Microstructural Instability of an Experimental Nickel-Based Single-Crystal Superalloy

Microstructural instability with the precipitation of topologically close-packed (TCP) phases of an experimental nickel-based single-crystal superalloy has been investigated. A significant amount of sigma phases are distinguished in the interdendritic region of the as-cast samples after thermal exposure at 900 degrees C for 1000 h. The sigma phases are preferentially precipitated at the periphery of coarse gamma/gamma ' eutectic, and their morphological evolution from needles to granules is observed. Microstructural analysis suggests that the local segregation of Cr and Ti at the periphery of coarse gamma/gamma ' eutectic accounts for the formation of sigma phases in the as-cast samples. After heat treatment with low solution temperature and short holding time, the dendritic segregation of alloying elements (i.e., W, Re, Ti and Ta) and the volume fraction of gamma ' phase in the interdendritic region are similar to that of the as-cast samples. However, no TCP phases are present in the interdendritic region of the heat-treated samples after thermal exposure, which is primarily ascribed to the elimination of local segregation of Cr and Ti near the coarse gamma/gamma ' eutectic. Moreover, small quantities of mu phases are precipitated in the secondary dendrite arm near the interdendritic region after thermal exposure, due to the increased volume fraction of gamma ' phase and the concomitant enrichment of W and Re in the gamma matrix.