Kinetics and thermodynamics of the α→γ m massive transformation in a Ti–47.5 at.% Al alloy

Abstract

Continuous cooling experiments, utilizing in situ, high-speed computer-controlled temperature and electrical resistivity measurements, were performed to study the kinetics and thermodynamics of the α→γ m massive transformation in a Ti–47.5 at.% Al alloy. Samples of the alloy were heated by controlled direct resistance heating in vacuum to the α-phase field and cooled at various rates either in vacuum or by controlling the flow of a helium jet quench. The results have shown that the α→γ transformation is sensitive to cooling rate and is accompanied by large discontinuous changes in resistivity and thermal arrest. Lamellar, feathery and massive γ m are observed with increasing cooling rate. Correlation of the resistivity–temperature–time data with microstructure allowed the determination of the cooling velocity range in which the massive transformation takes place, the reaction temperatures of this transformation as a function of cooling rate, and the growth rate of the massive γ m phase as a function of undercooling below T 0. The enthalpy and driving force associated with the formation of the massive γ m phase were determined and compared with those in other alloy systems; the experimental values of the driving force were also found to compare well with theoretically calculated values. Comparison of estimates of the activation enthalpy for boundary diffusion obtained with values for bulk diffusion established that the α→γ m massive transformation is controlled by interfacial rather than volume diffusion. Possible growth mechanisms of the massive phase, whether by a ledge process or by continuous growth, were examined and are discussed.