Crystal-to-amorphous phase transformation in ternary shape memory alloys

Abstract

This systematic study evaluates the role of important factors in crystal-to-amorphous phase transformation during severe plastic deformation in ternary shape memory alloys. Experiments coupled with thermodynamic calculations to obtain comprehensive interpretation on the amorphization in ternary shape memory alloys. Equiatomic binary NiTi together with ternary Ni–45Ti–5Hf and Ti–45Ni–5Cu (at%) alloys as alloy models were processed by high-pressure torsion (HPT) at room and cryogenic temperatures. The binary and Hf-added alloys revealed an amorphous structure with a few remaining nanocrystals, however, Cu-added ternary alloy showed resistance to the amorphization after HPT processing. The results revealed two key factors of preventing martensite-to-austenite transformation during deformation and the intrinsic potential of the material for amorphization drive crystal-to-amorphous phase transformation. Higher transformation temperatures, enthalpy and thermal hysteresis prevented martensite-to-austenite transformation, which therefore encouraged amorphization. In addition, a large negative heat of mixing, Gibbs energy for amorphization and a large atomic size mismatch are important factors in implementing crystal-to-amorphous phase transformation. This investigation is a step forward to a fundamental understanding of ternary or even compositionally complex shape memory alloys for microstructural engineering by providing a homogeneous ultrafine-grained structure due to amorphization and subsequent heat treatment to achieve superior shape memory effect and superelasticity.