The microstructure and martensitic transformation of Ti-13 V-3Al light weight shape memory alloy deformed by high-pressure torsion

Abstract

Ti-13 V-3Al (at%) high temperature shape memory alloy was considered attractive material due to its low density. The demand for the better shape memory performance is proposed to widen the scope of application. In the present study, Ti-13 V-3Al is processed by high-pressure torsion and annealing. The microstructure, martensitic transformation and mechanical properties were investigated. The severe plastic deformation introduced high density defects and decreased the stability of the α″ phase. The phase constitution is the sole β phase after high-pressure torsion, which was related to the Gibbs-Thomson effect. After the annealing treatment at 700 °C, numerous small parallel α″ martensite variants were observed, which was affected by the residual defects. More nucleation sites were provided, resulting that the reverse martensitic transformation was accomplished in a narrow temperature range. Moreover, the formation of (111) type I twinning become easier due to the assistance of the (111) stacking faults. The α″ martensite variants became larger and evolved to the V-shaped and the triangular self-accommodated morphologies as the annealing temperature increased to 800 °C. The twinning relationship in the different annealing temperature both were the (111) type I twinning. The properties were characterized by the tensile loading-unloading tests and microhardness tests. The shape memory effect was optimized and 4% full recoverable strain was obtained in the alloy annealed at 700 °C. The apparent hardness decreased from 323HV and 263HV as the annealing temperature raised from 700 °C to 800 °C.