Recrystallisation behaviour and grain structure in mechanically alloyed oxide dispersion strengthened MA956 steel

The recrystallisation mechanism and grain structure control in mechanically alloyed MA956 steel have been investigated. It is found that the pinning by grain boundary junctions themselves retards the onset of recrystallisation happening at extraordinary high temperatures. Although the material has significant alignment of the dispersoids, the dragged force for grain boundary migration is far less than the driving pressure from the release of stored energy during recrystallisation. The formation of a critical nuclei is considered to trigger the onset of recrystallisation, which strongly relied on the crystallographic texture of the unrecrystallised sample. As for the grain control, a refined grain structure can be obtained by preanneal before the final recrystallisation treatment. Although refined, it shows the anisotropic features due to the high yttria content in the steel. The other method to refine the recrystallisation structure is using the mixture of conventional MA956 with transformable MA957—Ni to form the metal—metal composite, in which the MA957—Ni component is to carry out the austenitic transformation refinement. In principle, a refined grain structure can be obtained, however, it is not highly reproducible. Detailed investigations of interdiffusion and homogenisation are needed before further progress is possible with the concept of a composite alloy.