Isothermal particle growth in two-phase titanium alloys

Abstract

Isothermal particle growth studies were carried out on several Ti−Mn and T−V alloys consisting of varying amounts of α and β phases at 973 K. It was found that the particle growth kinetics of the α and the β phases could be represented by simple equations in terms of time and volume percents of the phases. The growth process is presented as a two-way diffusion process where the solvent and the solute atoms move in opposite directions resulting in conversion of α to β and β to α leading to the growth of the particles. The Lifshitz-Wagner-Slyozov theory, which considers only solute diffusion for growth of particles, is slightly modified to incorporate the diffusion of both solute and solvent into the growth equation. The observation of the growth kinetics indicated that, under identical conditions, the growth of α particles in a β matrix was faster than the growth of β particles in an α matrix. Furthermore, for identical conditions, the growth kinetics of Ti−Mn alloys are faster than those of the Ti−V alloys. While the faster kinetics are consistent with higher bulk interdiffusivities of the Ti−Mn system, the magnitude of the difference is much smaller than that expected from the bulk interdiffusivity considerations. In addition, it was found that the growth exponent,n, values are in between the theoretically predictedn values for grain boundary diffusion control and bulk diffusion control. Based on these observations, it is suggested that a mixed mode diffusion mechanism consisting of bulk diffusion as well as grain boundary diffusion is controlling the growth process at 973 K. Details of the investigation and various growth models for α-β titanium alloys are presented.