Grain refinement under multi-axial forging in Fe–32%Ni alloy

The deformation behavior and the grain refinement mechanism of Fe–32%Ni alloy under multi-axial forging were studied by the flow stress curve analysis and the microstructure observation. The multi-axial forging was carried out at a strain rate of 10−2 s−1 under temperature of 773 and 1073 K, respectively. The results show that the general character of flow stress curve and the microstructure of Fe–32%Ni alloy sensitively depended on the deformation temperature. At the temperature of 773 K, the flow stress increased to a single peak with work hardening and then showed a steady-state-like one during further deformation. The new grains are formed in the process that the initial deformation leads to the formation of subgrain boundaries and further deformation causes the misorientations of subgrain boundaries to increase by usual intrinsic slip and the absorption of accommodated dislocations, in which way the new ultra-fine grains are formed homogeneously. The grain refinement mechanism is classified as continuous dynamic recrystallization (CDRX). At the temperature of 1073 K, the flow stress curve exhibited a single peak stress followed by work softening and then a steady state. The grain refinement process is that new grains that are almost defect-free are formed by nucleating at the bulged boundaries during deformation, then they grow and consume the deformed microstructure until the material is completely recrystallized, which process is classified as discontinuous dynamic recrystallization (DDRX).