Phase transformation of alloy 617B during 10000 h aging: An element redistribution-related process

The phase transformation characteristics of alloy 617B are investigated in the temperature range of 600-800 °C for up to 10000 h using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and precise chemical analysis. The γ′ phase is stable during 600-720 °C aging but degenerates quickly at 800 °C. The M23C6 carbide shows a plate-like shape and has a cube-on-cube orientation relationship with the γ matrix. The chemical composition and lattice parameter of phases are determined by precise chemical analysis. The phase evolution is an element diffusion-related process, and the element redistribution relationship is established here. In the precipitation and coarsening process, the γ′ phase absorbs the elements Al and Ti and rejects Cr and Mo elements into the matrix. M23C6 carbide is confirmed by chemical analysis to be rich in Cr and Mo. The rejected Cr and Mo elements from the γ′ phase can transfer into M23C6 carbide during the coarsening of M23C6 carbide. In addition, the Mo- and Cr-rich μ phase is detected, and the precipitation of the μ phase competes with M23C6 carbide for Mo and Cr. Hence, the interaction between phases should be taken into consideration for better control of microstructure stability in the alloy 617B.