Microstructural evolution during the α₂→α₂ + O transformation in Ti-Al-Nb alloys: phase-field simulation and experimental validation

The microstructural development during precipitation of a coherent orthorhombic phase (O-phase) from an α₂ matrix (DO₁₉) in a Ti-Al-Nb system is investigated through computer simulations using the phase-field approach and through experimental observations using transmission electron microscopy (TEM). Two compositions were considered in the simulations in order to examine 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)_alpha(2) or (001)_o planes. All the particles are interconnected by sharing their corners. In the alloy with lower volume fraction, the dominant morphology for the precipitates is thin plate. The spatial distribution of precipitates is highly non-uniform with the precipitates aggregating together to form various unique patterns to accomodate the elastic energy arising from the lattice misfit between the α₂ and O-phase. All the interfaces between the α₂ and O-phase are found to be undistorted habit planes of the type {470)_O, and the domain boundaries between different orientation variants of the O-phase are twin boundaries which are the strain-free planes {110)_O or {130)_O. The simulation predictions agree remarkably well with existing experimental observations and the concurrent TEM study.