Preparation, Microstructure And Mechanical Behaviors Of Ultrafine-grained Pure Ti TiNi Alloy

Pure Ti is a promising material in biomedical fields, because of its better biocompatibility, resistant corrosion and without toxic element. But the application of pure Ti is limited for its low strength. In recent decade, improving its strength and refinement of its structure by Severe Plastic Deformation (SPD) has attracted much attention. Lots of investigations on microstructure and mechanical properties of ultrafine-grained (UFG) pure Ti and Ti alloys have been carried out. However phase transformation behavior and properties related to phase transformations of UFG materials are few reported. TiNi shape memory alloy is characterized by multiple phase transformations, superior shape memory effect and super-elasticity associated with forward or reverse martensitic transformations, one of functional materials that have been widely utilized now. Phase transformation behaviors, super-elasticity and shape memory effect of UFG TiNi alloy have attracted much attention recently.Commercial pure Ti (Grade 3) and Ni-rich Ti-50.7.at%Ni alloy were selected, and bulk UFG Ti and UFG TiNi alloy were prepared by Equal Channel Angular Extrusion (ECAE) at 400℃and 500℃, respectively. High strength UFG Ti was manufactured by two-step SPD method i.e, ECAE and Cold Rolling (CR) processes at Liquid Nitrogen Temperature (LNT).Microscopic analysis indicates that submicron-grained microstructure is obtained in Ti subjected to eight passes ECAE at 400℃, less than 500 nm in size. After eight passes ECAE and CR at LNT, dislocation cell structures with a size of 100 nm～150 nm are formed in UFG Ti, and obvious (0002) texture revealed in XRD results.Tensile test at room temperature indicates that after four passes ECAE, strain hardening stage on its true stress-strain curves of UFG Ti (Grade-3) with high mass fraction of impurities is prolonged, and is much larger than that of pure Ti (Grade 0～2). There are two-stage plastic deformation, as uniform deformation and plastic instability on the engineering stress-strain curve, and necking occurred in the second stage. Both ultimate strength and elongation of UFG Ti (Grade 3) after four passes ECAE are higher than those of UFG Ti (Grade 0～2) reported in international articles. After eight passes ECAE and CR at LNT, the ultimate strength of UFG Ti (Grade 3) is improved to 1218 MPa, with an elongation of 12.6%.Compressive tests at RT or LNT reveal that the dependency of flow stress on temperature or strain rate of UFG Ti after eight passes ECAE is lower than that of coarse-grained (CG) Ti. At the strain rate range of 1×10-3～1×10-1/s, the strain rate sensitivity (m) of UFG Ti after eight passes ECAE is 0.026, lower than 0.056 of CG Ti.Microstructure observation of UFG Ti-50.7.at%Ni alloy indicates that microstructure after eight passes ECAE at 500℃is inhomogeneous, many smaller grains with a size of 200 nm～300 nm and some elongated grains with a size of 100 nm～200 nm were observed. Microstructure of UFG TiNi alloy after eight passes ECAE is stable, and the critical temperature for grains growth is determined as 550℃. Further CR treatment at RT, with an accumulative strain of 24%, the structure stability of UFG TiNi alloy decreased.TEM observation reveals that Ti3Ni4 phase precipitated in Ni-rich Ti-50.7.at%Ni alloy during the preheating treatment before each ECAE pass, but metastable Ti3Ni4 phase re-dissolved during sequent ECAE processes. It is suggested that defects and dislocations induced by severe plastic deformation of ECAE process supplies position for surplus Ni atoms and relatively high thermal-activity of Ni atoms at mediate temperature result in the re-dissolution of Ti3Ni4 precipitate.B2?R transformation is induced by ECAE process in ECAE treated Ti-50.7.at%Ni alloy. The B2?R transformation starting temperature (Rs) keeps almost unchanged with the increase of ECAE pass number, and higher than that of coarse-grained (CG) TiNi specimen aged at 500℃for 1 hour. After ECAE processes, B2?R transformation with steamed-bread shape exothermal peak took place within a larger temperature range. The appearance of R phase was not accordant in all ultrafine grains. It is observed in some grains where there are lots of dislocations, in contrast, no R phase observed in the grains with few dislocations. It is suggested that B2?R transformation of ECAE processed Ni-rich TiNi alloy is induced by internal stress fields formed during ECAE processes.Martensitic transformation (R?B19′) peak temperature (Mp) of Ti-50.7.at%Ni alloy decreases with the pass number of ECAE process in two stages. After 1～2 passes ECAE, the Mp is dramatically lowered for the accumulation of strain. During 3～8 passes ECAE, the Mp decreases slowly with the pass number of ECAE process, which is closely related to accumulative deformation strain imposed by ECAE and recovery or dynamic recovery caused by preheating process before ECAE or during ECAE.Super-elasticity behavior test shows that at room temperature, when the tensile strain is 1.5%, UFG Ti-50.7.at%Ni alloy after four passes ECAE shows complete super-elasticity behavior and the super-elasticity keeps stable during loading-unloading cycling for 10 times, no residual strain observed. Tensile strain increases to 4%, the residual strain in UFG TiNi alloy increases with the loading-unloading cycling number, but the increase rate is much lower than that of CG TiNi alloy. After 10 cycles, the residual strain is only 0.61% for UFG TiNi alloy, but 1.84% for CG TiNi alloy. Tensile strain increases to 6%, the increase rate of residual strain with the cycling number is almost the same for UFG TiNi and CG TiNi alloy. After 10 cycles, the residual strains in UFG and CG TiNi alloy are 2.66% and 2.75%, respectively. When the tensile strain increases to 8%, UFG TiNi alloy reveals no super-elasticity behavior. The maximum super elastic strain of UFG Ti-50.7.at%Ni alloy at room temperature is less than 4%. Shape memory effect test shows that when the bending deformation strain is smaller than 10%, UFG Ti-50.7.at%Ni alloy after four passes ECAE showes 100% one-way shape memory effect. The shape memory property is comparable to that of CG Ti-50.7.at%Ni alloy solution-treated and aged at 500℃at a same prestrain.