Nucleation and growth mechanism of α-lamellae of Ti alloy TA15 cooling from an α + β phase field

The mechanism of nucleation and growth of α-lamellae when a TA15 Ti alloy with an equiaxed structure is cooled from an α + β phase field were studied by end quenching experiments and thermal simulation tests using a Gleeble-3500. The results showed that for the Ti alloy TA15 the nucleation and growth of α-lamellae involved four steps, including nucleation of αGB, growth of αGB, nucleation of αWGB, and growth of αWGB. Widmanstätten α, αWGB, grew faster than grain boundary α, αGB, and equiaxed α in common cases. It was found for the first time that the mode of nucleation of αWGB for alloy TA15 was interface instability, i.e. αWGB nucleated through surface instability and the protuberance of αGB and equiaxed α, and the αWGB nucleus did not have an independent and complete surface. A new model of the nucleation of αWGB and phase transformation is proposed. The growth of αWGB in the TA15 alloy started from a small protuberance and spread into a β grain with a sectorial morphology, to become lamellar instead of spiculate or oblate cuboid in shape. The nucleation rate of αWGB determined the thickness of αWG, with, to some extent, an inverse relation between the nucleation rate and thickness of αWGB.