Morphology of discontinuous coarsening in fully lamellar Ti-44Al(at%) alloy

The morphology of discontinuous coarsening in the Ti3Al (alpha(2))/TiAl (gamma) fully lamellar Ti-44at%Al alloy has been studied. Three types of morphology were observed in discontinuously coarsened lamellar structures (secondary lamellae). Type-(I) lamellae have the low energy habit plane as their lamellar interfaces, and have the same lamellar direction as the original primary lamellae. Type-(II) lamellae have the same crystallographic orientation of the alpha(2) plates as that in the original primary lamellae, but have a different lamellar direction from the original primary lamellae, and have irregular faceted lamellar interfaces. Type-(III) lamellae have a different lamellar direction and a different crystallographic orientation of the alpha(2) plates from that in the original primary lamellae, but have the low energy habit plane as their lamellar interfaces. Geometric conditions for formation of these types of secondary lamellae were analyzed based on the Livingston and Cahn treatment [Acta Metall. 22, 495 (1974)], taking misorientation between the penetrated primary lamellae, the advancing secondary lamellae and the lamellar colony boundaries.