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In this paper, the variation of gamma prime precipitates along the creep specimen from the end to the fracture surface was studied, on which a necking phenomenon was observed. The morphologies and chemical compositions of gamma prime precipitates of crept DD417G alloy under the condition of 1000°C/115 MPa were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning TEM, with energy diffraction spectrum and X-ray diffraction. The results indicate that with a negative gamma/gamma prime lattice misfit the superalloy gamma prime rafted structure formed perpendicular to the tensile loading axis, but in the necked zone, the gamma prime rafts are thickened significantly and no longer perpendicular to the tensile loading axis, but tend to parallel to the tensile loading axis. Based on element diffusion and lattice rotation theories, this phenomenon was discussed. From the observation and analysis of dislocation configurations, it is evident that the tertiary creep deformation is mainly controlled by dislocation shearing mechanism. And, some γ′ particles also precipitate from the matrix during the creep test. [ABSTRACT FROM AUTHOR]

In this paper, a nickel based superalloy with good corrosion resistance is fabricated by directional solidification, and its microstructure and tensile properties were investigated. Microstructure observations reveal that besides the γ′ precipitates and γ matrix, some MC, Ni5Hf and M3B2 particles precipitate along the grain boundary. Tensile tests exhibit that the alloy has obvious anomalous yield and intermediate temperature brittleness (ITB) behaviour. The tensile properties depend on temperature significantly. Below 650°C, the yield strength decreases slightly but rises rapidly between 650 and 750°C, and after then, it decreases gradually with temperature increasing further. Examination by TEM exhibits that sharing of the γ′ by dislocation is almost the main deformation mechanism at low temperature, but the γ′ bypass dominates the deformation at high temperature and the transition temperature should be ∼800°C. In addition, the carbides and eutectic structure contribute partly to the ITB behaviours. [ABSTRACT FROM AUTHOR]