EBSD characterisation of massive γ nucleation and growth in a TiAl-based alloy

Electron backscattered diffraction (EBSD) has been carried out to obtain large orientation maps of massive γ (γm) structures in a Ti–46.8Al–1.7Cr–1.8Nb (at.%) alloy. γm grains presenting a Blackburn orientation relationship with one of the two parent α grains were detected along several α/α boundaries. According to previous studies, they were identified as original γm nuclei. Along a given α/α grain boundary, γm nuclei in relation with one α grain were always shown to be dominant with respect to the other. Moreover, while two twin variants are expected for this γm nucleus type, our study pointed out the predominance of one of the two twins. These clear selection phenomena are analysed in terms of favourable orientation relationship between the grain boundary plane and the habit plane of γm nucleus. An original analysis of EBSD maps pointed out that a large majority of the γm grains present in the microstructure can be grouped in as large ‘twin-domains’, inherited from the growth of these initially favoured Blackburn orientation related (OR) γm nuclei, through a mechanism of successive twinnings. This growth firstly occurs in the α grain having no Blackburn orientation relationship with the original nucleus, but later generations of γm twins can further re-enter into the original parent α grain in Blackburn orientation relationship. Although some misoriented or unfavourable Blackburn OR γm nuclei have also been detected close to the grain boundaries, they were always found to be dormant with respect to further growth. Depending upon the description on γm grains, a mechanism of their germination and growth is proposed.