Non-classical homogeneous precipitation mediated by compositional fluctuations in titanium alloys

This paper presents experimental evidence of homogeneous precipitation of the alpha-phase within the beta matrix of a titanium alloy, and then accounts for this phase transformation by a new, non-classical mechanism involving compositional fluctuations, based on the pseudo-spinodal concept [1]. This mechanism involves local compositional fluctuations of small amplitude which, when of a certain magnitude, can favor thermodynamically certain regions of the beta matrix to transform congruently to the alpha-phase but with compositions far from equilibrium. Subsequently, as measured experimentally using the tomographical atom probe, continuous diffusional partitioning between the parent beta- and product alpha-phases during isothermal annealing drives their compositions towards equilibrium. For a given alloy composition, the decomposition mechanism is strongly temperature dependent, which would be expected for homogeneous precipitation via the compositional fluctuation-mediated mechanism but not necessarily for one based on classical nucleation theory. The applicability of this mechanism to phase transformations in general is noted. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.