The effect of heat treatment variables on the phase transformations at 1420°c in ti-48a1 and Ti-48al-2Mn-2Nb alloys

The effect of heat treatment variables such as initial microstructure, isothermal reaction time and cooling rate on the phase transformations occurring at 1420°C in Ti-48A1 and Ti-48Al-2Mn-2Nb alloys was studied. The main effect of the initial microstructure, which comprised either lamellae of α₂and γ or equiaxed γ grains, was to alter the kinetics, through a change in the chemical driving force of the phase transformations. Therefore, the equiaxed γ grains transformed to α much faster than the lamellar structure and in the initially lamellar structure, growth of α resulted in the delineation of the initial dendritic structure formed during solidification. The effect of the rate of cooling from the heat treatment temperature on the final morphology of these alloys was drastic and resulted in a change in morphology from lamellar grains obtained on furnace cooling to a feathery and mottled morphology obtained on water quenching. TEM analysis of water quenched Ti-48Al-2Mn-2Nb revealed complex morphologies including a structure which consisted of equiaxed γ grains and residual α₂and abutting colonies of γ and α₂. Based on the TEM results, the early stages of formation of γ from α were studied and mechanisms of nucleation and growth discussed. The relative importance and the coexistence of massive and martensitic transformation products is also discussed.