Topologically close-packed phase precipitation in a nickel-base superalloy during thermal exposure

K465 superalloy was thermally exposed, respectively at 850, 950, 1000, 1050 and 1100 °C. During thermal exposure, topologically close-packed phases, such as σ and μ, are precipitated in interdendritic regions where rapid diffusion paths are provided and the TCP forming elements W, Mo, Cr, Co are concentrated. The exposure temperature is shown to have a great impact on the formation and growth of TCP phases. σ phase is readily formed at the lower annealing temperature (below 950 °C), having a basket weave or multi-layer structure whereas becomes instable at the higher exposure temperature above 950 °C. The disappearance of σ phase at higher temperature mainly results from the formation of M23C6 and M6C carbides because they can combine with the larger amounts of the σ forming element, Cr. The precipitation of μ phase is mainly depended on M6C carbide, and its morphology exhibits coarse needles or platelets. μ phase is stable above 950 °C, and the amount of μ phase reaches the peak value at 1000 °C, but is decreased above 1000 °C.