The mechanism of formation of a fine duplex microstructure in Ti-48Al-2Mn-2Nb alloys

The mechanism of formation of the fine duplex microstructure resulting from the α → γ transformation in water-quenched Ti-48Al-2Mn-2Nb alloys was studied using transmission and analytical electron microscopy. As-cast Ti-48Al-2Mn-2Nb alloys were heat treated in the α phase field and water quenched to room temperature. The resulting microstructure (referred to as a fine duplex microstructure) consisted of equiaxed grains and abutting lath colonies. Both the colonies and the grains were composed of the γ phase, twinned γ laths, and α₂ laths. It was found that the transformation from α to γ in the fine duplex microstructure took place through long range diffusional processes, and compctitive growth between the equiaxed and lath morphology occurred. Nucleation of they phase from the α matrix can occur through nucleation on stacking faults, followed by growth through the sympathetic nucleation and growth of new γ laths on a substrate lath. The observed misorientations and the interfacial structures between the laths were found to be consistent with such a mechanism. Compctition between such nucleation and growth mechanisms for the equiaxed and lath morphologies of γ leads to the formation of lath colonies (of γ and α₂) interspersed with equiaxed grains in these alloys. Formerly Visiting Scientist, Metals and Ceramics Division, Oak Ridge National Laboratory This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.