Microstructure Evolution And Mechanical Properties Of A Ti-35Nb-3Zr-2Ta Biomedical Alloy Processed By Equal Channel Angular Pressing (ECAP)

In this paper, the Ti-35Nb-3Zr-2Ta (wt%) β biomedical titanium alloy was preparedby arc-melting under an argon atmosphere. The ingot was re-melted three times to ensurecompositional homogeneity. By employing an equal channel angular pressing (ECAP)method, the Ti-35Nb-3Zr-2Ta alloy was extrude for one, two and four passes via routeBc under various temperatures from500℃to700℃. It can be concluded that the solutiontreated Ti-35Nb-3Zr-2Ta alloy had an nearly equiaxed β coarse grains with a size of80～120μm. The ultrafine nanograins of～300nm and600nm were achieved throughECAP method pressed at500and600oC via4pass, respectively. The evolution of shearbands was observed in specimens pressed at500℃which was beneficial to theaccommodation of large plastic strain. Furthermore, networks of shear bands involvinginitial bands propagation and new bands broadening was observed in specimen500℃4th pass. The unique herringbone morphology of twinned martensitic variants resultingfrom self-accommodation of martensite and energy minization effect was observed inspecimen600℃4th pass by TEM method.The microhardness values of the ECAP-processed β titanium alloy wereconsistently higher than that of the solution treated one. After4th pass, due to theuniform microstructure and equiaxed finer grains, the transverse and longitudinalmicrohardness values showed almost no difference. Moreover, compared with thesolution treated alloy, stress-strain curves of almost all specimens pressed at2nd and4thpass (except for500℃2nd pass) appeared a typical “double yielding” phenomenon dueto stress-induced martensite transformation of β phase to phase.The effects of different accumulative deformations and pressing temperature onsuperelasticity of the ECAP-processed Ti-35Nb-3Zr-2Ta alloys were investigated. Under the different accumulative deformation, the trend of superelastic behavior of theECAP-pressed billets was disparate. For specimens pressed at500℃, the εsewent upgradually which was the opposite trend of the εsein specimens pressed at600℃.However, the εsefirst increased and then decreased for specimens pressed at700℃. Also,the pressing temperature played an impact on the εse. The εseof600℃4th pass washigher than that of700℃4th pass, but lower than that of500℃4th pass, which wasrelated to uniform networks of shear bands and recrystallization at high temperature.Specimen500oC4th pass exhibited a tensile strength of765MPa, ductility of～16%, low elastic modulus of～59GPa and the εmseaxand εmraxas large as1.4%and2.7%respectively,indicating that the Ti-35Nb-3Zr-2Ta(wt%) β biomedical titanium alloyprocessed by ECAP was a great promising candidate for biomedical applications.