Quantitative microstructure analyses upon multistage martensitic transformation in an aged Ti-50.8 at.% Ni alloy

Quantitative microstructure analyses upon multistage martensitic transformation (MMT) in an aged Ti-50.8 at.% Ni alloy have been performed by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). The alloy used was heat-treated at 1273 K for 3.6 ks and then aged at 773 K for 3.6 ks under regulated and unregulated atmospheres. The specimen heat-treated under the regulated atmosphere shows the single stage martensitic transformation in addition to the R-phase transformation. That is, B2 → R → M. On the other hand, the specimen heat-treated under the unregulated atmosphere shows the MMT with the sequence of B2 → R → M1 → M2 → M3. There are three exothermic peaks related to the martensitic transformation in addition to the R-phase transformation peak. The R-phase transformation takes place at the intermediate part of grains and around the grain boundaries. Subsequently, the first (M1) and second (M2) martensitic transformations occur at the intermediate part of grains and around the grain boundaries, respectively. Finally, the third transformation (M3) appears at the center part of grains directly from the B2 phase without via the R-phase transformation. The proposed transformation sequence is quantitatively proved by comparing the EBSD image with DSC curve.