Metastable Phase Transformation of Beta Titanium Alloys by Slip Mechanism

Abstract

Extensive studies of the metastable structures in beta Ti alloys have investigated the ambiguity of the unstable phase. Their microstructural characteristics have been observed to have a modulated structure, and spot satellites of diffraction in TEM, however the formation mechanisms have not been successfully interpreted because of the resolution limit of the nano lattice structure. High resolution TEM was performed to observe the metastable nano structure and for analysis of the relationship between the nano lattice and spot satellites. A metastable structure was formed just before α precipitation by overlapping a new lattice onto the β matrix, inducing a modulated structure as a kind of Moire fringe. The new lattice structure of the 3a phase had 3 times the distance of the β lattice parameter and had lattice relationships with the β//3a phases of β(112)/(1100)/3a[⅓(112)] and β(222)/(1101)/3a[⅓(222)]. It was unreasonable to interpret the 3a phase as a definite precipitation of the  phase, because only the limited zone axes of <113> and <110> were present in the TEM diffraction results of the metastable structures. Other zone axes, such as <100> or <111> etc., were not able to break the distinct metastable features of the spot satellites. Rather than precipitation, the 3a phase was activated by the slip of <113>/(112) and <110>/(112) in the TEM diffraction. It exhibited 3 times the distance of the β lattice parameter, which could not be achieved by typical precipitation.