Variant selection during nucleation and growth of γ-massive phase in TiAl-based intermetallic alloys |
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Authors: | A. Sankaran E. Bouzy M. Humbert A. Hazotte |
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Affiliation: | Laboratoire d’Etude des Textures et Application aux Matériaux (LETAM), UMR CNRS/UPV-M 7078, Ile du Saulcy, 57045 Metz Cedex 1, France |
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Abstract: | This work deals with the mechanisms of nucleation and growth of γ-massive phase in TiAl-based intermetallic alloys. In particular, it focuses on the process of variant selection operating at both stages of the transformation. Small γ-massive domains produced by rapid cooling are extensively characterized by high-resolution electron backscatter diffraction. This large data set allows a statistical analysis of nucleation sites, according to different crystallographic configurations. It is established that, whatever the nucleation sites, i.e., grain boundaries or triple junctions, a coherent facet is always found presenting a Blackburn orientation relationship (BOR) between the γ nucleus and the α parent grain. Moreover, some γ nuclei can additionally present another semicoherent facet with an approximate BOR with the α host grain. A new nucleation mechanism, called “co-nucleation”, is highlighted for this type of double-faceted nucleus. Variant selection during nucleation is discussed for both types of nuclei, in terms of minimization of interface energy. In addition, it is shown that growth of γ-massive domains from their initial nucleus always involves successive {1 1 1} twinning. Variant selection also occurs either at the nucleus growth stage or during the development of successive twin generations, and is discussed in terms of interphase boundary mobility. |
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