Temperature evolution in shape memory alloy during loading in various conditions

Abstract

Aichi Institute of Technology, 1247 Yachigusa, Yakusa-cho, Toyota, 470-0392, Japan; tobushi@aitech.ac.jp Mechanical characteristics and temperature changes related to stress-induced martensite transformation developing in TiNi shape memory alloy have been presented. Exothermic martensite forward and endothermic reverse transformations have been recorded with use of three kinds of infrared cameras, including very fast and sensitive Therma-Cam Fenix DTS. It was found that the temperature distribution on the surface of the specimens was uniform during straining below the austenite start temperature, while investigating shape memory effect, whereas bands of higher temperature corresponding to localized martensitic transformation were recorded during the process carried out above the alloy austenite finish temperature. The shape memory effect (SME) and superelasticity (SE) are the main phenomena which appear in a shape memory alloy (SMA), depending on the test temperature T. They are controlled by two material parameters: the austenite finish (Af) and the austenite start (As) temperatures. If T is higher than Af, the SE appears, and if T is lower than As, the SME appears. The behavior is caused by the stress-induced reversible martensite transformation (MT) which takes place during the SMA loading and unloading. In the case of SE, almost complete reverse transformation occurs during the SMA unloading Figs 4-9. In the case of SME, quite significant residual strain is observed after the SMA unloading (Figs 2,3). The strain related to the martensitic phase disappears, if the specimen is heated after the unloading above the Af temperature. The energy storage and the energy dissipation due to the SE in SMA are very large and the recoverable stress and strain are quite large compared to the traditional metals. The described properties enable the SMA many applications, e.g., as damping elements, driving force of actuators or main parts of heat engines. The MT in general can be induced by variation in temperature or stress, so the SMA behavior depends on the thermomechanical loading conditions (1-4). The main point studied in the paper is the homogeneity of the martensite transformation process, carried out in various conditions, since the homogeneity usually assures higher reliability of the SMA applied systems.