Quantitative XRD analysis of the evolution of the TiNi phase transformation behaviour in relation to thermal treatments

Changes in the phase transformation behaviour in heat treated and thermally cycled NiTi shape memory alloy wires were quantified by X-ray diffraction as a function of temperature. The results were compared with those obtained from the electrical resistivity measurements. Comparing these two techniques helped to improve and clarify the interpretation of the electrical resistivity measurements independently of whether there is an R-phase in the alloy or not. The weight fraction diagrams enable us to detect simultaneous phase transformations between austenite, R-phase and martensite, and to clearly observe the effects of applying thermal treatments to SMA alloys. The results in the weight fraction diagrams show how thermal cycling at zero stress can either develop or reduce the R-phase, depending on the temperature of the heat treatment. The results show that the beginning of the transformation to martensite found in the weight fraction diagrams corresponds with different points in the electrical resistivity curves depending on the quantity of the R-phase that the alloy develops during the heat treatment or during thermal cycling at zero stress.