The massive transformation in titanium aluminides: Initial stages of nucleation and growth

Abstract

Rapid solidification by twin-anvil splat quenching captures the initial nucleation and growth of the α → γ _m massive transformation in titanium aluminides. Splat quenching Ti₅₂Al₄₈ and Ti₅₀Al₄₈Cr₂ from the liquid at slightly below the melting point produces an equiaxed α solidification structure. Solid-state cooling rates that approach 10⁶ K/s arrest the α → γ _m massive transformation with 1- to 5-µm-sized γ _m nuclei, especially in the Ti₅₀Al₄₈Cr₂ alloy. Classical massive-transformation heterogenous nucleation occurs at α:α grain boundaries with an orientation relationship of [111] _γ //[0001] _α and $\{ 110\} _\gamma //\{ 11\bar 20\} _\alpha$. The γ _m nucleus then grows into the adjacent α grain without the orientation relationship by forming an incoherent α:γ interface with {111} _γ facets. Orthogonal variants of the tetragonal c-axis in the γ _m product suggest that the massive transformation initially produces an fcc structure which subsequently orders into the L1₀ phase. Nucleation of γ _m is not only observed at α:α grain boundaries and triple points, but also within the α grains. The intragranular γ _m nucleation, which is believed to be heterogeneous, occurs with the same orientation relationship as for the intergranular nuclei. However, the intragrain nuclei do not form {111} _γ facets and retain a curved α:γ _m interface. Although analysis of the {111} _γ faceted growth using weak-beam dark-field (WBDF) imaging shows no evidence for any type of misfit-compensating dislocations, lattice imaging of the {111} _γ facets with high resolution transmission electron microscopy (HRTEM) reveals that the planar interface exhibits a slight curvature, produced by atomic steps of (111) planes. These experimental data have been used to estimate a ratio of ledge spacing (λ) to ledge height (h) for the {111} _γ facets as λ/h=41, which is similar to calculated values for a ledge growth mechanism of massive transformations in Cu-Zn and Ag-Cd alloys.