THE EFFECT OF HAFNIUM CONTENT AND CYCLING UNDER AN APPLIED AXIAL STRESS ON THE CREEP AND MARTENSITE STRAIN OF NITI BASED SHAPE-MEMORY ALLOYWIRES

The effect of cycling under an applied axial stress of 206.8 MPa on th e creep and martensite strain of a Ti rich NiTi shape memory alloy (SM A) and two ternary SMAs, having the compositions Ni49Ti51-xHfx With 1 at.% and 3 at.% Hf, is investigated. The wires used in this investigat ion have approximately 40% cold work (reduction in area) and a heat tr eatment (HT) temperature ranging from 300 degrees C to 600 degrees C. The results show, that for a given HT and an applied axial stress of 2 06.8 MPa, creep decreases with increasing Hf content. For the three al loys evaluated, creep increases with increasing HT temperature. Stress induced martensite strain during the transformation of austenite to m artensite is least when the HT temperature is lowest (300 degrees C), and for the three alloys heat treated at 300 degrees C, the martensite strain decreases with increasing Hf content. For the SMAs heat treate d at the highest HT temperature (600 degrees 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. Althoug h the martensite strain is stable from 1 to 100 transformation cycles for the three SMAs heat treated at 300 degrees C, the SMA with 3 at.% Hf retains stress induced martensite strain stability the best as the HT temperature increases.