High temperature creep of precipitation-strengthened Ni3Al-based alloy

High temperature creep of a precipitation-strengthened Ni3Al-based alloy with chemical composition Ni-20.2Al-8.2Cr-2.44Fe (at %) was investigated. The creep characteristics, including the stress exponent and activation energy for creep, are reported for the temperature range 1050-1150 K and applied approximately constant stresses ranging from 150 to 350 MPa. The steady-state creep rate is found to depend on the applied stress and temperature. The power law stress exponent for steady state creep is determined to be 4.1 +/- 0.1 and the apparent activation energy for creep is calculated to be 316 +/- 9 kJ mol(-1). Analysis of the creep data in terms of an effective stress normalised with respect to the temperature dependence of the elastic modulus gives a true activation energy for creep of 301 +/- 9 kJ mol(-1) and a stress exponent of 3. This activation energy for creep corresponds to the activation energy for lattice diffusion of Ni in Ni3Al. The kinetics of the steady-state creep deformation within the studied temperature range is proposed to be governed by the diffusion controlled climb of dislocation loops along the alpha-Cr precipitates. During the tertiary creep stage an intensive formation of large slip bands is observed. The alloy fails by growth and coalescence of voids within the columnar grains and cavities at the grain boundaries. (C) 1999 Elsevier Science Ltd. All rights reserved.