Preparation of metastable phase materials: nucleation and growth kinetics considerations

Separation of metastable phase, involving the melt undercooling before the onset of nucleation and the growth (solidification) velocity of competing phases, lies at the core of Rapid Solidification Processing. To clarify the dependences of metastable phase formation on the initial undercooling degree and solidification rate in the rapidly solidified alloys, three alloys, Ti50Al50, Ti47Al53 and Ti44Al56, were prepared and investigated by undercooling and laser resolidification experiments. The corresponding undercooling window and solidification velocity range for different competing phases were determined from the rapidly solidified samples by precise microstructural analysis. In Ti50Al50 and Ti47Al53 alloys, the primary phase changes from β to α and γ phase with increasing undercooling, and it evolves from α to γ phase with increasing solidification velocity. However, phase γ acts as the primary phase in the achievable undercooling and solidification velocity ranges of Ti44Al56 alloy. Two surface treatment methods, in view of nucleation and growth kinetics, were proposed to obtain metastable phase structures.