Characterization of the Hot Deformation Behavior and Microstructure Evolution of a New gamma-gamma ' Strengthened Cobalt-Based Superalloy

Recently, cobalt-based gamma-gamma' microstructured superalloys have attracted attention. However, studies on their processing behavior [i.e., processing maps (the variation of strain rate sensitivity (m) with temperature)] are limited. Thus, the high-temperature flow behavior of a gamma-gamma' Co-30Ni-10Al-5Mo-2Ta-2Ti-5Cr (at. pct) superalloy was investigated using isothermal compression tests between 1348 and 1498 K at strain rates from 0.001 to 10 s(-1). The m contour map was generated using the experimental flow stress values, which were used to locate the optimum hot workability and desired microstructural processing range. A strong dependence of m on the deformation parameters (temperature, strain rate, and strain) was observed. A maximum m value of around 0.3 at 1460 K to 1498 K and strain rates of 0.01 to 0.5 s(-1) was found. The deformed samples show a fully recrystallized microstructure at high m. Unstable domains showed the formation of cavities at the grain boundary triple points and cracks along the grain boundaries at high strain rates (1 to 10 s(-1)), corresponding to m < 0.10. A constitutive model was developed using an Arrhenius hyperbolic sine function, yielding an apparent activation energy of 540 +/- 30 kJ mol(-1) for hot deformation. This study indicates reasonable formability under certain conditions below the solvus, thus opening possibilities for further thermomechanical treatment. (C) The Minerals, Metals & Materials Society and ASM International 2018