Abstract

An investigation of the compressive stress-strain behaviour of a 50.4% Ni by weight NiTi shape memory alloy when subjected to both quasi-static and high strain rate deformation across a range of temperatures is reported. Testing was carried out using a Hounsfield universal testing machine and a split Hopkinson pressure bar system. Transformation stress dependence on both increasing strain rate and temperature is presented, together with observed values of strain rate sen- sitivity and derived entropy of transformation per unit volume. Since the discovery of the shape memory effect (SME) in Au-Cd alloys in the 1950's (1), shape memory alloys (SMAs) have found increasing popularity in the fields of both materials research and industrial application. Of the large variety of alloys that have been found to demonstrate SME behaviour, relatively few have shown sufficient promise for development into commercial applications, the exceptions being alloys centered around Nickel and Titanium, due to their superior memory, ability to recover substantial amounts of strain and the significant force that they are able to generate whilst undergoing shape change. Their SME properties are caused by a solid state phase transformation from the higher temperature (austenite) phase to lower temperature (martensite) phase. The properties of both single crystal and polycrystalline SMAs in the quasi-static strain rate region have been studied extensively (2, 3). However, a thorough understanding of their mechanical properties when subjected to dynamic loading has not been achieved, and of the small number of studies that have been conducted on NiTi alloys in the high rate range, the majority are restricted to room temperature measurements. For example, the high strain rate, small strain response of a 50.4 Ni - 49.6 Ti (at.%) alloy with an AF temperature of y 296 K has been observed under dynamic compressive loading by Nemat-Nasser et al. (4). They comment that the transition stress, which is defined as the critical stress for stress-induced martensite formation, is highly sensitive to the deformation strain rate, with a monotonic increase with increasing strain rate until approximately 1000 s x1 . At this point they observe a significant increase in the transition stress which is reminiscent of the high strain rate yield behavior of normal metals. In addition, Chen et al. (5) conducted an investigation into the compressive superelastic behavior of a 55.8% Ni-wt NiTi alloy with a quoted AF temperature between 5xC and 18xC. They also found that the transformation stress of the material depends highly on the strain rate. Conversely, Liu et al. (6) performed low and high strain rate compression measurements on a NiTi alloy with an AF temperature of 93xC. Their results indicate that the characteristics of the stress-strain curves for such an alloy are insensitive to the deformation rate for changes in strain rate up to 10 3 s x1 . Whilst these investigations offer valuable insight into the properties of NiTi SMAs during dynamic deformation, they are limited in the sense that the experimentation is conducted solely at room temperature. In addition, the results produced are for NiTi alloys that are either purely austenitie, e.g. (4) and (5), or purely martensitic, as is the case for example (6). The only

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