Abstract The effects of heat treatments and long term exposures of C-containing modifications of CMSX-4 were examined. Some of the alloys examined also contained additions of boron or nitrogen. Significant changes to as-cast carbide morphologies were observed. Long-term exposure caused significant MC carbide decomposition and formation of Cr-rich secondary carbides on or near to decomposing carbides. Increased topologically close-packed phase formation occurred in some cases, while, in other alloys no topologically close-packed phases were observed. All three modifications experienced more γ′ coarsening than baseline CMSX-4.