Microstructure evolution and its influence on deformation mechanisms during tensile creep of DD417G alloy

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 degrees C/115 MPa were studied using scanning electron microscopy ( SEM), transmission electron microscopy (TEM), scanning TEM, with energy diffraction spectrum and Xray 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 gamma' particles also precipitate from the matrix during the creep test.