Effect of solidification direction on microstructure and mechanical property of single crystal superalloy CMSX-4

The microstructure and mechanical property of single crystal superalloy CMSX-4 fabricated by downward di-rection solidification (DWS) and upward direction solidification (UWS) were systematically investigated by optical microscope (OM), scanning electron microscope (SEM), electron probe microanalyzer (EPMA), nano-indentation instrument and finite element simulation method. Meanwhile, the effect of remelting heat treat-ment on microstructure evolution and mechanical property of the superalloy was also explored. Compared to the traditional UWS process, the DWS process was superior in inhibiting the formation of freckle defects and alle-viating the segregation of difficult-diffusion elements. Moreover, the expected application of thin shell casting in the DWS process was also more beneficial to the control of freckles and the refinement of microstructure. The change of solidification direction would alter the solute distribution in the mushy zone. Different convection patterns were then formed under the action of gravity, thereby affecting the microstructure of the casting. In the following process of remelting heat treatment, the generation and elimination of incipient melting structures occurred in both UWS and DWS samples. However, more incipient melting structures in the DWS samples made longer elimination time required. After remelting heat treatment, the mechanical property of the DWS sample was comparable with that of the UWS one, since the element segregation in the final microstructure was reduced, and the size, morphology and uniformity of gamma' phase were also adjusted.