Equiaxed duplex nanocrystalline pure titanium, and preparation method and application thereof

Abstract

This invention belongs to the field of bio-implant material technology, and provides an equiaxed biphase nanocrystalline pure titanium, its preparation method, and its applications. The microstructure of the equiaxed biphase nanocrystalline pure titanium of this invention consists of a close-packed hexagonal phase and a face-centered cubic phase, with the close-packed hexagonal phase accounting for 60-80% of the volume and the face-centered cubic phase accounting for 20-40%. The equiaxed biphase nanocrystalline pure titanium of this invention is stable at room temperature, has a tensile strength higher than 1000 MPa, a yield strength higher than 900 MPa, and an elongation after fracture between 4-20%. It also exhibits excellent reprocessability, with strength and toughness far exceeding conventional coarse-grained pure titanium, surpassing conventional titanium alloys and novel titanium alloys with added rare elements such as niobium and tantalum. Utilizing simple and mature hydrostatic extrusion, multi-directional forging, and reversing rolling processes, it is low-cost and highly efficient, enabling large-scale, industrial production of bulk equiaxed biphase nanocrystalline pure titanium sheets.

Description

Technical Field

[0001] This invention relates to the field of bio-implant materials technology, and in particular to an equiaxed biphase nanocrystalline pure titanium, its preparation method, and its applications. Background Technology

[0002] Coarse-grained pure titanium has been widely used in medical devices and bio-implants due to its excellent corrosion resistance and biocompatibility. However, its practical applications are still significantly limited, mainly due to its insufficient mechanical properties, such as low strength and ductility, poor machinability, and significant anisotropy. From a microstructural perspective, conventional coarse-grained pure titanium is typically composed of a single hexagonal close-packed phase (HCP, α-titanium). The limited number of slip systems in the HCP phase results in limited plastic deformation capacity at room temperature. Furthermore, the grain size of conventional coarse-grained pure titanium is generally in the micrometer range, leading to relatively low strength according to the Hall-Petch relationship. Therefore, the disadvantages in mechanical properties, such as low strength and poor ductility, severely restrict the further application of coarse-grained pure titanium in industrial fields such as medical devices and bio-implants.

[0003] Refining grain size to the ultrafine or nanoscale can significantly improve the strength of materials. Compared to coarse-grained pure titanium, ultrafine-grained or nanocrystalline pure titanium exhibits a substantial increase in strength, reaching or even exceeding the strength levels of commonly used medical titanium alloys (TC4). However, improving strength through grain refinement often comes at the cost of ductility, resulting in ultrafine-grained or nanocrystalline pure titanium showing a significant tendency to embrittlement, which further limits its engineering applications and widespread adoption. A possible solution to this problem is to introduce a second phase into single-phase ultrafine-grained / nanocrystalline pure titanium, constructing a two-phase microstructure. Two-phase or multiphase materials typically possess excellent strength and toughness because the synergistic deformation between different phases significantly enhances the material's work hardening ability, while phase interfaces and grain boundaries themselves are important sources of strengthening. Therefore, two-phase or multiphase materials can generally achieve better ductility while maintaining high strength.

[0004] Typical dual-phase titanium alloys consist of an α-phase (hexagonal close-packed phase, HCP) and a β-phase (face-centered cubic phase, FCC). However, the stable existence of the FCC phase in titanium alloy systems usually depends on the addition of FCC phase-stabilizing elements such as Nb, Mo, and V, and obtaining a stable dual-phase microstructure through specific heat treatment processes. The FCC phase is a high-temperature stable phase; for pure titanium without alloying elements, it is extremely difficult for the FCC phase to exist stably at room temperature, as documented in "Phase transformation induced titring in commercially pure titanium: An in-situ study." To date, there are no reports on bulk dual-phase nanocrystalline pure titanium, either in academic research or industrial applications. Existing research mainly relies on methods to introduce a second phase into pure titanium that induce solid-state phase transformations under ultra-high temperature and ultra-high pressure conditions, such as the method reported in "Study on High-Pressure Solid-State Phase Transformation of Pure Titanium." However, this method has extremely demanding requirements for equipment and process conditions. Furthermore, the pure titanium grains remain at the micrometer scale during processing, making further refinement difficult to achieve the desired comprehensive mechanical properties. More importantly, it remains difficult to obtain room-temperature stable biphase nanocrystalline pure titanium. In summary, the preparation of bulk biphase nanocrystalline pure titanium still faces two mutually restrictive technical challenges: firstly, it is difficult to stabilize the FCC phase to form a biphase structure under conventional conditions; secondly, the nanocrystalline structure is prone to rapid growth at high temperatures, disrupting the biphase structure.

[0005] Therefore, there is an urgent need to develop biphase nanocrystalline pure titanium with simple processes, mature technology, practical feasibility, and promising industrial applications, so as to achieve a stable biphase microstructure under normal conditions and better meet the application needs of pure titanium in engineering and industrial fields. Summary of the Invention

[0006] The purpose of this invention is to provide an equiaxed biphase nanocrystalline pure titanium, its preparation method, and its application, addressing the shortcomings of existing technologies.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution: The present invention provides an equiaxed biphase nanocrystalline pure titanium, wherein the microstructure of the equiaxed biphase nanocrystalline pure titanium consists of a close-packed hexagonal phase and a face-centered cubic phase, wherein the volume ratio of the close-packed hexagonal phase is 60-80% and the volume ratio of the face-centered cubic phase is 20-40%.

[0008] Preferably, the grain size of the close-packed hexagonal phase is 50~100nm, and the grain size of the face-centered cubic phase is 30~90nm.

[0009] The present invention also provides a method for preparing the aforementioned equiaxed dual-phase nanocrystalline pure titanium, comprising the following steps: 1) The coarse-grained pure titanium rods are sequentially subjected to hydrostatic extrusion and multi-directional forging to obtain pure titanium blocks; 2) The pure titanium block is sequentially subjected to triaxial pressing and reversing rolling; Step 2) involves heat treatment while applying triaxial pressure.

[0010] Preferably, the hydrostatic extrusion is a multi-stage hydrostatic extrusion, and the diameter reduction in each stage of hydrostatic extrusion is 1~4 mm. The strain after multiple hydrostatic extrusions is 1.6~3.2.

[0011] Preferably, the multi-directional forging is a multi-pass multi-directional forging, with a deformation of 1~4mm in each pass and a temperature of -196~30℃ in each pass. Each forging pass cycles sequentially along the X, Y, and Z directions, with more than 6 forging passes in each direction.

[0012] Preferably, the triaxial pressurization pressure is 0.5~2GPa, and the pressurization rate to reach the pressurization pressure is 15~25MPa / min; The triaxial pressurization process involves applying pressure along the X, Y, and Z directions.

[0013] Preferably, the heat treatment temperature is 200~400℃, and the heating rate to the heat treatment temperature is 1~10℃ / min; The pressure holding and heat preservation time for the triaxial pressurization is 30~120 minutes.

[0014] Preferably, the reversing rolling is a multi-pass reversing rolling, with a rolling pressure of 0.4~2mm per pass, a roll speed of 10~40mm / s, and a rolling temperature of -196~30℃ per pass.

[0015] Preferably, the rolling direction of each pass is cyclically along the X and Y directions, and the strain after the multi-pass reversing rolling is 0.5~0.8.

[0016] This invention also provides the application of the aforementioned equiaxed biphase nanocrystalline pure titanium in medical devices and biological implants.

[0017] The beneficial effects of this invention are: 1) The equiaxed dual-phase nanocrystalline pure titanium of the present invention can exist stably at room temperature, with a tensile strength higher than 1000 MPa, a yield strength higher than 900 MPa, and an elongation after fracture between 4% and 20%. It has high strength, good toughness, and excellent reprocessability. It can withstand continuous deformation with strain of 0.5 to 2.0 without cracking. Its strength and toughness are far higher than conventional coarse-grained pure titanium, and exceed conventional titanium alloys and new titanium alloys with added rare elements such as niobium and tantalum. The equiaxed dual-phase nanocrystalline pure titanium that can exist stably at room temperature can be prepared by simple and mature hydrostatic extrusion, multi-directional forging and reversing rolling processes. The process is simple, low-cost and efficient, and can be used for large-scale and industrialized production of bulk equiaxed dual-phase nanocrystalline pure titanium plates.

[0018] 2) The high strength of the equiaxed dual-phase nanocrystalline pure titanium of the present invention is due to the fact that the two ultra-fine phases provide an interface with extremely high density, and the high density of dislocations is introduced by the intense plastic deformation process in the preparation process. The grain boundary strengthening and dislocation strengthening work together to endow the equiaxed dual-phase nanocrystalline pure titanium with high strength and high toughness.

[0019] 3) The reason why the equiaxed dual-phase nanocrystalline pure titanium of the present invention has excellent reprocessability is that it has extremely high internal stress, FCC phase nucleation point and recrystallization nucleation point. During the processing, the internal stress and high dislocation density can act as driving forces to drive the formation and growth of new phases and new grains. At the same time, phase transformation and dynamic recrystallization coordinate the applied strain, further ensuring the reprocessability. Attached Figure Description

[0020] Figure 1 The image shows the backscattered electron diffraction pattern of pure titanium after treatment in Example 1. The area indicated by the arrow is the face-centered cubic phase, and the blank area is the close-packed hexagonal phase. Figure 2 The image shows the backscattered electron diffraction pattern of pure titanium after treatment in Example 2. The area indicated by the arrow is the face-centered cubic phase, and the blank area is the close-packed hexagonal phase. Figure 3 The image shows the backscattered electron diffraction pattern of pure titanium after treatment in Example 3. The area indicated by the arrow is the face-centered cubic phase, and the blank area is the close-packed hexagonal phase. Figure 4 The image shows the backscattered electron diffraction pattern of pure titanium after treatment in Example 4. The area indicated by the arrow is the face-centered cubic phase, and the blank area is the close-packed hexagonal phase. Figure 5 The image shows the backscattered electron diffraction pattern of pure titanium after treatment with Comparative Example 1. The area indicated by the arrow represents the face-centered cubic phase, and the blank area represents the close-packed hexagonal phase. Figure 6The image shows the backscattered electron diffraction pattern of pure titanium after treatment with Comparative Example 2. The area indicated by the arrow represents the face-centered cubic phase, and the blank area represents the close-packed hexagonal phase. Figure 7 The grain size and FCC volume ratio of the nanocrystalline pure titanium prepared in Examples 1-4 and Comparative Examples 1-2 are shown. Detailed Implementation

[0021] The present invention provides an equiaxed biphase nanocrystalline pure titanium, wherein the microstructure of the equiaxed biphase nanocrystalline pure titanium consists of a close-packed hexagonal phase and a face-centered cubic phase, wherein the volume ratio of the close-packed hexagonal phase is 60-80% and the volume ratio of the face-centered cubic phase is 20-40%.

[0022] In this invention, the volume percentage of the close-packed hexagonal phase in the microstructure of the equiaxed dual-phase nanocrystalline pure titanium is preferably 65-75%, more preferably 70%; and the volume percentage of the face-centered cubic phase is preferably 25-35%, more preferably 30%.

[0023] In this invention, the grain size of the close-packed hexagonal phase is preferably 50-100 nm, more preferably 60-90 nm, and even more preferably 70-80 nm; the grain size of the face-centered cubic phase is preferably 30-90 nm, more preferably 40-80 nm, and even more preferably 50-70 nm.

[0024] The present invention also provides a method for preparing the aforementioned equiaxed dual-phase nanocrystalline pure titanium, comprising the following steps: 1) The coarse-grained pure titanium rods are sequentially subjected to hydrostatic extrusion and multi-directional forging to obtain pure titanium blocks; 2) The pure titanium block is sequentially subjected to triaxial pressing and reversing rolling; Step 2) involves heat treatment while applying triaxial pressure.

[0025] In this invention, the hydrostatic extrusion is preferably a multi-pass hydrostatic extrusion, and the diameter reduction of each pass of hydrostatic extrusion is preferably 1~4mm, more preferably 2~3mm; The strain after multiple hydrostatic extrusion is preferably 1.6 to 3.2, more preferably 2 to 2.8, and even more preferably 2.1 to 2.5. Hydrostatic extrusion performs large plastic deformation on the bar, effectively refining and elongating the grains to an ultrafine grain state.

[0026] In this invention, the formula for calculating the strain in the hydrostatic extrusion process is ε1=ln(D0) 2 / D 2 ), where D0 is the diameter of the coarse-grained pure titanium rod before hydrostatic extrusion, in mm; D is the diameter of the coarse-grained pure titanium rod after hydrostatic extrusion, in mm; ε1 is the strain.

[0027] In this invention, the hydrostatic extrusion is preferably performed at room temperature.

[0028] In this invention, after the hydrostatic extrusion is completed, the coarse-grained pure titanium rod is preferably cut to obtain a pure titanium block, which is then subjected to multi-directional forging.

[0029] In this invention, the multi-directional forging is preferably multi-pass multi-directional forging, and the deformation amount of each forging pass is preferably 1~4mm, more preferably 2~3mm; the temperature of each forging pass is preferably -196~30℃, more preferably -168~0℃, and more preferably -120~-48℃. The forging direction of each pass is preferably cyclical along the X, Y, and Z directions, with more than 6 forging passes in each direction, more preferably more than 7 passes, and even more preferably more than 8 passes. Forging further refines the grains and introduces higher internal stress; multi-directional forging can avoid cracking problems caused by deformation in one direction.

[0030] In this invention, the pressure of the triaxial pressurization is preferably 0.5~2GPa, more preferably 0.6~1.6GPa, and even more preferably 1~1.2GPa; the pressurization rate to the pressurization pressure is preferably 15~25MPa / min, more preferably 18~22MPa / min, and even more preferably 20MPa / min; The preferred method for triaxial pressurization is to apply pressurization along the X, Y, and Z directions.

[0031] In this invention, the heat treatment temperature is preferably 200~400℃, more preferably 250~350℃, and even more preferably 300℃; the heating rate to the heat treatment temperature is preferably 1~10℃ / min, more preferably 3~8℃ / min, and even more preferably 5℃ / min. The holding and heat treatment time for the triaxial pressurization is preferably 30~120 min, more preferably 60~100 min, and even more preferably 80 min. The triaxial pressurization and holding treatment promotes the nucleation sites of the FCC phase; at the same time, heat treatment is performed to prevent the material from breaking and restore the deformability of nanocrystalline pure titanium.

[0032] In this invention, the reversing rolling is preferably a multi-pass reversing rolling, and the rolling pressure of each pass is preferably 0.4~2mm, more preferably 0.5~1.8mm, and more preferably 0.8~1.5mm; the rolling speed of the rolls is preferably 10~40mm / s, more preferably 20~30mm / s, and more preferably 25mm / s; the rolling temperature of each pass is preferably -196~30℃, more preferably -168~0℃, and more preferably -120~-48℃.

[0033] In this invention, the rolling direction of each pass is preferably cyclically along the X and Y directions. The strain after multi-pass reversing rolling is preferably 0.5~0.8, more preferably 0.6~0.75, and even more preferably 0.65~0.7. By introducing larger cumulative strain through reversing rolling, dynamic recrystallization and phase transformation processes are further promoted under high internal stress and high dislocation density, significantly reducing the anisotropy of the bulk. The newly formed grains during reversing rolling will grow to a certain extent and exist stably in the form of face-centered cubic phase.

[0034] In this invention, the formula for calculating the strain during the reversing rolling process is ε = (t0-t) / t0, where t0 is the thickness of the pure titanium sheet before rolling (in mm); t is the thickness of the pure titanium sheet after rolling (in mm); and ε is the strain.

[0035] This invention also provides the application of the aforementioned equiaxed biphase nanocrystalline pure titanium in medical devices and biological implants.

[0036] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0037] The raw material used in the embodiments and comparative examples of the present invention is coarse-grained pure titanium rod of grade TA2, and the technical standard conforms to GB / T 3620.1-2007.

[0038] Example 1

[0039] At room temperature, coarse-grained pure titanium rods with a diameter of 40 mm were subjected to multiple hydrostatic extrusion passes. Each pass reduced the diameter by 1-2 mm, resulting in pure titanium rods with a diameter of 14 mm after multiple hydrostatic extrusion passes. The formula ε1=ln(D0) is used. 2 / D 2 Calculate the strain, where D0 is the diameter of the coarse-grained pure titanium rod before hydrostatic extrusion (in mm); D is the diameter of the coarse-grained pure titanium rod after hydrostatic extrusion (in mm); ε1 is the strain. The strain after multiple hydrostatic extrusions is 2.1.

[0040] The pure titanium rod after hydrostatic extrusion was cut to obtain a three-dimensional dimension of 8×10×12 mm. 3 Ultrafine-grained pure titanium blocks were obtained. At -196℃, the ultrafine-grained pure titanium blocks underwent multi-pass multi-directional forging. Each forging pass sequentially cyclically followed the X, Y, and Z directions, with 8 forging passes in each direction and a deformation of 1 mm per pass. After multi-directional forging, nanocrystalline pure titanium blocks were obtained.

[0041] The multi-directional forged nanocrystalline pure titanium block was subjected to triaxial pressing, with the pressure increased to 0.6 GPa at a rate of 20 MPa / min and held at 0.6 GPa for 60 min. Simultaneously, the temperature was increased to 250 °C at a rate of 5 °C / min and held at 250 °C for 60 min. After the holding and pressing were completed, the triaxial pressing process was repeated in the opposite direction, ensuring that triaxial pressing and holding were performed in all three directions (X, Y, and Z). After the triaxial pressing and heat treatment, a nanocrystalline pure titanium block with a thickness of 8 mm was obtained.

[0042] At -196℃, a nanocrystalline pure titanium block under triaxial pressure was subjected to multi-pass reversing rolling. The initial gap between the rolling mill rolls was 8 mm, the roll speed was 20 mm / s, and the reduction in pressure per pass was 0.5 mm. After each pass, reversing rolling was performed cyclically along the X and Y directions. After multi-pass reversing rolling, a nanocrystalline pure titanium sheet with a thickness of 2 mm was obtained. The strain was calculated using the formula ε = (t0 - t) / t0, where t0 is the thickness of the pure titanium sheet before rolling (in mm), t is the thickness of the pure titanium sheet after rolling (in mm), and ε is the strain. The strain after multi-pass reversing rolling was 0.75.

[0043] After the nanocrystalline pure titanium plate was naturally cooled to room temperature, its microstructure was observed using backscattered electron diffraction, and the phase composition and corresponding grain size were recorded.

[0044] Figure 1 The image shows the backscattered electron diffraction pattern of the nanocrystalline pure titanium plate prepared in Example 1. The areas indicated by arrows represent the face-centered cubic phase, while the blank areas represent the close-packed hexagonal phase. Figure 1 As can be seen, the microstructure of the nanocrystalline pure titanium plate exhibits an equiaxed dual-phase nanocrystalline structure, composed of HCP and FCC. The HCP accounts for approximately 68% of the volume, with an average grain size of 82 nm; the FCC accounts for approximately 32% of the volume, with an average grain size of 65 nm. The different depths of the face-centered cubic phase FCC in the figure are due to their different crystal orientations.

[0045] It can be seen that after the treatment in Example 1, equiaxed biphase nanocrystalline pure titanium that exists stably at room temperature was obtained.

[0046] The mechanical properties of the equiaxed dual-phase nanocrystalline pure titanium prepared in Example 1 were tested according to GB / T 228.1-2021. The results showed that the yield strength was 950 MPa, the tensile strength was 1060 MPa, and the elongation after fracture was 12%. The comprehensive mechanical properties were better than those of titanium alloy TC4.

[0047] The equiaxed biphase nanocrystalline pure titanium (i.e., nanocrystalline pure titanium sheet) prepared in Example 1 was continuously cold-rolled at room temperature until the deformation reached 90%, and no cracking occurred, indicating that the equiaxed biphase nanocrystalline pure titanium has excellent reprocessability.

[0048] Example 2

[0049] The difference from Example 1 is that the multi-pass multi-directional forging is carried out at 25°C.

[0050] Figure 2 The image shows the backscattered electron diffraction pattern of the nanocrystalline pure titanium plate prepared in Example 2. The areas indicated by arrows represent the face-centered cubic phase, and the blank areas represent the close-packed hexagonal phase. Figure 2 As can be seen, the microstructure of the nanocrystalline pure titanium plate exhibits an equiaxed dual-phase nanocrystalline structure, composed of HCP and FCC. The HCP accounts for approximately 76% of the volume, with an average grain size of 90 nm; the FCC accounts for approximately 24% of the volume, with an average grain size of 79 nm. The different depths of the face-centered cubic phase FCC in the figure are due to their different crystal orientations.

[0051] It can be seen that after the treatment in Example 2, equiaxed biphase nanocrystalline pure titanium that exists stably at room temperature was obtained.

[0052] The mechanical properties were tested as in Example 1. The results showed that the yield strength was 935 MPa, the tensile strength was 1025 MPa, and the elongation after fracture was 13%.

[0053] In this embodiment, by changing the processing temperature of multi-directional forging, the average grain size of the obtained equiaxed dual-phase nanocrystalline pure titanium increases, while the volume fraction of FCC decreases. This indicates that the high-density dislocations and extremely high internal stress introduced during the low-temperature deformation process have a better effect on grain refinement and FCC nucleation.

[0054] Example 3

[0055] The difference from Example 1 is that the pressure held during the triaxial pressurization process is 1 GPa, and the temperature held is 350°C.

[0056] Figure 3 The image shows the backscattered electron diffraction pattern of the nanocrystalline pure titanium plate prepared in Example 3. The areas indicated by arrows represent the face-centered cubic phase, while the blank areas represent the close-packed hexagonal phase. Figure 3 As can be seen, the microstructure of the nanocrystalline pure titanium plate exhibits an equiaxed dual-phase nanocrystalline structure, composed of HCP and FCC. The HCP accounts for approximately 65% ​​of the volume, with an average grain size of 85 nm; the FCC accounts for approximately 35% of the volume, with an average grain size of 68 nm. The different depths of the face-centered cubic phase FCC in the figure are due to their different crystal orientations.

[0057] It can be seen that after the treatment in Example 3, equiaxed dual-phase nanocrystalline pure titanium that exists stably at room temperature was obtained.

[0058] The mechanical properties were tested as in Example 1. The results showed that the yield strength was 948 MPa, the tensile strength was 1045 MPa, and the elongation after fracture was 12%.

[0059] In this embodiment, by increasing the holding pressure and holding temperature during triaxial pressurization, the average grain size of the obtained equiaxed dual-phase nanocrystalline pure titanium was slightly increased, and the volume fraction of FCC was slightly improved. This indicates that simultaneously increasing the holding pressure and holding temperature during the triaxial pressurization process has a certain effect on grain refinement and FCC nucleation.

[0060] Example 4

[0061] The difference from Example 1 is that the temperature of the multi-pass reversing rolling is 25°C.

[0062] Figure 4 The image shows the backscattered electron diffraction pattern of the nanocrystalline pure titanium plate prepared in Example 4. The areas indicated by arrows represent the face-centered cubic phase, while the blank areas represent the close-packed hexagonal phase. Figure 4 As can be seen, the microstructure of the nanocrystalline pure titanium plate exhibits an equiaxed dual-phase nanocrystalline structure, composed of HCP and FCC. The HCP accounts for approximately 72% of the volume, with an average grain size of 100 nm; the FCC accounts for approximately 28% of the volume, with an average grain size of 89 nm. The different depths of the face-centered cubic phase FCC in the figure are due to their different crystal orientations.

[0063] Therefore, after the treatment in Example 4, equiaxed biphase nanocrystalline pure titanium that exists stably at room temperature was obtained.

[0064] The mechanical properties were tested as in Example 1. The results showed that the yield strength was 918 MPa, the tensile strength was 1006 MPa, and the elongation after fracture was 14.5%.

[0065] In this embodiment, by increasing the temperature of the reversible rolling process, the average grain size of the obtained equiaxed dual-phase nanocrystalline pure titanium increased, while the volume fraction of FCC decreased. This indicates that the processing temperature of the reversible rolling process has an impact on grain refinement and FCC nucleation.

[0066] Comparative Example 1

[0067] The difference from Example 1 is that hydrostatic extrusion is not performed; instead, the coarse-grained pure titanium rod is directly cut to obtain a three-dimensional dimension of 8×10×12 mm. 3 Ultrafine-grained pure titanium bulk material.

[0068] Figure 5 The image shows the backscattered electron diffraction pattern of the pure titanium sheet prepared in Comparative Example 1. The areas indicated by arrows represent the face-centered cubic phase, while the blank areas represent the close-packed hexagonal phase. Figure 5It can be seen that the microstructure of the pure titanium plate consists of elongated ultrafine grains (HCP), with an average grain size of 280 nm. Only a small amount of FCC was observed, accounting for about 5% of the volume. This shows that Comparative Example 1 did not form a stable equiaxed two-phase microstructure.

[0069] The mechanical properties were tested as in Example 1. The results showed that the yield strength was 875 MPa, the tensile strength was 952 MPa, and the elongation after fracture was 17.8%. The overall mechanical properties were significantly lower than those of the equiaxed dual-phase nanocrystalline pure titanium in Example 1.

[0070] In this comparative example, multi-directional forging, triaxial pressing, and reversible rolling were performed directly on coarse-grained pure titanium rods with micrometer-scale grain size. The strain at each step was low, resulting in limited grain refinement and failing to produce nanocrystalline pure titanium. Furthermore, the internal stress was insufficient, and no significant phase transformation or dynamic recrystallization occurred during the reversible rolling process, preventing the formation of a two-phase structure. These results indicate that hydrostatic extrusion to refine micrometer-scale coarse-grained pure titanium to ultrafine grains is a crucial step in preparing equiaxed two-phase nanocrystalline pure titanium.

[0071] Comparative Example 2

[0072] The difference from Comparative Example 1 is that, during the reversing rolling process, the reduction in pressure per pass is 0.5 mm. After multiple passes of reversing rolling, a nanocrystalline pure titanium sheet with a thickness of 0.5 mm is obtained. The strain is calculated using the formula ε = (t0-t) / t0, where t0 is the thickness of the pure titanium sheet before rolling (in mm), t is the thickness of the pure titanium sheet after rolling (in mm), and ε is the strain. The strain after multiple passes of reversing rolling is 0.94.

[0073] Figure 6 The image shows the backscattered electron diffraction pattern of the pure titanium sheet prepared in Comparative Example 2. The areas indicated by arrows represent the face-centered cubic phase, while the blank areas represent the close-packed hexagonal phase. Figure 6 It can be seen that the microstructure of the pure titanium plate consists of equiaxed ultrafine-grained HCP, with an average grain size of 120 nm. Only a small amount of equiaxed FCC was observed, accounting for approximately 7% of the volume. Therefore, Comparative Example 2 did not form a stable equiaxed dual-phase microstructure. The different depths of the face-centered cubic FCC in the figure are due to their different crystal orientations.

[0074] The mechanical properties were tested as in Example 1. The results showed that the yield strength was 908 MPa, the tensile strength was 996 MPa, and the elongation after fracture was 13.5%. The overall mechanical properties were significantly lower than those of the equiaxed dual-phase nanocrystalline pure titanium in Example 1, but better than those of Comparative Example 1.

[0075] Compared with Comparative Example 1, increasing the strain of reversible rolling resulted in finer grains and higher strength. However, due to the high rolling strain, the pure titanium sheet had defects such as edge cracks and through cracks, which affected its subsequent reprocessability.

[0076] Figure 7 The HCP grain size and FCC volume ratio of the nanocrystalline pure titanium prepared in Examples 1-4 and Comparative Examples 1-2 are shown.

[0077] As can be seen from the above embodiments, the present invention provides equiaxed biphase nanocrystalline pure titanium, its preparation method and application. The microstructure of the equiaxed biphase nanocrystalline pure titanium consists of a close-packed hexagonal phase and a face-centered cubic phase. The volume proportion of the close-packed hexagonal phase is 60-80%, and the grain size is 50-100 nm; the volume proportion of the face-centered cubic phase is 20-40%, and the grain size is 30-90 nm. By sequentially performing hydrostatic extrusion, multi-directional forging, triaxial pressing and reversing rolling, and reasonably controlling the processing temperature and strain of each step, equiaxed biphase nanocrystalline pure titanium that is stable at room temperature is obtained. The tensile strength is higher than 1000 MPa, the yield strength is higher than 900 MPa, the elongation after fracture is between 4-20%, and it has high strength, good toughness and excellent reprocessability.

[0078] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An equiaxed dual-phase nanocrystalline pure titanium, characterized in that, The microstructure of the equiaxed dual-phase nanocrystalline pure titanium consists of a close-packed hexagonal phase and a face-centered cubic phase, wherein the volume ratio of the close-packed hexagonal phase is 60-80% and the volume ratio of the face-centered cubic phase is 20-40%.

2. The equiaxed dual-phase nanocrystalline pure titanium according to claim 1, characterized in that, The grain size of the close-packed hexagonal phase is 50~100nm, and the grain size of the face-centered cubic phase is 30~90nm.

3. The method for preparing equiaxed dual-phase nanocrystalline pure titanium according to claim 1 or 2, characterized in that, It includes the following steps: 1) The coarse-grained pure titanium rods are sequentially subjected to hydrostatic extrusion and multi-directional forging to obtain pure titanium blocks; 2) The pure titanium block is sequentially subjected to triaxial pressing and reversing rolling; Step 2) involves heat treatment while applying triaxial pressure.

4. The method for preparing equiaxed dual-phase nanocrystalline pure titanium according to claim 3, characterized in that, The hydrostatic extrusion is a multi-stage hydrostatic extrusion, with the diameter reduction in each stage being 1-4 mm. The strain after multiple hydrostatic extrusions is 1.6~3.

2.

5. The method for preparing equiaxed dual-phase nanocrystalline pure titanium according to claim 3 or 4, characterized in that, The multi-directional forging is a multi-pass multi-directional forging, with a deformation of 1~4mm in each pass and a temperature of -196~30℃ in each pass. Each forging pass cycles sequentially along the X, Y, and Z directions, with more than 6 forging passes in each direction.

6. The method for preparing equiaxed dual-phase nanocrystalline pure titanium according to claim 5, characterized in that, The triaxial pressurization pressure is 0.5~2GPa, and the pressurization rate to reach the pressurization pressure is 15~25MPa / min; The triaxial pressurization process involves applying pressure along the X, Y, and Z directions.

7. The method for preparing equiaxed dual-phase nanocrystalline pure titanium according to claim 6, characterized in that, The heat treatment temperature is 200~400℃, and the heating rate to the heat treatment temperature is 1~10℃ / min; The pressure holding and heat preservation time for the triaxial pressurization is 30~120 minutes.

8. The method for preparing equiaxed dual-phase nanocrystalline pure titanium according to claim 6 or 7, characterized in that, The reversing rolling is a multi-pass reversing rolling, with a rolling pressure of 0.4~2mm per pass, a roll speed of 10~40mm / s, and a rolling temperature of -196~30℃ per pass.

9. The method for preparing equiaxed dual-phase nanocrystalline pure titanium according to claim 8, characterized in that, The rolling direction of each pass is cyclically along the X and Y directions, and the strain after the multi-pass reversal rolling is 0.5~0.

8.

10. The application of the equiaxed biphase nanocrystalline pure titanium according to claim 1 or 2 in medical devices and biological implants.

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