Discontinuous coarsening of the primary $α_2$/γ lamellar microstructure in Ti-44 at% Al alloy was studied in a temperature range between 900℃ and 1200℃, using optical microscope, scanning electorn microscope and transmission electron microscope. The primary lamellar structure was observed in samples which had been solution treated at 1360℃ in an α single phase region and which had been air cooled or furnace cooled. The lamellar spacing was finer in the case of air-cooled specimen as compared to that of furnace cooled specimen.
On holding the specimen at isothermal aging temperatures lower than the eutectoid temperature, the inter-lamellar spacing of the primary lamellar first decreases and then increased to establish a constant lamellar spacing at each temperature in a later stage of aging. The primary lamellar spaicng tends to continuously increase with the aging time at temperatures higher than the eutectoid temperature. This is in accordance with the absence of the discontinuous coarsening at these high temperatures.
Discontinuously coarsened lamellar(secondary lamellar) cells nucleated at the original α grain boundaries and grew into one grain by means of grain boundary migration. The lamellar morphology of cells was observed to be irregular at an early stage of aging(i.e., near nucleation stage) and to become regular at a later stage of aging. The migration velocity of the moving boundary increased with heat treatment temperature, but the velocity decreases with reaction time at a given temperature. Decreasing rate of the migration velocity was larger at higher temperature. At all heat treatment temperatures air-cooled specimen had a higher migration velocity than furnace cooled specimen because of their finer primary lamellar spacing and a larger supersaturation of the matrix.
The growth rates as well as the lamellar spacing and γ volume fraction of the secondary lamellar cells have been measured. The results have been analysed using the kinetic model of Fournelle. The free energy change has been calculated by assuming that both the TiAl and $Ti_3Al$ have a stoichiometry.
The result yields that the activation energy for the cell growth is ~ 256 KJ/mole, which is somewhat smaller than the activation energy for Ti diffusion in TiAl.
