Transformation temperature changes due to second phase precipitation in NiTi-based shape memory alloys

This study investigated changes in the martensite start temperature (Ms) of NiTi-based shape memory alloys as a result of second phase precipitation. Precipitation of the second phase leads to a change in the matrix chemical composition and may alter the electron concentration (cv) of the matrix. When the electron concentration of the matrix increases, the transformation temperature decreases, whereas the Ms temperature rises when the cv of the matrix decreases. Additionally, if the precipitation does not result in a significant variation of the cv of the matrix, the variation in Ms is small (<15 °C). In the present work it is shown that alteration of the Ms temperature as a result of precipitation is mainly due to the change in electron concentration of the matrix. The extrinsic effect of the precipitates on Ms is also discussed on the basis of their quantities in the as-quenched and aged microstructures and the strain energy induced in the matrix.