Characterization of mechanisms of hot deformation of as-cast nickel aluminide alloy

Abstract

The hot deformation behavior of as-cast $Ni_3Al$ alloy has been characterized on the basis of its flow stress variation obtained by isothermal constant true strain rate compression testing in the temperature range 1100-1250°C and strain rate range $0.001-10 \hspace{2mm}s^-1$. The mechanisms of hot working have been evaluated using four generations of materials modeling techniques, which included shape of stress-strain curves, kinetic analysis, processing maps and dynamical systems approach. The material exhibited a steady-state flow behavior at slower strain rates but flow softening associated sometimes with broad oscillations, was observed at higher strain rates. The flow stress data did not obey the kinetic rate equation over the entire regime of testing but a good fit has been obtained in the intermediate range of temperatures (1150-1200°C). In this range, a stress exponent value of 6.5 and an apparent activation energy of about 750 kJ/mol have been evaluated. Microstructural investigations have shown that the matrix $\gamma'$ phase undergoes dynamic recovery in the presence of harder $\gamma$ colonies The processing maps revealed four different domains out of which three are interpreted to represent cracking processes. The fourth domain, which has a peak efficiency of about 44%, occurred at $1250^oC/0.001 \hspace{2mm} s^-1$. Microstructural observations revealed that this domain represents dynamic recrystallization (DRX) of $\gamma$ phase and is desirable for hot working the material. The material exhibits flow instabilities when deformed in the intermediate temperature regime at strain rates higher than $1 \hspace{2mm}s^-1$ and these are manifested as shear localization