength and reduces the toughness. For aircraft, a cold environment is an inescapable operating environment. Usually, a large margin of mechanical properties is ensured in the design of structural materials. How to ensure that the TC18 titanium alloy structural material can ensure both room-temperature performance and low-temperature performance is an urgent problem to be solved. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for improving the low-temperature mechanical properties of TC18 titanium alloy, which can not only ensure the room-temperature performance of TC18, but also significantly improve the low-temperature performance.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions:

[0006] A method for improving the low-temperature mechanical properties of TC18 titanium alloy, comprising:

[0007] (1) In the forging process, control the deformation amount of the TC18 titanium alloy forging to be 50%-70%, break the grain boundary α phase, reduce the low-temperature impurity segregation; adjust the β grain ratio, promote the transformation of β grains to the <100> orientation, reduce the elastic modulus, and optimize the deformation ability;

[0008] (2) In the heat treatment process, adopt a double annealing process, heat up to 830-870°C in the furnace for solution treatment for 1 hour, cool in the furnace to 700-800°C for heat preservation for 1-3 hours, and then air-cool; then age at a temperature of 500-600°C for 6-8 hours.

[0009] Control the thickness (1.3 - 1.6 μm) and aspect ratio (7:1) of the primary lamellar α-phase, and optimize the critical shear stress ratio; promote the uniform precipitation of the secondary α-phase to form a duplex lamellar structure.

[0010] Adjust the critical shear stress ratio (basal / cylindrical / conical slip) through forging deformation, initiate the conical slip mechanism at low temperature, and coordinate deformation; generate a uniform strain gradient of the duplex lamellar structure (primary α + secondary α) through heat treatment processes, and combine the characteristics of suppressing dislocation slip at low temperature to promote twin generation and enhance work hardening ability.

[0011] Through the synergistic effects of grain orientation optimization (<100> β grains), critical shear stress adjustment, and duplex lamellar structure design, the present invention significantly improves low-temperature toughness while maintaining room-temperature strength. It has been verified that the TC18 titanium alloy processed by the method of the present invention has significantly improved yield strength and toughness at -70°C. Brief Description of the Drawings

[0012] Figure 1 It is the microstructural diagram of the present invention.

[0013] Figure 2 It is the tensile performance of the TC18 titanium alloy of the present invention at room temperature.

[0014] Figure 3 It is the tensile performance of the TC18 titanium alloy of the present invention at -70°C. Detailed Description of the Invention

[0015] A method for improving the low-temperature mechanical properties of TC18 titanium alloy provided in this embodiment includes: First, control the deformation amount in the forging process to be 50% - 70%, break the grain boundary α, and adjust the proportion of <100> β grains; the discontinuous grain boundary α reduces the segregation of impurity elements at low temperature and reduces stress concentration, which is the premise for improving low-temperature performance.

[0016] Secondly, through double annealing: 830 - 870°C for 1 h, furnace cool to 700°C - 800°C and hold for 1 - 3 h, then air cool. Strict temperature and time control the thickness of the primary lamellar α to be 1.3 - 1.6 microns and the aspect ratio to be about 7:1, and then age at 500 - 600°C for 6 - 8 h to ensure the uniform precipitation of the secondary α, obtaining the TC18 alloy with a duplex lamellar structure.

[0017] In this embodiment, the lattice friction force is first increased at low temperature, dislocation slip is suppressed, and the critical shear stress of α phase basal plane, cylindrical surface and conical surface slip will increase to varying degrees, but their ratio is controlled by solution treatment at 830-870°C, so that the ratio of critical shear stress increases compared to room temperature. The increase in shear stress before slip occurs plus the increase in the critical shear stress ratio, coupled with the crystal orientation that is conducive to conical surface slip, will start the conical surface slip that is not easy to start at room temperature, which is conducive to coordinated deformation. Then during the forging process, as the deformation amount increases, the crystal orientation of β will move to <100> β shift, due to <100> The lower elastic modulus of the β grains, and <100> The α phase crystal orientation precipitated from the β grains during heat treatment is in a soft orientation, so it is more conducive to deformation. Finally, the double-lamellar structure will significantly homogenize the strain gradient. In addition, low temperature inhibits dislocation slip and reduces the stacking fault energy. The elements in the primary α lamellae of appropriate size are redistributed, and the content of each element in different phases is as follows: Figure 1 As shown, it is conducive to the formation of twins, and the synergy of multiple deformation mechanisms significantly improves the work hardening ability and is conducive to uniform deformation. The combined effect of multiple factors significantly improves the low-temperature performance (above -70°C).

[0018] right Figure 1 The spectrum analysis was performed at three locations in the middle, and the results are shown in Table 1. The significant increase in the Al equivalent in the primary α layer is conducive to the formation of twins.

[0019]

[0020] The tensile test of TC18 titanium alloy was carried out to obtain the tensile properties at room temperature and -70℃, such as Figure 2 and Figure 3 As shown, it can be seen that the TC18 titanium alloy processed by the method of this embodiment significantly improves the low-temperature mechanical properties without sacrificing the room temperature mechanical properties.

[0021] The above description is only a preferred implementation manner of the present invention, but the protection scope of the present invention is not limited thereto, and any modification and replacement based on the technical solution and inventive concept provided by the present invention should be included in the protection scope of the present invention.

Claims

1. A method for improving the low temperature mechanical properties of TC18 titanium alloy, characterized in that: include: By controlling the forging deformation and heat treatment process, the grain orientation of TC18 titanium alloy is optimized, the critical shear stress ratio is adjusted, and a double-lamellar α phase structure is generated, thereby improving its low-temperature performance; Wherein, the forging deformation is controlled at 50-70%; The heat treatment process refers to solution treatment at a solution temperature of 830-870°C for 1 hour, furnace cooling to 700°C-800°C, insulation for 1-3 hours, and air cooling; aging at an aging temperature of 500-600°C for 6-8 hours.

2. A method for improving the low temperature mechanical properties of TC18 titanium alloy according to claim 1, characterized in that: The optimization of the grain orientation of TC18 titanium alloy refers to adjusting the proportion of β grains and promoting the orientation of β grains. <100> Change of orientation.

3. The method for improving the low temperature mechanical properties of TC18 titanium alloy according to claim 1, characterized in that: The adjustment of the critical shear stress ratio refers to adjusting the critical stress ratio of the slip system to promote the initiation of low-temperature cone slip.

4. The method for improving the low temperature mechanical properties of TC18 titanium alloy according to claim 1, characterized in that: The generation of the double-lamellar α phase structure refers to controlling the thickness of the primary lamellar α phase to be 1.3-1.6 μm and the aspect ratio to be 7:1, and the secondary α phase to be uniformly precipitated to form a double-lamellar structure.

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