Effect of thermal cycling on multistage martensitic transformation in aged ti-50.8 at% ni alloy

The effect of thermal cycling on multistage martensitic transformation (MMT) in aged Ti-50.8 at% Ni alloy was investigated. The specimens were solution-treated at 1273 K for 3.6 ks and then aged at 773 K for 3.6 ks in vacuum without atmosphere regulation. Upon cooling the aged specimens clearly showed quadruple-stage transformation denoted as B2 → R → M1 → M2 → M3. The peak temperatures of exothermic reactions R M1 and M2(corresponding to the R-phase M1 and M2 transformations respectively) in the differential scanning calorimetry (DSC) cooling curve were rather stable up to 100 thermal cycles since there were fine Ti₃Ni₄ precipitates with high distribution density in the intermediate and grain boundary regions. These precipitates prevented the formation of dislocations during thermal cycling. On the other hand the peak temperature of exothermic reaction M3 (corresponding to the M3 transformation) drastically decreased with increasing the number of thermal cycles since there were large Ti ₃Ni₄ precipitates with low distribution density. Many dislocations were observed in the central regions of grains after 100 thermal cycles.