Phase Transformations And Mechanical Behaviors Of Ni-free Ti Based Shape Memory Alloys ECAP Processed And Aged

Ni-Ti shape memory alloys (SMAs) are widely used in biomedical applicationsdue to their superior properties such as low elastic modulus, shape memory effect andsuperelasticity. However, since Ni-Ti alloy contains Ni of about50at.%, the possibilityof Ni-hypersensitivity has been pointed out because of the Ni extricated in the bodyfluid. New practical shape memory alloys consisted of non toxic elements, especiallyNi-free, have been a hotspot in biomedical materials area. Ti-Mo based SMAs andTi-Nb based SMAs are the two kinds of important Ni-free alloys.Ti-xMo-4Nb-2V-3Al(x=7.6～11mass%) alloys are newly developed Ti-basedbiomedical materials by Memry corporation in USA. These alloys exhibit low Young′smodulus, good superelasticity behavior when Mo element content is about10mass%.In the present study, the effect of aging on the microstructure, deformation behaviorand superelasticity of Ti-9.8Mo-3.9Nb-2V-3.1Al alloy and the shape memory effect(SME) of Ti-9.5Mo-4Nb-2V-3Al alloy were investigated. Ti-9.8Mo-3.9Nb-2V-3.1Alalloy and Ti-25at.%Nb alloy were processed by Equal Channel Angular Pressing(ECAP) and aging treatment to obtain ultra-fine grain (UFG) microstructure, and thesuperelasticity of these alloys with UFG microstructure were investigated.Microscopic analysis indicates that β+ωathdual phases microstructure wereobtained in solution treated Ti-9.8Mo-3.9Nb-2V-3.1Al alloy, and the shape of ωathphase is lamina. After aging at300℃for30minutes, ellipsoidal shape ωisophase witha size about10nm precipitated in the matrix. As aged at400℃for30minutes,needle-like phase about100nm in length precipitated and ellipsoidal shape ωisophase increased about20nm in size. After aging at500℃for30minutes, lots of phase precipitated and dispersed homogeneously in the matrix. However, as agingtemperature increased to600℃, phase precipitated along grain boundary dominantly, and a few phase precipitated inside grains. When the aging temperaturewas700℃, the amount of phase along the grain boundary decreased and there waslittle phase precipitated inside grains.Tensile test indicates that deformation behavior of Ti-9.8Mo-3.9Nb-2V-3.1Alalloy can be divided into three groups. For Group I, the stress-strain curves show workhardening after yielding. The specimens aged at350℃,400℃and450℃for30minutes belong to group I, showing a similar deformation behavior to that of STspecimen, with obvious work hardening after yielding. For group II, the stress-straincurves show little work hardening after yielding. Specimens aged at250℃,300℃and500℃for30minutes belong to Group II. For group III, the stress-strain curvesshow two yielding phenomena and a high work-hardening rate after first yielding, butvery small after the second yielding. Specimens aged at600℃and700℃for30minutes belong to group III. The solution treated Ti-9.8Mo-3.9Nb-2V-3.1Al alloyexhibits good superelasticity with a recoverable strain of2.8%and a remained strain of1.2%. After aging at400℃for30minutes, the specimen exhibits excellentsuperelasticity with a recoverable strain of3.2%and a remained strain of0.8%.However, after aging at300℃and500℃, the superelasticity lost completely. Asaging temperature increased to600℃and700℃, the superelasticity was restored witha recoverable strain of3.3%. Because of the strengthening effect of ωathphaseprecipitates, the solution treated specimen exhibits high yielding strength. Dispersed phase homogeneously in the matrix can increase the strength strongly.The martensitic transformation temperature (Ms) of Ti-9.8Mo-3.9Nb-2V-3.1Alalloy is-56℃.When the temperature is lower than250℃, the resistance ratiodecrease during heating on the resistance (ρ) vs. temperature (T) curves, because ofthe ωathphase dissolving into the matrix. As the temperature is in the range of250～380℃,the resistance ratio increases during heating because of the precipitation of ωisophase. The resistance ratio decreases again during heating from380℃to450℃with the dissolving of ωisoand the precipitation of α phase. When the temperature isin the range of450～600℃, the resistance ratio increases because of the precipitationof α phase from β phase. As the temperature is in the range of600～850℃,resistanceratio decreases because of the dissolving of α phase by α→β transformation. The αphase to β phase transformation completed at850℃. A multifunctional alloy with a nominal chemical composition ofTi-9.5Mo-4Nb-2V-3Al (mass%) was prepared by cold crucible levitation meltingtechnique, which exhibits shape memory effect. The microstructure of solution treatedTi-9.5Mo-4Nb-2V-3Al alloy is β+ωath+α" three phases. This alloy exhibits doubleyielding stage during tensile deformation. There is a stress induced martensitictransformation (SIM) of β α" during the first yielding stage. Deformed specimensexhibit shape memory effect after heated at223℃for5minutes, and the shaperecoverable strain is2.2%. DSC and ρ-T experiments indicated that α" induced bystress which can transform to parent phase (β) during heating. And the start temperatureof transformation (As) is92℃. After aged at300～400℃for30～60minutes,Ti-9.5Mo-4Nb-2V-3Al alloy exhibites superelastic behavior. The recoverable strain is3.2%for specimen after aged at300℃for60minutes unloading from tensile strain of4%. However, after aging at300℃for120minutes, the superelasticity lostcompletely.Ti-9.8Mo-3.9Nb-2V-3.1Al alloy was processed by ECAP to obtain the UFGmicrostructure. After one pass ECAP at400℃, the micrograph is lath microstructureand the strength increases obviously. The yielding strength (σ0.2) is1296MPa and thetensile strength1355MPa. The Young′s modulus increases to88.9GPa. After twopasses ECAP, the yielding strength (σ0.2) is more than1500MPa with Young′s modulus104.9GPa, but the plasticity decreases sharply. When materials were flash annealed at700℃for10s after1pass ECAP, the Young′s modulus decreased to63GPa. Afterone pass ECAP at300℃, The yielding strength and ultimate tensile strength are1265MPa and1310MPa, respectively. It is noted that there were cracks on the surface ofspecimen after2pass ECAP.The microstructure analyses of Ti-25at.%Nb alloy after4passes ECAP at550℃indicates that the grains of Ti-25at.%Nb alloy were elongated, but no obtained UFG.The grains were refined obviously as the process temperature decreased to400℃.After2passed ECAP at400℃, most grains were elongated, and some equiaxed UFGare observed with grain size about500nm. The equiaxed UFG can be obtained after4passes ECAP at400℃with a grain size about300nm. The strength andsuperelasticity can be improved by ECAP+aging treatment. After2,4passes ECAPprocessing and aged at300℃for60minutes, the strain can be restored completely when unloaded from tensile strain of1.5%. The Ti-25at.%Nb alloy after ECAP andaged at300℃for30minutes exhibits good superelastic stability at a tensile strain of1.5%.