Microstructure evolution of a multiphase superalloy processed by severe plastic deformation

This paper presents an overview and some original results about the microstructure evolution of an Ultra Fine Grained (UFG) nickel-iron based alloy INCONEL 718 processed by Severe Plastic Deformation (SPD). The ultrafine grain structure of this alloy that contains a high density of gamma' and gamma precipitates was characterized by Scanning Transmission Electron Microscopy (STEM). We propose a comparison between two SPD processes, High Pressure Torsion (HPT) and Multiple Forging (MF). The grain refinement is much more pronounced by HPT but intermetallic particles are partly dissolved during SPD. The UFG structure after MF is obviously very different and exhibits a much better thermal stability especially because second phase particles do not reprecipitate during post-deformation annealing.