Kinetics, thermodynamics and nucleation of the alpha to gamma(m) massive transformation in Ti-Al alloys.

摘要

The primary objective of this thesis was to study fundamental aspects of massive and other phase transformations from the {dollar}alphatogamma{dollar} phase in TiAl alloys using a novel technique for studying reaction kinetics, coupled with detailed quantitative analysis.; A novel computer-controlled temperature and electrical resistivity measurement system (TERMS) for studying phase transformations was constructed. Using this device, the variation in electrical resistivity with temperature on heating and cooling was established for the first time for Ti-(25-52)Al alloys. The results show that the currently accepted Ti-Al phase diagram is, by and large, accurate, except for minor modifications.; The results of studies of Ti-(47.5,48)Al alloys have shown that the {dollar}alphatogamma{dollar} transformation is sensitive to cooling rate and accompanied by large discontinuous changes in resistivity and thermal arrest. In these alloys, lamellar, Widmanstatten and massive {dollar}gamma{dollar} are observed with increasing cooling rate. Correlation of the resistivity-temperature-time data with microstructure has allowed the determination of start temperatures and the growth rate of the massive {dollar}gammasb{lcub}rm m{rcub}{dollar} phase as a function of undercooling below the T{dollar}sb{lcub}rm o{rcub}{dollar} temperature. The occurrence of the massive transformation in the two-phase {dollar}alpha+gamma{dollar} region has been rationalized.; The enthalpy values and driving forces for the massive transformation in Ti-47.5Al were determined experimentally and compared with the massive transformations in other systems as well theoretically modeled values for TiAl. The estimates for the activation enthalpy for boundary diffusion ({dollar}sim{dollar}120 kJ/mole) obtained established that the massive transformation in TiAl alloys is interface rather than bulk diffusion controlled. These results are interpreted in terms of the different models for interface migration and the growth process, whether a continuous or ledge-type mechanism, is discussed.; Finally, the nucleation rates of massive {dollar}gamma{dollar} at grain faces, edges and corners of prior {dollar}alpha{dollar} grain boundaries were determined experimentally at three different undercoolings from stereological measurements and compared with theoretical calculations. It revealed that facetted grain face nucleation and incoherent grain corner nucleation are highly likely. These results are discussed in light of the existing TEM evidence for incoherent interfaces and for coherent nucleation of the massive {dollar}gammasb{lcub}rm m{rcub}{dollar} phase in Ti-Al alloys.