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

摘要

This study investigated changes in the martensite start temperature (M_s) 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 (c_v) of the matrix. When the electron concentration of the matrix increases, the transformation temperature decreases, whereas the M_s temperature rises when the c_V of the matrix decreases. Additionally, if the precipitation does not result in a significant variation of the c_v of the matrix, the variation in M_s is small (<15 deg C). In the present work it is shown that alteration of the M_s 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 M_s is also discussed on the basis of their quantities in the as-quenched and aged microstructures and the strain energy induced in the matrix.