Phase Transformations and Super-Elasticity of a Ni-rich TiNi Alloy with Ultrafine-Grained Structure

A Ni-rich Ti-50.7.at%Ni alloy was processed by Equal Channel Angular Extrusion (ECAE) at 500 degrees C. After 8 passes ECAE, microstructure was refined to sub-micron scale, approximately 0.2 mu m similar to 0.3 mu m. 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 re-dissolved during sequent ECAE processes. After ECAE treatment, the B2 -> R transformation occurred within a larger temperature range. Comparing with the solution-treated TiNi specimen, the martensitic transformations start (Ms) and peak temperatures (Mp) of TiNi specimens ECAEed were dramatically lowered. Super-elasticity characteristics of TiNi alloy were tested by tensile loading and unloading cycles. The results reveal that at a tensile strain of 4% or smaller, ultrafine-grained (UFG) TiNi alloy processed by 4 passes ECAE shows better super-elasticity than solution-treated sample. Microstructure evolution and its effect on phase transformations and super-elasticity characteristics have been discussed.