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

The elastic interactions among alpha precipitates and their effects on variant selection are investigated. The stress field around a semi-coherent alpha lath is first calculated using phase field microelasticity theory and then the precipitation process of beta -> alpha + beta in Ti-6Al-4V is simulated using a 3-D phase field model. The orientations of secondary alpha variants induced by a primary (pre-existing) alpha variant obtained from the phase field simulations are consistent with those of variants within commonly observed alpha clusters in experiments. The formation of alpha/alpha orientation relationship of the [11 (2) over bar0](alpha)/60 degrees-type, [(10) over bar 553](alpha)/63.26 degrees-type and [0001](alpha)/10.53 degrees-type frequently observed in experiments could be a direct consequence of autocatalysis during nucleation and growth of alpha variants. Moreover, the competition among alpha variants nucleated simultaneously around a primary alpha lath for limited number of preferred nucleation sites also results in the selection of variants possessing specific crystallographic orientation with respect to the primary a precipitates. (C) 2016 Elsevier B.V. All rights reserved.