Hot workability and microstructure control in Co20Cr15W10Ni cobalt-based superalloy

Hot isothermal compression tests were conducted on Cobalt-based superalloy Co-20Cr-15W-10Ni using a thermo-mechanical simulator to study the hot workability and microstructure evolution over a wide range of temperatures and strain rates. Strain hardening was observed at lower strain rates and flow softening at higher strain rates. Microscopy revealed presence of deformation bands at a temperature of 1323 K and a strain rate of 10 s(-1); whereas at temperatures above 1423 K, fine recrystallized grains were observed at all strain rates. Dynamic recrystallization (DRX) initiates at 1373 K. At 1473 K, sudden increase in the recrystallized grain size (DDRX) is noticed, indicating grain growth. Processing maps have been developed to delineate the regions of stable and unstable metal flow during high temperature deformation. Using Ziegler's continuum principles and processing maps, regions of stable metal flow was observed in the temperature range of 1323e1500 K and strain rate range of 0.1-1 s(-1). Activation energy (Q(avg)) for the alloy was found to be 421 kJ mol(-1). As Zener Holloman parameter (Z) increases, D-DRX decreases indicating higher nucleation, restricted grain growth, when deformation resistance is high. Constitutive equation and relation between 'Z' and D-DRX for the alloy has been established. (C) 2016 Elsevier B.V. All rights reserved.