High strain rate properties of a near equi-atomic NiTi shape memory alloy
thesisposted on 09.05.2014 by Andrew J. Patman
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The effects of strain rate and testing temperature on the mechanical response of a near equi-atomic NiTi alloy have been investigated. All experiments have been conducted in compression, at testing temperatures of room .temperature (-20°C), 30°C, 40°C and 50°C. Quasi-static experiments were performed using a Hounsfield HK50 universal testing machine, and high strain rate measurements were obtained using the split Hopkinson pressure bar technique. The primary differences in the behaviour of the material within these deformation rate regimes appeared to be the presence of a possible transformation inhibition mechanism that occurs for high rates of strain, which manifests itself as an accommodation of applied load after the onset of transformation, increased strain rate sensitivity at high rates, and temperature dependence not evident at low rates. Initial material characterisation was achieved' through microhardness testing, DSC, DMTA, X-ray and electron diffraction, resulting in clarification of the transformation temperatures, martensitic volume fraction and microstructure of the alloy. A post experiment X -ray investigation was also performed in order to establish the microstructural response of the material to deformation. From the stress-strain data collected, the strain rate sensitivity and entropy of· transformation of the alloy have been calculated. The application of a standard Arrhenius type equation has also been attempted, in order to estimate the material parameters of activation volume, and the free energy of transformation in the absence of stress. This model was found to be reasonably representative of the response of the alloy, although the results calculated demonstrated a high degree of intrinsic error.