Stress response behaviour of and microstructural evolution in Nimonic PE16 LCF tested at temperatures up to 923 K

Abstract

The cyclic stress strain behaviour of nickel base superalloy, Nimonic PE16 has been studied at a constant strain rate of 4 × 10-3s-1 at different strain amplitudes as a function of γ′ particle size for temperatures up to 650°C. Fatigue softening after initial hardening was observed in all samples containing γ′ particles up to an average size of 30 nm tested at room temperature while a saturation in the cyclic stress response behaviour after initial hardening was observed in samples containing γ′ particles above an average size of 21 nm tested at 650°C. Detailed TEM investigation was carried out to understand the observed fatigue behaviour. Fatigue softening was noticed to be associated with shearing of γ′ particles and localisation of slip while saturation behaviour was associated with Orowan looping and non-localisation of slip. Particle size at which the mechanism of deformation changes from shearing to looping decreases with increase of test temperature. This behaviour is explained on the basis of the effect of test temperature on the line tension of dislocations. In the case of shearable precipitates, width of deformation band decreases with increase of temperature reaching that of single slip lines at 923 K. Change of deformation microstructure with testing temperature, γ′ free bands at room temperature, fine γ′' particles within the bands at 673 K and sheared γ′ at 923 K, are explained on the basis of combined effect of mechanical shearing and fatigue enhanced diffusion within bands. Fatigue life decreases with increase of test temperature.