The effect of γ′ particle size on the deformation mechanism in an advanced polycrystalline nickel-base superalloy

The deformation mechanisms under tensile loading in a 48 vol.% γ′ polycrystalline nickel-base superalloy (RR1000) have been studied in-situ using neutron diffraction at 20°C, 500°C and 750°C. In addition, post-mortem microstructural studies were carried out on deformed samples using an ultra high resolution field emission gun scanning electron microscope (FEGSEM). Deformation studies were carried out on three different model microstructures with a uni-modal γ′ mean particle size of 80 nm, 120 nm and 250 nm. The elastic response of γ and γ′ during in-situ loading was measured by neutron diffraction and load transfer from γ to γ′ was observed during plastic deformation at high temperature in samples with a coarse γ′ mean particle size. It was found that as the testing temperature increases, load transfer can be observed first only for the coarse γ′ microstructure and at 750°C for the medium and coarse γ′ microstructure showing that there is a combined particle size/temperature dependency for γ to γ′ load transfer. No significant load transfer was detectable in samples with a fine mean γ′ particle size at any temperature. In some cases a region of plastic deformation without load transfer was succeeded by γ to γ′ load transfer when a certain level of plastic straining had been exceeded. FEGSEM studies of the samples plastically deformed at 500°C showed sheared particles only in the fine γ′ microstructure but not in samples with coarse γ′. The data recorded during the in-situ loading experiment demonstrate that such experiments are suitable for detecting changes of the deformation mode. But it is only in combination with post mortem electron microscopy studies that the load transfer observed can be related to a specific change of slip mode. So far, the experimental data suggest that fine γ′ is sheared during plastic deformation at room and high temperature up to 750°C whereas in coarse γ′ Orowan looping is the most likely deformation mechanism at high temperature although cutting by strongly coupled dislocation might also explain the observed load transfer.