HIGH-RESOLUTION ELECTRON-MICROSCOPE STUDY OF LAMELLAR BOUNDARIES IN TI-RICH TIAL POLYSYNTHETICALLY TWINNED CRYSTALS

The lamellar structure of Ti-rich TiAl has been investigated on an atomistic scale by high-resolution electron microscopy (HREM). The interfaces studied include those of TiAl[110BAR]/TiAl[110BAR], TiAl[011BAR]/TiAl[011BAR], TiAl[110BAR]/TiAl[011BAR], TiAl[110BAR]/Ti3Al[1120BAR] and TiAl[011BAR]/Ti3Al[1120BAR]. Interfaces between differently oriented TiAl variants are atomistically flat except for the presence of ledges two (111)TiAl planes high, and in most cases contain no Ti3Al phase. Interfaces between TiAl and Ti3Al are also atomistically flat and perfectly coherent but contain somewhat more ledges two (111)TiAl planes high. Close examination of HREM images taken at TiAl[110BAR]/Ti3Al[1120BAR] interfaces has revealed that the atomic rows composed ot Ti atoms only are aligned across the interface between the two phases. The formation mechanism of the lamellar structure are the factors governing the ductility of TiAl are briefly discussed based on the results obtained.