9731349 Vasudevan This research applies high resolution transmission electron microscopy in a study of the massive transformation in TiAl- based intermetallic alloys. Two important issues are addressed related to the mechanisms of massive transformations in TiAl alloys: (1) the nature of the massive -parent interphase interfacial structure, and (2) the role of defect structures. The study focuses on alloys with aluminum contents between 46-47 at. %, which allows retention of large areas of the massive-parent interfaces. Orientation relationships that develop during nucleation and the early stages of growth of the massive phase are studied in samples whose reaction kinetics has been determined. The structures of the parent -massive phases and the types of interfaces that between them are determined in samples undergoing different undercoolings, reaction start temperatures, and growth rates. The state of coherency of the interfaces is determined and from these studies atomistic models of the structures of the massive-parent interphase interfaces are developed. The atomic structures of defects (dislocations and stacking faults) are determined as well, and their role in the growth of the massive product assessed. %%% New alloys based on the TiAl compound are of great importance for application in high temperature propulsion systems because of their unique combination of properties. Mechanical properties of these materials are controlled by their microstructures, which is the focus of this research. ***
