A study on diffusion couples of Ti and polysynthetically twinned (PST) Ti–Al: I. Microstructure characterization

The microstructural evolution in multi-phase diffusion couples of polycrystalline Ti and polysynthetically twinned (PST) Ti–49.3 at.% Al is studied, with the diffusion direction parallel to the lamellar planes. A reaction layer or reaction zone (RZ) of polycrystalline α2-Ti3Al phase, having a wavy interface with the PST Ti–Al crystal, forms along the PST/Ti bonding interface and exhibits a deeper penetration in the α2 lamellae, consisting of fine α2 and secondary γ laths, than in the primary γ lamellae. Furthermore, high-resolution transmission electron microscopy observations reveal that the RZ/PST interface at the fine α2 and secondary γ lathes is microscopically flat. The lamellar thickness and the mass balance across the RZ/PST interface are believed to be the major factors that lead to these different behaviors in the penetration depth of the RZ.