【摘要】 为获得兼具高屈服强度、良好塑性与低弹性模量的Ti-25Nb-3Zr-3Mo-2Sn(TLM)医用钛合金，本文采用室温多道次等通道角挤压（ECAP）技术，结合X射线衍射（XRD）、透射电子显微镜（TEM）、金相显微镜（OM）及拉伸试验，系统研究了TLM钛合金在ECAP变形过程中的显微组织演变、变形机制及强韧化机制。结果表明，1道次变形后，组织中存在大量板条，其内部含有高密度位错，并析出应力诱导马氏体（SIM）α″相与等温ω相，此时抗拉强度达到761 MPa；随着变形道次增加，等温ω相逐渐消失，马氏体临界诱发应力升高，α″相发生向β相的马氏体逆转变，SIM α″相的体积分数不断降低，变形机制逐渐转变为位错滑移主导。ECAP变形后，TLM钛合金的强度显著提升，4道次变形后抗拉强度达到808 MPa，一方面归因于变形引入的大量位错所引起的位错强化，另一方面源于部分SIM α″相析出所产生的第二相强化。变形后合金的弹性模量降低到26.6 GPa，其主要原因为变形使马氏体相变温度升高、 β相稳定性降低，同时低弹性模量SIM α″相的析出和大量位错的引入亦对此有所贡献。更多还原

【Abstract】 Ti-25 Nb-3 Zr-3 Mo-2 Sn（TLM） titanium alloy is a new multifunctional near-β biomedical third-generation titanium alloy developed by Northwest Institute. Equal channel angular pressing（ECAP） is a severe plastic deformation method that can effectively improve materials’ strength. To improve its strength, reduce the elastic modulus, and obtain good elongation, four passes of ECAP at room temperature were used to perform on TLM titanium alloy after solid solution at 850 ℃ for 1.5 h in this paper. The extrusion speed of ECAP was 2.5 mm·s^-1. The C mode, in which longitudinal rotation by 180° was required after each pass before deformation, was selected for ECAP. The internal channel angle of ECAP die was 90°, and the external arc angle was 20°. The equivalent strain after each pass of deformation was 1.16. D8 Advance A25 X-ray diffractometer（XRD） was used to determine the phase of the samples with a scanning angle of 20°～90°, a test voltage of 40 kV, and a current of 40 mA. Microstructural observations were taken using an GX51-Olympus optical microscope（OM）. For transmission electron microscopy（TEM, JEM-2100 Plus, 200 kV） observations, samples were mechanically polished to a thickness <100 μm, then punched to form thin disks that had a diameter of 3 mm. Transparent areas were produced using the ion thinner（Leica EM RES102）. Uniaxial tensile tests were carried out with an INSTRON 8801 universal testing machine at a quasi-static strain rate of 1×10^-3 s^-1. XRD patterns showed that the solid solution sample was composed of β matrix and quenched α’ phase. In the first pass of ECAP, α’ phase disappeared, and stress-induced martensite α″ phase precipitated. As the number of ECAP passes increased, the stress-induced martensite α″ phase gradually decreased from 16.7% in the first pass to 2.0% in the fourth pass. The gradual reduction of the stress-induced martensite α″ phase was due to two reasons: the occurrence of reverse transformation of martensite and the increase in the required stress for stress-induced martensite. It was seen from stress-strain curves that after four passes of ECAP, the tensile strength increased from 556.4 MPa to 808 MPa, an increase of 45.2%. Obviously, the "double yield" phenomenon disappeared after the first pass of ECAP. At the fourth pass, the elastic modulus decreased to 26.6 GPa. In the "double yield phenomenon", the first yield point was the critical inducing stress point of stress-induced martensite（SIM）. The appearance of the second yield point was due to the occurrence of plastic deformation. The reason for the disappearance of the "double yield" phenomenon was the reduction in the content of stress-induced martensite α″ phase and the increase in the critical inducing stress of martensite. The reason for the reduction in elastic modulus was divided into two parts. The decrease in elastic modulus was partly due to the fact that large plastic deformation raised the martensitic transformation temperature and reduced the stability of β phase, and the other part was that the precipitation of the low elastic modulus SIMα″ phase and the generation of a large number of dislocations led to the decrease in lattice stability, thereby reducing the elastic modulus. The reason for the increase in yield strength was also divided into two parts. One was the strengthening due to the precipitation of stress-induced martensite α″ phase, and the other was the generation of a large number of dislocations due to deformation. The dislocations entangled with each other and hindered the movement of dislocations, making plastic deformation difficult and thus improving the alloy strength. From the metallographic photos, it was seen that a large number of slip bands appeared and the grains were obviously elongated after the first pass of ECAP. After the second pass, the grains showed an equiaxed trend. After the fourth pass, the grain size was more uniform. In TEM images, diffraction spots of α″ and ω phases could be observed. There were a large number of lath structures. The dislocation density was significantly increased compared to the solid solution sample. After four passes of ECAP, the lath width was reduced from 100～400 nm in the first pass to 50～100 nm. Moreover, ω phase disappeared, and α″ phase gradually decreased. TEM images showed that no twins were found in the grains. The main characteristics were a large number of dislocations and laths, which indicated that twinning was not the main deformation mechanism during 90° ECAP process of TLM titanium alloy. ECAP deformation mechanism of TLM titanium alloy at room temperature consisted of dislocation slip, strain-induced martensite α″ phase, and isothermal ω phase. As ECAP passes increased, ω phase gradually disappeared, the martensite induction stress increased, the reverse transformation of α″ to β martensite occurred, and the volume fraction of SIM α″ phase continuously decreased. The deformation mechanism was finally dominated by dislocation slip.更多还原