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Nickel-based superalloys are extensively used in aero-engin due to their combined high strength, toughness, corrosion and creep resistance at elevated temperatures. Strain rate, temperature, and strain are important factors influencing the microstructural evolution of nickel-based superalloys. In this work, a typical nickel-based superalloy, GH4738 alloy, is selected to study the dynamic compressive deformation behavior of this material. Split Hopkinson pressure bar (SHPB) compression test was performed on GH4738 superalloy at strain rates of 1000~7500 s-1 with temperature ranges from RT to 500 °C. The yield strength of GH4738 superalloy decreases with increasing temperature and increases with increasing strain rate; however, at the temperature of 500 °C and the strain rate of 7500 s-1, it drops sharply. In order to understand the microscopic deformation behavior of GH4738 superalloy, parallel specimens were prepared with SHPB at frozen strains of -0.02, -0.05, -0.10, -0.20 and -0.25 at a strain rate of 3000 s-1 for the cases of RT, 400 °C and 500 °C, respectively. Neutron diffraction technique was employed to characterize the evolution of lattice constants and elastic lattice strains. We define the horizontal lattice mismatch as the lattice misfit at the γ/γ' interface that is perpendicular to the SHPB compressed direction, and the vertical lattice mismatch as the lattice misfit parallel to the SHPB compression direction. As the frozen strain increases, the horizontal lattice mismatch exhibits positive values and an increasing trend, while the vertical lattice mismatch changes from positive to negative values; the elastic lattice strain of the γ' phase consistently increases, while that of the γ phase remains almost unchanged. The lattice strains of the {111} and {220} planes are negative at 400 °C and 500 °C but positive at RT; the lattice strain of the {200} plane alternates between positive and negative values from RT to 500 °C, while that of the {311} plane remains negative throughout this temperature range. However, at a frozen strain of -0.25, the lattice strain of the {311} plane exhibits a significant rebound at both RT and 500 °C, indicating generation of significant intergranular stresses in the material. Dislocation configurations are characterized using transmission electron microscopy (TEM) to interpret the underlying mechanism. At RT, plastic deformation is dominated by γ-γ' co-deformation, with defects manifesting as parallel slip bands and stacking faults. Lattice misfit is effectively relaxed due to the formation of dislocation networks at γ/γ' interfaces, resulting in minimal residual lattice strain at RT. At 500 °C, dislocation density increases substantially because both γ and γ' phases readily undergo plastic deformation under thermal activation. Under such conditions, dislocation networks fail to compensate for lattice distortions induced by defect multiplication, resulting in high lattice misfit and residual lattice strain. At 400 °C, the alternating dominance of dislocation climb and slip induces fluctuations in both lattice misfit and residual lattice strain. Due to slow dislocation density accumulation, {hkl} lattice strains continuously increase. This contrasts with the RT and 500 °C scenarios, where rising dislocation density partially recovers elastic lattice distortion and even induces {hkl} lattice strain rebound at high strains (ε = -0.20~-0.25).
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Keywords:
- nickel-based superalloy /
- split Hopkinson pressure bar test /
- neutron diffraction /
- residual stress
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