Cp and fractal phase transformation in the shape memory alloy Ni52Ti

Abstract

Measurement of the temperature dependence of the molar heat capacity C p for an Ni52Ti alloy was carried out using a differential scanning calorimeter (DSC) and an enthalpy technique. This technique is based on stepqise heating or cooling. The heat of transition was determined by integrating ΔC p between the measured C p value of the sample in the transition region and the C p value of the parent phase at an equivalent temperature. The latter C p value was determined by linear extrapolation, C p = A + BT, from the higher temperature range of stability of the parent CsCl-type structure. The experimental data for the molar heat capacities C p show a fractal phase transformation. If the heat of transformation is divided into three parts, i.e. chemical enthalpy, elastic and inelastic frictional energy, the proportion of transformed martensitic variants correlates with the chemical enthalpy. M s, i , M f, i , A s, i and A f, i decrease with an increase in the generation number i. This is because the elastic energy and friction energy increase with grain size. From the changes in M s, i , M f, i , A s, i , and A f, i with generation number, the mid-temperature T 0 between M s and A f can be calculated for a complete transformation in polycrystalline alloys but not for single crystals. The undercooling ΔT and the elastic entropy ΔS el are discussed.