Microstructures and tensile properties of a grain-size gradient nickel-based superalloy

This work systematically investigated the microstructure characteristics and tensile mechanical properties of a grain-size gradient nickel-based superalloy. The plastically deformation mechanism and the failure behavior of the gradient superalloy at different temperatures (room temperature, 650 °C, 750 °C) were discussed. Results show that the microstructure of nickel-based gradient superalloy is mainly composed of γ matrix phases, γ′ phases and dispersed carbides. The microstructure of the gradient superalloy exhibits equiaxed grains with a few annealing twinning structures. The average grain size changes uniformly from 94.3 µm of the coarse-grain side to 4.9 µm of the fine-grain side. The room temperature tensile strength of the grain-size gradient superalloy is about 1635 MPa, the yield strength is about 1160 MPa and the elongation is around 21%. When testing at 650 °C, the tensile and yield strength of the specimens declined to 1414 MPa and 1034 MPa, respectively, but the elongation raises to 36%. The strength of the specimens continues to decrease at temperature of 750 °C, whereas the elongation decreases back to around 21%, exhibiting intermediate temperature brittle phenomenon (ITB). Analysis concludes that the micro-twinning mechanism dominates at 750 °C in the grain-size gradient nickel-based superalloy. Micro-twins formed by intense interaction of dislocations with γ′ phases lead to the reduction of ductility at 750 °C.