Variation in Martensitic Transformation Temperatures and Defect Density upon Thermal Cycling of Ti-Hf-Ni-Cu Alloys with Various Grain Sizes

Abstract

The variation in martensitic transformation temperatures, electrical resistivity, and dislocation density were studied during 500 thermal cycles in Ti_40.7Hf_9.5Ni_44.8Cu₅ alloy with grain sizes ranging from 130 to 0.16 µm. The maximum transformation temperature change was observed in the sample with an average grain size of 130 µm, and the minimum temperature change was found in the sample with a grain size of 0.16 µm. It was shown that during thermal cycling of Ti_40.7Hf_9.5Ni_44.8Cu₅ alloy the transformation temperatures can decrease, increase, or remain constant, independently of the grain size. Analysis of the dislocation density variation measured by the Williamson–Hall method and the resistivity variation determined in the austenitic and martensitic states showed that these quantities change nonmonotonically with increasing number of thermal cycles. In Ti_40.7Hf_9.5Ni_44.8Cu₅ samples with grain sizes of 16 and 0.16 µm, the dislocation density and resistivity increased during the first 50–200 cycles and then decreased, without any effect on the transformation temperature variation. The obtained results show that the change in dislocation density is not the only reason for the change in transformation temperatures during thermal cycling.