Phase Transformation during the Continuous Cooling in Near α Titanium Alloy Ti60

The phase transformation mechanism of near α titanium alloy Ti60 was investigated during the continuous cooling from β phase field, at the cooling rate from 80 °C·s−1 to 0.1 °C·s−1. The initial and final temperatures and kinetics of the β→α′ and β→α diffusion type transformations were analyzed by a dilatometric method. Over the cooling rate of 50 °C·s−1, the original β phase rapidly transformed to α′ martensite at 910 °C in the form of acicular platelets without any β phase left. A great amount of dislocations and (101) stacking faults were found inside the marten site. Under cooling rate of 5 °C·s−1, α phase nucleates along the grain boundary as well as inside the grain, while at the cooling rate below 1 °C·s−1, the nucleation occurred only along the grain boundary. A small amount of retained β phase was observed along the α platelet boundary under TEM, while the ordering transformation of α phase occurred inside the platelet, resulting in forming of α2 phase. Therefore, the cooling rate determines the alloy phase transformation mechanism and the inner structure of product phase.