ON THE SECONDARY RECRYSTALLIZATION OF MA754

The origin of the strong [100] fibre texture observed after the second ary recrystallisation at 1300 degrees C of ODS MA754 has been investig ated by FEG-SEM EBSD. The as-received grain size is bimodal, with occa sional grains (embryos) that possess a six-fold size advantage over th e ''matrix'' grains. The origin of the strong [100] fibre texture exhi bited by the extruded alloy is shown to be unlikely to arise from a mo bility advantage derived from its orientation relationship with the ma trix. The matrix is weakly textured and calculations of average misori entation reveal that the [100] migrating boundaries have no special at tributes. A mean field analysis has shown that secondary recrystallisa tion can take place in the presence of a stable dispersion once a grai n with a critical size advantage is established. Under these circumsta nces, abnormal grains would not require an energy or mobility advantag e, since they would not be competing with normal grain growth (which r emains pinned). The origin of the strong texture originates from the t riggering event, which is envisaged as a process of solute breakaway f acilitated by the reduction in segregation with temperature. The exist ence of local mesotextures around successful [100] grains is postulate d. The theory can account for some of the observed transformation char acteristics such as the orientation dependence of the triggering event , and is supported by the observation that higher annealing temperatur es allow grains to grow which have orientations other than [100]. (C) 1998 Acta Metallurgica Inc.