EFFECTS OF HAFNIUM, HEAT TREATMENT AND CYCLING UNDER AN APPLIED STRESS ON THE TRANSFORMATIONS OF COLD WORKED NiTi-BASED SHAPE MEMORY ALLOYS

摘要

The effect of thermal cycling under a constant tensile load on the transformation temperatures (TTs) of NiTi-based shape memory alloys (SMAs) is investigated. Three SMAs are examined in this study: a near equiatomic binary Ni_(49)Ti_(51) alloy and two ternary Ni_(49)Ti_(51)-xHfx alloys with 1 at% and 3 at% Hf. The SMAs are in the form of wires with 40% cold work (reduction in area) and heat treated between 300℃ and 600℃. These SMA wires are thermally cycled between their martensite (M) and austenite (A) phases for 100 cycles under an axial tensile stress of 206.8MPa (30Ksi) in air. Results show that the effect of thermal cycling on the M and A TTs depends on heat treatment (HT) temperature and composition in a complex manner. For example, the M TT, of the binary NiTi SMA heat treated between 300℃ and 450℃, increases during thermal cycling. However, with HT temperatures between 500℃ and 600℃, the M TT decreases slightly during thermal cycling. For the NiTi-based SMAs that contain 1 at% and 3 at% Hf, the M TT increases during thermal cycling for HT temperatures up to 500℃, and the M TT decreases during thermal cycling when heat treated at 600℃. These results are due to changes in internal stress and structure, such as dislocation density and arrangement, which are affected by HT temperature and thermal cycling. The influence of Hf content on the changes in the M and A TTs during thermal cycling is also shown.