Microstructural validation of processing maps using the hot extrusion of P/M Nimonic AP-1 superalloy

The microstructural features of hot-extruded Nimonic AP-1 superalloy powder are presented in this paper. Experiments have been designed to validate the processing maps of Nimonic AP-1 microstructurally using a commercial extrusion process. The time average mean strain rate\xi t, has been used as the basis for selection of the extrusion parameter viz., ram speed and extrusion ratio. Extrusion was conducted under non-isothermal conditions. The microstructural evaluation of the material extruded in the dynamic recrystallization (DRX) domain of the processing map exhibited homogeneous deformation and a recrystallized structure, free from cracks and shear bands. Adiabatic shear bands and inter-crystalline cracks seen in the microstructures of other extruded products have been correlated accurately with the instability and cracking regimes of the processing map

Processing map for controlling microstructure in hot working of hot isostatically pressed powder metallurgy NIMONIC AP-1 superalloy

The hot deformation behavior of hot isostatically pressed (HIP) NIMONIC AP-1 superalloy is characterized using processing maps in the temperature range 950-degrees-C to 1200-degrees-C and strain rate range 0.001 to 100 s-1. The dynamic materials model has been used for developing the processing maps which show the variation of the efficiency of power dissipation given by [2m/(m +1)] with temperature and strain rate, with m being the strain rate sensitivity of flow stress. The processing map revealed a domain of dynamic recrystallization with a peak efficiency of 40 pct at 1125-degrees-C and 0.3 s-1, and these are the optimum parameters for hot working. The microstructure developed under these conditions is free from prior particle boundary (PPB) defects, cracks, or localized shear bands. At 100 s-1 and 1200-degrees-C, the material exhibits inter-crystalline cracking, while at 0.001 s-1, the material shows wedge cracks at 1200-degrees-C and PPB cracking at 1000-degrees-C. Also at strain rates higher than 10 s-1, adiabatic shear bands occur; the limiting conditions for this flow instability are accurately predicted by a continuum criterion based on the principles of irreversible thermodynamics of large plastic flow