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

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 degrees 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. (C) 2009 Elsevier Ltd. All rights reserved.