Kinetics of alpha precipitation in Ti-6 wt % Cr and Ti-11 wt % Mo

Abstract

The morphology and kinetics of the precipitation of the alpha phase produced by two different heat treatment routes, namely, (a) direct isothermal decomposition and (b)β-quenching and subsequent ageing, were studied. In isothermally decomposed samples theβ′ (supersaturated) →α + β transformation was seen to occur mainly through the discontinuous growth of the transformed zone consisting of groups of parallel side plates from the grain boundary regions towards the interior of the grain. Unlike for the case of a regular discontinuous precipitation, here the transformed regions are not separated from the untransformed by an incoherent interface and the growingα-plates do obey a fixed orientation relationship with the grain from which they are evolved. The theory of cellular reaction has been applied to explain the growth rate of the duplex (α + β) region. The overall reaction kinetics were analysed on the basis of the Johnson-Mehl formulation and were found to be consistent with that of a discontinuous precipitation reaction, where grain boundary nucleation sites were saturated at an early stage of the transformation. The structure of theβ-quenched samples showed a uniform distribution of athermal omega particles which acted as precursors to theα-precipitates. As a consequence, the reaction rate was greatly enhanced andα-precipitation in the quenched and aged samples was seen to occur continuously in the entire body of the grain.