New Interpretation of the Electrical Resistivity Measurements for Obtaining NiTi SMA Stress-Free Transformation Temperatures

The objective of this study is to clarify and to improve the interpretation of the ER measurements used to obtain NiTi SMA stress-free transformation temperatures for the austenite to martensite transformation process. To achieve this objective, the transformation temperatures of NiTi SMA wires are measured by ER using the Ling and Kaplow interpretation and are compared and complemented by weight fraction diagrams. The weight fractions are obtained from XRD profiles using the Parametric Rietveld refinement and adequate software. As a result of comparing both techniques, a new interpretation of the ER curves is proposed. This new interpretation is based on the shape of NiTi ER curve, which depends on the quantity of R-phase in the NiTi SMA. The quantity of the R-phase is obtained by weight fraction diagrams. According to the findings presented here, a new criterion for R-phase and martensite transformation temperatures is proposed.