The Effect of Hafnium Content and Cycling under an Applied Axial Stress on the Creep and Martensite Strain of NiTi Based Shape Memory Alloy Wires

Abstract
The effect of cycling under an applied axial stress of 206.8 MPa on the creep and martensite strain of a Ti rich NiTi shape memory alloy (SMA) and two ternary SMAs, having the compositions Ni49Ti51-xHfx with 1 at.% and 3 at.% Hf, is investigated. The wires used in this investigation have approximately 40% cold work (reduction in area) and a heat treatment (HT) temperature ranging from 300°C to 600°C. The results show, that for a given HT and an applied axial stress of 206.8 MPa, creep decreases with increasing Hf content. For the three alloys evaluated, creep increases with increasing HT temperature. Stress induced martensite strain during the transformation of austenite to martensite is least when the HT temperature is lowest (300°C), and for the three alloys heat treated at 300°C, the martensite strain decreases with increasing Hf content. For the SMAs heat treated at the highest HT temperature (600°C), the martensite strain decreases with an increasing number of transformation cycles, and the martensite strain decreases least for the SMA with 3 at.% Hf. Although the martensite strain is stable from 1 to 100 transformation cycles for the three SMAs heat treated at 300°C, the SMA with 3 at.% Hf retains stress induced martensite strain stability the best as the HT temperature increases.