Hot working characteristics of nimonic 75, 80A and 90 superalloys: a comparison using processing maps

Abstract

The hot-working characteristics of Nimonic 75, 80A and 90 are studied in the temperature range 850–1200°C and strain rate range 0.001–100 s⁻¹ using hot-compression tests. Processing maps for hot working are developed on the basis of the variations of efficiency of power dissipation (η=2m/m+1) where m is the strain-rate sensitivity with temperature and strain rate, interpreted using a dynamic materials model. The maps of all these alloys exhibit a high-temperature domain representing the process of dynamic recrystallization (DRX), with a peak efficiency of power dissipation of about 38% occurring at 0.1 s⁻¹. The DRX temperature for Nimonic 75, 80A and 90 are 1200°C, 1150°C and 1130°C respectively. The DRX features of these alloys are compared with those of Nichrome (Ni 20Cr) and Nimonic 105. Carbide dissolution is found to be essential for the occurrence of DRX in all of these alloys, which helps in making interstitial carbon available for increasing the rate of dislocation generation for DRX nucleation. The optimum conditions for good workability are more restricted with higher order of the Nimonic series of alloys. Flow instability due to adiabatic shear-band formation occurs in these alloys at strain rates above 1 s⁻¹.