Stability of the B2 ↔ B19′ martensitic transformation on thermal cycling of Ti-Hf-Ni-Cu thin ribbons

Abstract

The B2 ↔ B19′ martensitic transformation was studied on thermal cycling of the Ti40.7Hf9.5Ni49.8-xCux (x = 5, 10 at.%) thin ribbons. The temperatures, as well as the enthalpy of the transformation, were found to be stable on thermal cycling. The stability of the B2 ↔ B19′ transformation was attributed to the formation of a specific grain structure during the crystallization of the initially amorphous ribbons. The grains were characterized by an average size of 800 nm, which increased the yield limit for dislocation slip to 700 MPa. Moreover, one martensite corresponding variant pair (CVP) formed on cooling inside each grain. It was assumed that the formation of one CVP martensite plate within the grain on cooling, as well as a high yield limit for dislocation slip, might have been responsible for the stability of the martensitic transformation temperatures and enthalpy on thermal cycling.