Grain coarsening of PM nickel-base superalloy by critical strain annealing

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Critical strain annealing has been investigated for the of development of coarse grain structures in PM Astroloy. The need for this type of treatment arises because normal grain coarsening is severely retarded by carbides in the prior particle boundaries (PPBs). It was found that a strain of ∼ 2.2% produced coarse grain structures (∼ 360 μm) on subsequent annealing for 2 h at 1250°C. Smaller amounts of deformation could also initiate the nucleation of recrystallization but here the driving force was insufficient to cause the migration of the recrystallization front through the PPBs, resulting in no increase in grain size beyond the size of the prior powder particles.