The variation in the martensitic transformation during thermal cycling of the cast Ti-Hf-Ni-Cu shape memory alloys under or without stress

This paper examines the variation in martensitic transformation temperatures during thermal cycling with or without constant stress. Five cast Ti50.2−xHfxNi49.8−yCux (x =1  or  9.5  at.%, y =1,  5  or  10  at.%) alloys with various chemical compositions were subjected to ten thermal cycles in stress-free state or under 100 MPa (for x =1 at.%) or 200 MPa (for x = 9.5 at.%). It was found that during thermal cycling without stress, the transformation temperatures decreased in all alloys; however, the smallest variation was observed in the Ti49.2Hf1Ni44.8Cu5 and Ti40.7Hf9.5Ni39.8Cu10 alloys. Thermal cycling under stress showed that the variation in the transformation temperatures might be smaller or larger than on thermal cycling without stress. It was shown that an increase in the intensity of the variation in transformation temperatures on thermal cycling under stress did not depend on the chemical composition of the alloy, yield limit for dislocation slip or variation in the dislocation density. This showed that the variation in the dislocation density was not the main reason for the variation in the transformation temperatures during thermal cycling. It was concluded that the thermal cycling stability shown by the Ti-Hf-Ni-Cu alloys during thermal cycling without stress is not maintained during thermal cycling under stress.

1. Russian Science Foundation - 23-19-00280