MORPHOLOGY AND GROWTH-KINETICS OF DISCONTINUOUS COARSENING IN FULLY LAMELLAR TI-44 AL (AT.PERCENT) ALLOY

The morphology of discontinuous coarsening in the Ti3Al(alpha(2))/TiAl (gamma) fully lamellar Ti-44 at.% Al alloy has been studied. Three typ es of morphology were observed in discontinuously coarsened lamellar s tructures (secondary lamellae). Type-(I) lamellae have the low energy habit plane as their lamellar interfaces, and have the same lamellar d irection as the original primary lamellae. Type-(II) lamellae have the same crystallographic orientation of the al plates as that in the ori ginal primary lamellae, but have a different lamellar direction from t he original primary lamellae, and have irregular faceted lamellar inte rfaces. Type-(III) lamellae have a different lamellar direction and a different crystallographic orientation of the eel plates from that in the original primary lamellae, but have the low energy habit plane as their lamellar interfaces. The growth kinetics of these three types of lamellae are discussed by modifying the Livingston and Cahn treatment [11] in order to obtain the dependence of the secondary lamellar morp hologies on misorientation between the penetrated primary lamellae, th e advancing secondary lamellae and the lamellar colony boundaries. (C) 1997 Acta Metallurgica Inc.