Effects of Heat-treatment Temperature on Shape Memory Characteristics of a Tape-shaped Ti-Ni Alloy Element Fabricated by a Centrifugal Casting Method

Abstract

A heat-engine using Ti-Ni shape memory alloy (SMA) driven by low-temperature waste heat energy has been investigated for waste heat recovery potential system. We had proposed the use of the tape-shaped SMA element because of the cooling efficiency improvement of the SMA element. However, the manufacturing cost of the tape-shaped SMA fabricated by arc melting method is higher than that of a wire or coil shaped SMA. To reduce the manufacturing cost, we fabricated a tape-shaped SMA element by the centrifugal casting method using an alloy fabricated by the powder metallurgy method, and investigated the effect of heat-treatment temperature on shape memory characteristics of this tape-shaped Ti-Ni SMA specimen fabricated by the centrifugal casting method. The mechanical and shape memory properties of tape-shaped SMA specimen show a minimum at 803 K, and it increases as temperature rises up to 953 K, above which it decreases. The fluctuation of mechanical and shape memory properties are caused by the variation of dislocation density, secondary phase and precipitated phase. From these results, it was found that the optimum heat-treatment temperature of the tape-shaped Ti-Ni SMA specimen is 953 K. Moreover, the output properties of SMA heat-engine driven by an SMA element fabricated by the centrifugal casting method, is superior to the output properties of the heat-engine using an SMA element fabricated by arc-melting method.