Characterization of order domains in γ-TiAl by orientation microscopy based on electron backscatter diffraction

A new approach to resolve the slight tetragonality of L1₀-ordered γ-TiAl by electron backscatter diffraction (EBSD) is presented. The phase has a c/a ratio of only about 2% larger than unity. The corresponding EBSD patterns therefore exhibit cubic pseudosymmetry. As a consequence, different order variants cannot be easily distinguished on the basis of their EBSD patterns. Automated orientation mapping results in frequent misindexing. In the past, either this problem was overcome by identifying order domains by relatively laborious transmission electron microscopy, or the order domain structure was ignored altogether by using a generic face-centered cubic structure to solve for the crystal orientations, accepting a significant loss of microstructural information. The presented approach is based on the detection of the minor tetragonal distortion of the diffraction patterns by an accurate measurement of backscatter Kikuchi band positions. To this end an accurate pattern center calibration together with high-accuracy parameters for pattern acquisition and indexing are required. Together with a modified indexing algorithm, the order domains in a lamellar microstructure of Ti–45.9Al–8Nb (at%) could be reliably identified. The occurrence of superlattice reflections in the Kikuchi patterns was used to validate the technique. The developed method was successfully applied to create a crystal orientation map of Ti–45.9Al–8Nb (at%) with a fully resolved domain microstructure.