Microstructural changes during superplastic deformation of powder-consolidated nickel-base superalloy IN–100

Abstract

Changes in the microstructure occurring during superplastic deformation of the powder-consolidated nickel-base superalloy IN–100 at 1311 K are reported. The initial microstructure was shown to contain a range of recovered and recrystallized grains together with some warm-worked unrecrystallized grains of high dislocation density. Heating to the test temperature and holding produced no changes in microstructure other than dissolution of M23C6 precipitates. Prolonged annealing at the test temperature without deformation led principally both to a reduction in the volume fraction of warm-worked areas and removal of some dislocations from recrystallized grains. Deformation in the high strainrate regime (region III) produced grain refinement associated with a marked increase in dislocation density and some cavitation. Some randomization of the distribution of MC carbides on the prior particle boundaries occurred which implied that substantial grain-boundary sliding had taken place. In the regime of optimal superplasticity (region II) structural modification occurred gradually. After deformation at the lowest strain rates (region I) a reduction in dislocation density was again observed with most grains dislocation free. Extensive dislocation activity was noted at all types of phase boundaries during deformation in regions I and II. From the observations it is concluded that deformation in regions I and II takes place by grain-boundary sliding with accommodation by both slip and diffusion with the diffusional component increasing as the strain rate decreases.