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 gamma (gamma(m)) structures in a Ti-46.8AI1.7Cr-1.8Nb (at.%) alloy. gamma(m) grains presenting a Blackburn orientation relationship with one of the two parent a grains were detected along several alpha/alpha boundaries. According to previous studies, they were identified as original gamma(m) nuclei. Along a given alpha/alpha grain boundary, gamma(m) nuclei in relation with one a grain were always shown to be dominant with respect to the other. Moreover, while two twin variants are expected for this gamma(m) 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 gamma(m) nucleus. An original analysis of EBSD maps pointed out that a large majority of the gamma(m) 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) ym nuclei, through a mechanism of successive twinnings. This growth firstly occurs in the a grain having no Blackburn orientation relationship with the original nucleus, but later generations of gamma(m) twins can further re-enter into the original parent a grain in Blackburn orientation relationship. Although some misoriented or unfavourable Blackburn OR ym 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 ym grains, a mechanism of their germination and growth is proposed. (c) 2005 Elsevier Ltd. All rights reserved.