Origin of three-stage transformation in a severely cold-rolled and annealed Ti51Ni40Cu9 shape memory alloy

Severely cold-rolled Ti51Ni40Cu9 shape memory alloy annealed at 500 °C × 72 h and 650 °C × 1 h exhibits three-stage martensitic transformation owing to the grain-size effect, i.e., the specimen has small grains near the rolling surfaces and large grains in the central region. The different grain-size distribution is attributed to the inhomogeneous cold-rolling effect. The three-stage transformation has three pairs of transformation peaks which are associated with B21 ↔ B191 transformation of large grains, B22 ↔ B192 transformation of small grains and (B191 + B192) ↔ B19′ transformation of both large and small grains.