TEM characterization of planar defects in a massively transformed TiAl alloy

The microstructure of a massively transformed Ti-49at.%Al alloy has been studied by conventional transmission electron microscopy (CTEM) and high resolution TEM (HREM). A high density of planar defects, namely complex anti-phase domain boundaries (CAPDB) and thermal micro-twins (TMT) have been observed. CTEM images and diffraction patterns showed that two anti-phase related gamma-matrix domains were generally separated by a thin layer of a 90 degrees-domain, for which the c-axis is rotated 90 degrees over a common cube axis with respect to those of the gamma-matrix domains. HREM confirmed the presence of two crystallographically different types of 90 degrees-domains being associated with the CAPDB. Furthermore, interactions between the CAPDB and TMT have been observed. Local faceting of the generally wavy, non-crystallographic CAPDB parallel to the {111} twinning planes occurred due to interaction with the TMT. The relaxation of the CAPDB onto {111} required diffusion which is proposed to be enhanced locally in the presence of the dislocations associated with the formation of TMT during the massive transformation.