The Effect of γ' Particle Size on the Deformation Mechanism in an Advanced Polycrystalline Nickel-Base Superalloy

Abstract

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.

The Effect of Lattice Misfit on Deformation Mechanisms at High Temperature

“…Previous work on RR1000 has shown that as the precipitate size decreases, onset of load partitioning occurs at increased levels of plastic strain [8]. The diffraction data for Alloy 720Li shows a similar trend.…”

“…To date, most work has attempted to relate γ' size distributions to mechanical properties [9] but without considering misfit. It has also been shown that deformation temperature [10] and precipitate size [8] have very significant impact on the level of load partitioning between γ and γ' during deformation in both Alloy 720Li or RR1000. In this paper, the focus is on the effect of the γ/γ' lattice misfit.…”

Back-Stress and Its Evolution during Primary Creep in Particle Strengthened Nickel Superalloys

“…Neutron and synchrotron X-ray diffraction experiments coupled with in-situ loading are extremely helpful in understanding phase-level and grain-level lattice strain and stress in Ni-base superalloys [11][12][13][14][15]. Preuss et al coupled neutron diffraction with tensile loading to study the role of γ' particles during high temperature deformation.…”

The Effect of Primary γ' Distribution on Grain Growth Behavior of GH720Li Alloy

“…The chemical composition of this alloy is related to that of GH720 (UDIMET 720) from which it is derived, but differs from it with respect to the chromium, carbon and boron levels. Till now, a lot of studies [1][2][3][4][5][6] have been done for GH720Li alloy including the solutioning and precipitating of γ prime phase, the ingot to billet conversion process control, the heat treatment schedule optimizing, the mechanical behavior under service conditions, etc. However, the microstructure of GH720Li consists of primary, secondary and tertiary γ′ due to its special heat treatment schedule.…”