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℃∙s^-1 to 0.1℃∙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℃∙s^-1, the original β phase rapidly transformed to α' martensite at 910℃ in the form of acicular platelets without any β phase left. A great amount of dislocations and (101̄1) stacking faults were found inside the marten site. Under cooling rate of 5℃∙s^-1, α phase nucleates along the grain boundary as well as inside the grain, while at the cooling rate below 1℃∙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.