Modelling of deformation and microstructural changes in P/M Rene 95 under isothermal forging conditions

The changes in microstructure induced by forging and their influence on flow strength of hot isostatically pressed P/M Rene 95 as revealed by constant strain rate compression tests under simulated isothermal forging conditions are discussed. Results are presented for initially fine (7 μm) and coarse (50 μm) grained compacts tested at temperatures of 1050, 1075 and 1100 °C and at strain rates in the range from 10−4 to 1 s−1. Under these test conditions, both the fine and coarse-grained compacts recrystallize and their grain size is refined during plastic deformation. This grain refinement gives rise to softening in both materials. Ultimately, their microstructures transform into the same equiaxed fine-grained microduplex structure at which point their flow strength becomes identical. Continued deformation at that point produces no further change in grain size or flow strength. In this regime of deformation, the microduplex grain size and flow strength are independent of the original microstructure but are conditioned by the strain rate at a given temperature. The steady state grain size increases whereas the steady flow strength decreases with a decrease in strain rate and/or an increase in temperature. It is shown how changes in microstructure and flow strength during isothermal forging can be modelled in P/M Rene 95 compacts by means of established deformation models for predicting peak flow strength, using the steady state deformation data as a boundary condition for the evolution of microstructure and flow strength and a model for deformation-induced recrystallization during forging that has been recently developed for this class of materials.