Formation sequence of intermetallics and kinetics of reaction layer growth during solid state reaction between titanium and aluminum

Understanding on the reaction kinetics associated with the formation of intermetallic compounds and the mechanisms involved in evolution of the reaction product at different temperatures due to solid state reaction in Friction Stir Welding (FSW) of dissimilar materials is very important in selecting optimum operating condition to control intermetallic compounds for safety-critical applications. In this study, the evolution sequence of intermetallic compounds and reaction kinetics during the solid state reaction between Ti and Al has been investigated. It was found that the mechanical mixing of Al and Ti affects evolution of reaction layers (RL) and intermetallic compounds. The thickness of the RL increases with reaction temperature and titanium trialuminide (Al3Ti) was found to be the only ordered phase that is formed at a higher temperature (650 degrees C). The activation energy for the growth of the reaction layer was found to be significantly lower when compared to conventional welding. The sequence of formation of these intermetallic compounds is proposed based on kinetics and thermodynamic principles such as inter-diffusion and free energy of formation. The variation in stoichiometry at the Al/Ti interfaces due to mechanical mixing as occurred during FSW and difference in diffusivity of Al and Ti through reaction layers are considered responsible for the phase evolution and growth of these layers. The mechanism of the reaction with the reduced activation energy was attributed to fragmentation, mechanical mixing, development of dislocation and twinning in materials, activation of atoms due to severe deformation and grain boundary diffusion.