A method for preparing full beta phase fine grain / super fine grain titanium alloy based on alpha" martensite reverse phase change

Abstract

The application discloses a method for preparing full beta phase fine grain / superfine grain titanium alloy based on alpha" martensite reverse phase change, and is characterized in that the method comprises the following steps: firstly, pre-deforming titanium alloy is obtained by rolling bulk metastable beta titanium alloy at room temperature; secondly, short-time annealing heat treatment is carried out; thirdly, secondary rolling is carried out at room temperature; and fourthly, fine grain / superfine grain titanium alloy is obtained by secondary short-time annealing heat treatment. In the application, micron-level alpha" martensite laths are introduced by pre-deformation, reverse phase change of alpha" to beta is realized by combining with primary short-time annealing heat treatment, the size of beta phase grains is reduced, then nanometer-level alpha" martensite phase is introduced by secondary rolling, the size of beta phase grains is further reduced by taking the alpha" martensite phase as the nucleation core of beta grain recrystallization, and finally, fine grain / superfine grain titanium alloy is obtained by combining with secondary short-time annealing heat treatment. The titanium alloy has beta single-phase organization, the minimum grain size is submicron, the superelasticity performance of the titanium alloy is improved, and the titanium alloy is suitable for metastable alloy with deformation martensite phase change.

Description

Technical Field

[0001] This invention belongs to the field of ultrafine microstructure technology of β titanium alloys, specifically relating to a method for preparing full β-phase fine / ultrafine-grained titanium alloys based on α" martensite reverse phase transformation. Background Technology

[0002] Ultrafine-grained metals refer to polycrystalline materials with an average grain size distribution in the submicron range (100 nm to 1000 nm). When the average grain size is reduced to below 100 nm, nanocrystalline metals can be obtained. The most significant microstructural characteristic of ultrafine-grained metals and alloys is that when the average grain size is less than 1 μm, the proportion of grain boundaries in the microstructure increases rapidly. Therefore, they are considered interface-controlled materials, and the properties of grain boundaries (high-angle / low-angle, specific / random, equilibrium / non-equilibrium, etc.) have begun to dominate some important material properties. For the definition of bulk ultrafine-grained metals, a homogeneous microstructure and equiaxed grains are also required, i.e., most grain boundaries are high-angle grain boundaries. Due to these microstructural characteristics, ultrafine-grained metals often exhibit unique and excellent mechanical behavior and physicochemical properties, and are gradually being promoted in the industrial applications of structural materials, biomedical materials, and superconducting materials.

[0003] Ultrafine-grained titanium alloys are broadly classified as ultrafine-grained metals because they contain both β and α phases. Refining the microstructure significantly improves the strength of titanium alloys; however, this ultrafine microstructure often comes at the cost of reduced plasticity. In a narrower sense, ultrafine-grained titanium alloys refer to β-titanium alloys. These alloys can be stabilized to room temperature through water quenching, allowing for the control of the β single-phase structure to achieve ultrafine-grained β-titanium alloys. In this case, the alloy phase composition consists only of the β phase, without the α phase. Ultrafine-grained β-titanium alloys are primarily targeted at superelastic titanium alloys. This is because titanium alloys exhibit relatively small martensitic transformation strain and low critical stress for dislocation slip initiation. Therefore, refining the β grains is necessary to increase the critical stress for dislocation initiation, thereby improving superelasticity. High superelasticity is more easily achieved in the β single-phase state (where the precipitated phase does not occupy matrix space).

[0004] Achieving ultrafine matrix grains in β-titanium alloys has long been a challenge in both production and scientific research. While severe plastic deformation (SPD) methods, such as high-pressure torsion and equal-channel extrusion, can introduce high-density defects into the alloy, followed by grain refinement through heat treatment, these methods are costly and result in small sample sizes, making industrial-scale production difficult. Furthermore, the β-phase transformation temperature is relatively high; to obtain single-phase β grains, annealing above the β-transformation temperature is necessary. At this temperature, the diffusion rate of alloying elements is high, leading to extremely rapid β grain growth, making it difficult to obtain ultrafine-grained titanium alloys in the β-single-phase state, thus limiting the development and application of superelastic titanium alloys. Summary of the Invention

[0005] The technical problem to be solved by this invention is to address the shortcomings of the prior art by providing a method for preparing fine-grained / ultra-fine-grained titanium alloys based on α" martensite reverse phase transformation. This method employs a two-stage rolling and two-stage short-time annealing heat treatment process to obtain an intermediate transformation microstructure that combines martensitic phase size and β-phase crystal structure. This allows for control of grain growth, resulting in fine-grained / ultra-fine-grained titanium alloys with a minimum grain size in the submicron range and a β-phase single-phase microstructure at room temperature. This improves the superelastic properties of the titanium alloy and solves the problem of rapid β-phase grain growth at high temperatures, making it difficult to obtain fine-grained and ultra-fine-grained β-phase titanium alloys through conventional heat treatment.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a method for preparing all-β phase fine-grained / ultra-fine-grained titanium alloys based on α" martensite reverse phase transformation, characterized in that the method includes the following steps:

[0007] Step 1: The blocky metastable β titanium alloy is initially rolled at room temperature, and the cumulative deformation of the initial rolling is 40% to 50%, to obtain a pre-deformed titanium alloy; the microstructure of the pre-deformed titanium alloy consists of two phases: β phase and lath-shaped α" martensite phase, and the width of the α" martensite phase is 1μm to 10μm.

[0008] Step 2: The pre-deformed titanium alloy obtained in Step 1 is subjected to a first short-time annealing heat treatment above the β phase transformation point to obtain an α"→β reverse phase transformation structure; the α"→β reverse phase transformation structure is a β single phase, the β grain morphology is lath-shaped, and the lath width is 1μm~20μm;

[0009] Step 3: The pre-deformed titanium alloy after short-time annealing heat treatment in Step 2 is subjected to secondary rolling at room temperature, and the cumulative deformation of the secondary rolling is 86% to 90%, and α" deformed martensite is introduced again; the oxide scale of the pre-deformed titanium alloy after short-time annealing heat treatment is removed before the secondary rolling.

[0010] Step 4: The pre-deformed titanium alloy after the second rolling in Step 3 is subjected to a second short-time annealing heat treatment above the β phase transformation point to obtain a fine-grained / ultra-fine-grained titanium alloy with a β single-phase structure. The grain size of the β phase is 0.2μm to 7μm, of which the grain size of the ultra-fine-grained titanium alloy is 0.2μm to 1μm, and the grain size of the fine-grained titanium alloy is greater than 1μm and does not exceed 7μm.

[0011] The above-mentioned method for preparing full β-phase fine / ultra-fine grain titanium alloy based on α" martensite reverse phase transformation is characterized in that the blocky metastable β titanium alloy in step one can obtain the α" martensite phase through cold deformation, and the thickness of the blocky metastable β titanium alloy before the first rolling is not more than 10 mm, and it is polished to a smooth and flat surface before the first rolling.

[0012] The above-mentioned method for preparing full β-phase fine / ultrafine-grained titanium alloys based on α" martensite reverse phase transformation is characterized in that the initial short-time annealing heat treatment in step two uses a temperature within 100°C above the β-phase transformation point of the metastable β titanium alloy, an annealing time of less than 60s, and water cooling. This invention precisely controls the temperature and time of the initial short-time annealing heat treatment, thereby controlling the introduced β grain size, i.e., the α"→β reverse phase transformation microstructure parameters, ensuring that the β grain morphology is lath-like with a lath width of 1μm to 20μm.

[0013] The above-mentioned method for preparing full β-phase fine / ultra-fine grain titanium alloys based on α" martensite reverse phase transformation is characterized in that the reduction per pass of the secondary rolling in step three is 0.1 mm to 0.2 mm, and the thickness of the pre-deformed titanium alloy after secondary rolling is 1 mm ± 0.5 mm. Since the thermal conductivity of titanium alloys is only 16% of that of steel, short-time annealing cannot guarantee uniform heating of the sample surface and core, which is not conducive to obtaining a uniform microstructure. This invention ensures the obtaining of fine / ultra-fine grain titanium alloys with uniform grain size distribution by controlling the final rolling thickness.

[0014] The above-mentioned method for preparing full β-phase fine / ultra-fine grain titanium alloys based on α" martensite reverse phase transformation is characterized in that the secondary short-time annealing heat treatment in step four is carried out at a temperature of 830℃~850℃, with an annealing time of 30s~80s, and water cooling. This invention sets the temperature of the secondary short-time annealing heat treatment within 100℃ above the β phase transformation point, and, combined with controlling the annealing time, ensures the complete occurrence of the α"→β reverse phase transformation while avoiding grain growth. The use of water cooling avoids the problems of low thermal conductivity of titanium alloys and grain growth during slow cooling.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] 1. This invention first introduces fine micron-sized α" martensite phase into the titanium alloy matrix through initial rolling, and then performs a high-temperature α"→β reverse phase transformation through initial short-time annealing heat treatment to obtain an intermediate transformation structure that combines martensite phase size and β phase crystal structure. This intermediate transformation structure is then subjected to secondary rolling and secondary short-time annealing heat treatment to introduce nano-sized α" martensite phase again. These fine martensite phases serve as nucleation sites for β grain recrystallization, further reducing the β phase grain size, thereby effectively controlling grain growth and obtaining fine / ultra-fine grain titanium alloys with a minimum grain size of submicron, thus improving the superelastic properties of the titanium alloy.

[0017] 2. Based on the theory of reverse phase transformation of deformed martensite, this invention can refine the β phase grains of metastable β titanium alloys to the submicron scale through simple rolling and annealing processes. It is applicable to all alloys that can undergo martensitic phase transformation through deformation.

[0018] 3. The fine-grained / ultra-fine-grained titanium alloy prepared by this invention is in a β single-phase state. Compared with the dual-phase structure containing precipitates, this single-phase state is beneficial to improving the recoverable strain of the superelastic titanium alloy. At the same time, the strength of the alloy is improved after grain refinement, which is expected to achieve the integration of structural properties (strength and plasticity) and functional properties (superelasticity).

[0019] 4. The preparation method of the present invention has low equipment requirements, relatively simple processing technology, high repeatability, and is suitable for large-scale industrial production.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1a This is a physical image of the Ti21Nb8Mo titanium alloy forging billet used in Embodiment 1 of the present invention.

[0022] Figure 1b This is the electron backscatter diffraction pattern of the Ti21Nb8Mo titanium alloy forging blank used in Example 1 of the present invention.

[0023] Figure 2 The X-ray diffraction pattern is shown for the pre-deformed titanium alloy prepared in Example 1 of this invention.

[0024] Figure 3a The electron backscatter diffraction image-inverse pole figure of the pre-deformed titanium alloy prepared in Example 1 of the present invention.

[0025] Figure 3b This is a diagram showing the martensitic phase composition of the pre-deformed titanium alloy in Example 1 of the present invention.

[0026] Figure 4 The X-ray diffraction pattern of the pre-deformed titanium alloy prepared in Example 1 of the present invention after initial short-time annealing heat treatment is shown.

[0027] Figure 5 The image shown is an electron backscatter diffraction image-orientation imaging diagram of the pre-deformed titanium alloy prepared in Example 1 of this invention after initial short-time annealing heat treatment.

[0028] Figure 6a The image shown is an electron backscatter diffraction image-orientation imaging diagram of the pre-deformed titanium alloy prepared in Example 1 of this invention after undergoing a second short-time annealing heat treatment.

[0029] Figure 6b This is a grain boundary diagram of the pre-deformed titanium alloy prepared in Example 1 of the present invention after undergoing a second short-time annealing heat treatment.

[0030] Figure 7 This is a cyclic loading-unloading curve of the ultrafine-grained titanium alloy prepared in Example 1 of the present invention.

[0031] Figure 8 The image shown is an electron backscatter diffraction image-orientation imaging diagram of the pre-deformed titanium alloy prepared in Example 2 of this invention after undergoing a second short-time annealing heat treatment.

[0032] Figure 9 The image shown is an electron backscatter diffraction image-orientation imaging diagram of the pre-deformed titanium alloy prepared in Example 3 of this invention after undergoing a second short-time annealing heat treatment.

[0033] Figure 10 Electron backscatter diffraction image-orientation imaging of the pre-deformed titanium alloy prepared in Example 4 of the invention after undergoing a second short-time annealing heat treatment.

[0034] Figure 11 This is a schematic diagram of the grain size determination method using the section method. Detailed Implementation

[0035] Example 1

[0036] This embodiment includes the following steps:

[0037] Step 1, Pre-deformation: Wire cutting is used on a Ti21Nb8Mo (wt.%) titanium alloy forging billet with an initial grain size of 98.8 μm (e.g.) Figure 1a and Figure 1b A block sample with a thickness × width × length of 10mm × 20mm × 30mm was obtained (as shown) and polished to a smooth and flat surface. The polished block sample was then subjected to a two-step initial rolling process at room temperature. First, it was rolled to a thickness of 7mm ± 0.4mm with a reduction of 0.2mm per pass. Then, the reduction was reduced to 0.1mm per pass and rolling continued until a thickness of 5mm ± 0.2mm was achieved. The roll speed was 3m / min, resulting in a pre-deformed titanium alloy. The microstructure of the pre-deformed titanium alloy consisted of two phases: a β phase and a lath-like α" martensite phase (as shown). Figure 2 As shown), the width of the α" martensite phase is 1μm to 10μm (e.g. Figure 3a and Figure 3b (as shown);

[0038] Step 2: Precise annealing to achieve the α"→β reverse phase transformation: The pre-deformed titanium alloy obtained in Step 1 is subjected to an initial short-time annealing heat treatment with a vacuum degree of 5×10⁻⁶. -3 Pa, temperature 830℃, annealing time 50s, water cooling, to obtain α"→β reverse phase transformation structure; the α"→β reverse phase transformation structure is a β single phase (e.g. Figure 4 As shown), the β-grain morphology is lath-like, with lath widths ranging from 1 μm to 20 μm (e.g., Figure 5 (as shown);

[0039] Step 3: Secondary Deformation to Introduce Nanoscale α" Martensite: The pre-deformed titanium alloy after short-time annealing heat treatment in Step 2 is subjected to secondary rolling at room temperature, with a cumulative deformation of 87.6% and a reduction of 0.1 mm per pass, rolled to a thickness of 1 mm ± 0.5 mm, thereby introducing α" deformed martensite again; the oxide scale of the pre-deformed titanium alloy after short-time annealing heat treatment is removed before the secondary rolling.

[0040] Step 4: Second α"→β reverse phase transformation to further refine β phase grains: The pre-deformed titanium alloy after the second rolling in Step 3 is subjected to a second short-time annealing heat treatment with a vacuum degree of 5×10 -3 Pa, temperature 830℃, annealing time 20s, water cooling, to obtain a β single-phase microstructure with a β phase grain size of 0.23μm (e.g., Figure 6a and Figure 6b (as shown) is an ultrafine-grained titanium alloy.

[0041] Figure 7 This is a cyclic loading-unloading curve of the ultrafine-grained titanium alloy prepared in this embodiment. Figure 7 It can be seen that this ultrafine-grained titanium alloy exhibits excellent superelasticity, with a recoverable strain of up to 3.5%.

[0042] Example 2

[0043] The difference between this embodiment and Embodiment 1 is that the annealing time for the secondary short-time annealing heat treatment in step four is 30 seconds, resulting in a grain size of 0.51 μm in the β phase of the obtained ultrafine-grained titanium alloy (e.g., ...). Figure 8 (As shown).

[0044] Example 3

[0045] The difference between this embodiment and Embodiment 1 is that the temperature of the secondary short-time annealing heat treatment in step four is 850℃, the annealing time is 80s, and the grain size of the β phase in the obtained fine-grained titanium alloy is 6.8μm (e.g., Figure 9 (As shown).

[0046] Example 4

[0047] The difference between this embodiment and Embodiment 1 is as follows: In step one, a metastable β-titanium alloy of Ti-62.95Zr-4.82Sn-2.02Mo (wt.%) is selected, and it is rolled to a thickness of 6mm ± 0.2mm with a reduction of 0.1mm per pass. The cumulative deformation of the first rolling is 40%. In step three, the cumulative deformation of the second rolling is 90%, with a reduction of 0.2mm per pass. In step four, the temperature of the second short-time annealing heat treatment is 830℃, and the annealing time is 60s. The grain size of the β phase in the obtained ultrafine-grained titanium alloy is 1.84μm (e.g., ...). Figure 10 (As shown).

[0048] The grain size measurement method in Examples 1 to 4 of this invention is the section method, and its principle is as follows: Figure 11 As shown.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing all-β phase fine-grained / ultra-fine-grained titanium alloys based on α" martensite reverse phase transformation, characterized in that, The method includes the following steps: Step 1: The blocky metastable β titanium alloy is initially rolled at room temperature, and the cumulative deformation of the initial rolling is 40%~50% to obtain a pre-deformed titanium alloy; the microstructure of the pre-deformed titanium alloy consists of two phases: β phase and lath-shaped α" martensite phase, and the width of the α" martensite phase is 1μm~10μm. Step 2: The pre-deformed titanium alloy obtained in Step 1 is subjected to a first short-time annealing heat treatment above the β phase transformation point to obtain an α"→β reverse phase transformation structure; the α"→β reverse phase transformation structure is a β single phase, the β grain morphology is lath-like, and the lath width is 1μm~20μm; the temperature used for the first short-time annealing heat treatment is within 100℃ above the β phase transformation point of the metastable β titanium alloy, the annealing time is less than 60s, and the cooling condition is water cooling; Step 3: The pre-deformed titanium alloy after short-time annealing heat treatment in Step 2 is subjected to secondary rolling at room temperature, and the cumulative deformation of the secondary rolling is 86%~90%, and α" deformed martensite is introduced again; the oxide scale of the pre-deformed titanium alloy after short-time annealing heat treatment is removed before the secondary rolling. Step 4: The pre-deformed titanium alloy after secondary rolling in Step 3 is subjected to a second short-time annealing heat treatment above the β phase transformation point to obtain a fine-grained / ultra-fine-grained titanium alloy with a β single-phase structure. The grain size of the β phase is 0.2μm~7μm, wherein the grain size of the ultra-fine-grained titanium alloy is 0.2μm~1μm, and the grain size of the fine-grained titanium alloy is greater than 1μm and not more than 7μm. The temperature used for the second short-time annealing heat treatment is 830℃~850℃, the annealing time is 30s~80s, and the cooling condition is water cooling.

2. The method for preparing all-β phase fine-grained / ultra-fine-grained titanium alloys based on α" martensite reverse phase transformation according to claim 1, characterized in that, The blocky metastable β titanium alloy described in step one can be cold deformed to obtain the α" martensite phase, and the thickness of the blocky metastable β titanium alloy before the first rolling is no more than 10 mm, and it is polished to a smooth and flat surface before the first rolling.

3. The method for preparing all-β phase fine-grained / ultra-fine-grained titanium alloys based on α" martensite reverse phase transformation according to claim 1, characterized in that, In step three, the reduction in each pass of the secondary rolling is 0.1mm~0.2mm, and the thickness of the pre-deformed titanium alloy after secondary rolling is 1mm±0.5mm.

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