Effect of autocatalysis on variant selection of α precipitates during phase transformation in Ti-6Al-4V alloy

The elastic interactions among α precipitates and their effects on variant selection are investigated. The stress field around a semi-coherent α lath is first calculated using phase field microelasticity theory and then the precipitation process of β→α+β in Ti-6Al-4V is simulated using a 3-D phase field model. The orientations of secondary α variants induced by a primary (pre-existing) α variant obtained from the phase field simulations are consistent with those of variants within commonly observed α clusters in experiments. The formation of α/α orientation relationship of the [112¯0]_α/60°-type, [10‾553]_α/63.26°-type and [0001]_α/10.53°-type frequently observed in experiments could be a direct consequence of autocatalysis during nucleation and growth of α variants. Moreover, the competition among α variants nucleated simultaneously around a primary α lath for limited number of preferred nucleation sites also results in the selection of variants possessing specific crystallographic orientation with respect to the primary α precipitates.