Thermal cycling stability of the B2 ←→ B19' transformation in the Ti-Hf-Ni-Cu alloys with cast and ultrafine-grained structures

Martensitic transformations were studied during thermal cycling of Ti40.7Hf9.5Ni44.8Cu5 and Ti40.7Hf9.5Ni39.8Cu10 alloys with cast and ultrafine-grained structures the latter being formed by crystallization of amorphous ribbons. The variation of transformation temperatures and resistivity were analyzed over 500 thermal cycles through the temperature range of martensitic transformation. The results showed that in the samples with cast structure, transformation temperatures significantly decreased with the number of cycles, while in ribbons with ultrafine-grained structure, all transformation temperatures were constant. In Ti40.7Hf9.5Ni44.8Cu5 ribbons, the variation in resistivity was significantly less than in the cast samples, while in the Ti40.7Hf9.5Ni39.8Cu10 alloy the rho(N) curves were comparable. This is due to the fact that yield limit for dislocation slip in Ti40.7Hf9.5Ni44.8Cu5 ribbons was twice larger than in the cast state, hence the dislocation density in ribbon sample was hardly changed on thermal cycling while it increased in the cast sample. In Ti40.7Hf9.5Ni39.8Cu10 alloy the dislocation yield limit was the same in ribbons and cast samples, hence, the dislocation density increased on thermal cycling in the same way. As the transformation temperatures were constant in ribbon samples of both alloys, then one may conclude that the value of the dislocation yield limit and the variation in dislocation density on thermal cycling had no effect on the variation in transformation temperatures. It was assumed that a high thermal cycling stability of the martensitic transformation in ribbon samples were due to only one plate with corresponding variant pairs of martensite twins formed in each grain on cooling.