Multiscale γ variant selection in a quaternary near-γ Ti–Al alloy

Abstract

Classical electron back-scattered diffraction in a scanning electron microscope (EBSD/SEM) technique and a novel approach based on computer-aided collection and indexing of TEM Kikuchi patterns were used to study the γ variant and γ|γ misorientation distributions in lamellar grains of a near-γ TiAl alloy. Coarse and fine lamellar structures obtained by continuous cooling at two different rates were investigated. The two methods were complementary and reveal the presence of one-twin-dominant zones (OTDZ) in all grains of both samples. In agreement with previous works, these OTDZ were proposed to result from a mechanism of self-accommodation through twin-variant selection at the nucleation stage. Moreover, it has been pointed out that one γ -order variant often appears to be dominant in a given OTDZ. As the previous mechanism can hardly explain this experimental evidence, the presence of back-stresses due to grain confinement is proposed as a possible explanation. The statistical analysis of γ|γ misorientation distributions, de-correlated from the effect of the non-random distribution of γ variants, reveals that γ|γ interfaces in true-twin relationship are clearly predominant in the coarse lamellar structure, but not in the fine one. This difference is discussed on the basis of different growth mechanisms recently underlined and whose respective influences depend on the transformation driving force, i.e. on the cooling rate.

Acknowledgements

The authors are indebted to Dr S. Naka, who offered the heat treatment facilities of ONERA research centre. They are grateful to Dr J. J. Fundenberger (LETAM) and Dr A. Morawiec (Institute of Metallurgy and Materials, Kraków) for their help in the development and use of EP software, as well as to Dr M. Humbert (LETAM) for fruitful discussions. S. R. Dey thanks the Regional Council of Lorraine for partial financial support of his PhD research.

Notes

¹ Note that the representation used in figure 4 was first proposed in 8.