The Comparison of Martensitic Transformation Sequences Between Aged Ti-50.6 and Ti-51.0 at.% Ni Alloys by in-situ SEM Observations

The comparison of martensitic transformation sequences between aged Ti-50.6 and Ti-51.0 at.%Ni alloys  have been investigated by differential scanning calorimetry (DSC) and in-situ scanning electron microscopy (SEM). Ti-50.6 and 51.0 at.% Ni alloys  were selected for evaluation of Ni concentrations. These alloys were heat-treated at 1223 K for 3.6 ks and then aged at 773 K for 3.6 ks. Although the Ti-50.6 and 51 at. % Ni alloys revealed the triple stage transformations during cooling, but the results of in-situ SEM observations showed that the sequence of these transformations was completely different from each other. Although, the distribution and size of Ti3Ni4 precipitates observed in various compositions were different from each other, but the heterogeneity in distribution and size of precipitates was a common feature of them. For both of the compositions, martensitic transformation sequences are quantitatively proved by comparing DSC measurements and SEM observations. The calculations show good agreement between DSC cooling curve and SEM observation results.