The effect of prior annealing on high temperature flow properties of and microstructural evolution in SPF grade IN718 superalloy

High temperature flow properties of superplastic forming grade IN718 superalloy, annealed at 1198–1273 K for 15 min–5 h, were investigated. Tensile specimens of grain sizes (do) 2.9–16.9 μm were deformed at constant strain rate of 5 × 10−5 s−1 and by differential strain rate test technique over the strain rate range of ∼2.5 × 10−6 to 3 × 10−2 s−1 at 1198 K. The stress–strain curves exhibited flow hardening followed by flow softening. While tensile ductility was found to increase the peak stress generally decreased with the decrease in grain size. Flow hardening was attributed to concomitant grain growth in the fine-grained (do = 2.9–4.3 μm) samples and to deformation-induced precipitation in the coarse grained (do = 16.9 μm) sample. The occurrence of flow softening, in these two cases, was due to cavitation and precipitate coarsening. log(stress)–log(strain rate) curves delineated superplastic behavior with strain rate sensitivity index (m) ∼0.7 and the maximum ductility was found to be 579%. However, the higher strain rates and coarser grains led to a loss of superplasticity, with m ≈ 0.2.