Necking behavior and microstructural evolution during high strain rate superplastic deformation of IN718 superalloy

Necking, one of the most special characteristics for IN718 alloy during the superplastic deformation with respect to other conventional superplastic alloys, dominates the failure of the alloy in the high strain rate superplastic deformation (10−2-10−1 s−1). We have systematically investigated the necking procedure by tensile experiment (950 °C and 10−2 s−1), with particular emphasis on the microstructural evolution. We have found that the grains were refined by the increasing low angle grain boundaries before necking, and a positive gradient of grain size was present from the necking zone to the grip. After necking, the low angle grain boundaries have been found transformed to the high angle grain boundaries continually without apparent change on the grain size. We deem that the dislocation motion plays the main role before necking and the propagation of necking is closely related to the extent of dynamic recrystallization, the unique deformation mechanism of IN718 alloy.