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

The hot deformation behavior of as-cast Ni3Al alloy has been characterized on the basis of its flow stress variation obtained by isothermal constant t rue strain rate compression testing in the temperature range 1100-1250 degr eesC and strain rate range 0.001-10 s(-1) The mechanisms of hot working hav e been evaluated using four generations of materials modeling techniques, w hich included shape of stress-strain curves, kinetic analysis, processing m aps and dynamical systems approach. The material exhibited a steady-state f low behavior at slower strain rates but flow softening associated sometimes with broad oscillations, was observed at higher strain rates. The flow str ess data did not obey the kinetic rate equation over the entire regime of t esting but a good fit has been obtained in the intermediate range of temper atures (1150-1200 degreesC). 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 unde rgoes dynamic recovery in the presence of harder gamma colonies The process ing maps revealed four different domains out of which three are interpreted to represent cracking processes. The fourth domain, which has a peak effic iency of about 44%, occurred at 1250 degreesC/0.001 s(-1). Microstructural observations revealed that this domain represents dynamic recrystallization (DRX) of gamma phase and is desirable for hot working the material. The ma terial exhibits flow instabilities when deformed in the intermediate temper ature regime at strain rates higher than 1 s(-1) and these are manifested a s shear localization. (C) 2000 Elsevier Science Ltd. All rights reserved.