Influence of thermomechanical processing parameters on microstructural evolution of a gamma-prime strengthened cobalt based superalloy during high temperature deformation

Abstract

Recent reports on γ′-L12 strengthened cobalt base superalloys showed the potential of this class of alloys to replace nickel-base superalloys for high-temperature applications. The studies on thermo-mechanical processing behavior of these new class of alloys are limited. Thus, the present study investigates the thermo-mechanical processing behavior of Co-30Ni-10Al-2Nb-4Ti-12Cr (all in at. %, referred to as Co10Al2Nb) γˈ strengthened cobalt base superalloy at different conditions of temperature and strain rates. The high temperature uniaxial compression tests were carried out using Gleeble 3800® in the temperature range of 1298–1448 K and the strain rate range of 0.001 to 10 s-1. The flow curves exhibited three distinct flow behavior characteristics: work hardening, softening, and steady state. Microstructures of deformed samples revealed the presence of fine strain-free dynamic recrystallization (DRX) grains in lower strain rate regime (0.001 to 0.1 s-1) and flow localization features like shear bands, cracks along the grain boundary in high strain rate regime (especially at 10 s-1) for all the temperatures. The strain rate sensitivity (m) map along with instability ( ) map revealed two distinct domains, one with m ~0.15 to 0.3 (0.001 to 1 s-1), and other domain with m LT0.13 ( GT1 s-1) for the temperature range studied. DRX was observed to be the main restoration process at m ~ 0.15 to 0.3, whereas the flow localization was dominant at m LT 0.13. The microstructure at different strain levels revealed that discontinuous dynamic recrystallization (DDRX) was the prevalent deformation mechanism for the present alloy. The strain field distribution obtained from the FEM simulation for different deformation conditions correlated with DRX microstructure evolution was also presented.