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

Abstract 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 α GB , growth of α GB , nucleation of α WGB , and growth of α WGB . Widmanstatten α, α WGB , grew faster than grain boundary α, α GB , and equiaxed α in common cases. It was found for the first time that the mode of nucleation of α WGB for alloy TA15 was interface instability, i.e. α WGB nucleated through surface instability and the protuberance of α GB and equiaxed α, and the α WGB nucleus did not have an independent and complete surface. A new model of the nucleation of α WGB and phase transformation is proposed. The growth of α WGB 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 α WGB determined the thickness of α WG , with, to some extent, an inverse relation between the nucleation rate and thickness of α WGB .