Microstructure evolution of a Ni-Cr-W superalloy during long-term aging at high temperatures

The effects of long-term exposure to high temperature (800 and 900 °C for up to 20,000 h) on the microstructure evolution of a Ni-Cr-W superalloy (Alloy 230) were investigated. After long-term aging at 800 °C, extensive precipitation of the secondary Cr-rich M23C6 carbides was observed. In addition, the internal dissociation of W-rich primary η-M6C carbides was observed, such as, W-rich M6C → α-W, Cr-rich M23C6, and W-depleted Ni-base matrix. After long-term aging at 900 °C, precipitation of Cr- and Ni-rich phase and α-W phase was observed in some areas at the expense of the secondary Cr-rich M23C6 carbides. Meanwhile, the dissociation of the primary M6C carbides was less significant at 900 °C. Long-term aging at both temperatures resulted in a decrease in tensile elongation while an increase in strength was minimal after aging at 900 °C. The degradation of tensile property depended on the evolution of Cr-rich M23C6 carbides, Cr- and Ni-rich precipitates, and α-W phases but not on the primary M6C carbides.