Microstructural evolution during the alpha(2)->alpha(2)+O transformation in Ti-Al-Nb alloys: Phase-field simulation and experimental validation

The microstructural development during precipitation of a coherent orthorho mbic phase (O-phase) from an alpha (2) matrix (DO19) in a Ti-Al-Nb system i s investigated through computer simulations using the phase-held approach a nd through experimental observations using transmission electron microscopy (TEM). Two compositions were considered in the simulations in order to exa mine the influence of volume fraction of the O-phase on the microstructure. It is found that in the alloy with higher volume fraction of the O-phase, the precipitates have either square or rectangular shapes on (0001)(alpha2) or (001)(O) planes. All the particles are interconnected by sharing their corners. In the alloy with lower volume fraction, the dominant morphology f or the precipitates is thin plate. The spatial distribution of precipitates is highly non-uniform with the precipitates aggregating together to form v arious unique patterns to accommodate the elastic energy arising from the l attice misfit between the alpha (2) and O-phase. All the interfaces between the alpha (2) and O-phase are found to be undistorted habit planes of the type {470)(O), and the domain boundaries between different orientation vari ants of the O-phase are twin boundaries which are the strain-free planes {1 10)(O) or {130)(O). The simulation predictions agree remarkably well with e xisting experimental observations and the concurrent TEM study. (C) 2000 Ac ta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved .