Effect of grain size on ductility and anomalous yield strength of micro- and submicrocrystalline TiAl

Abstract

Results obtained recently indicate that the TiAl intermetallic compound brittleness at room temperature (RT) and its limited ductility at elevated temperatures is caused by extremely fast localization of deformation. This is primarily due to the limited character of its slip systems, impeded cross-slip, and sequential transition of deformation from one grain to another, as well as formation of dislocation barriers typical of ordered alloys. One of the ways for improving the intermetallic compound ductility is to refine its microstructure, because it can provide for a more uniform straining. On the other hand, the growth of the grain boundary area hinders rack propagation, thus stimulating plastic flow. This is why it was supposed that there must be a certain critical value of grain size under which intermetallics can become ductile. This supposition was supported by results reported for NiAl where, at 400C, a brittle to ductile transition was observed as the grain size decreased. However, recent attempts to improve the RT ductility of some intermetallics by refining their microstructure had no noticeable effect. Even if a certain critical grain size truly exists, it is probably smaller than that described in those papers. In this connection, the authors suggested that intermetallics couldmore » probably be made ductile by converting then into a nanocrystalline state. The present paper contains new data on the mechanical behaviour of submicrocrystalline TiAl as well as a detailed discussion of the grain size effect on the relative elongation to rupture and yield strength of the intermetallic compound under study over a broad range of temperatures.« less