The effect of grain boundary misorientation on the intergranular M23C6 carbide precipitation in thermally treated Alloy 690

Abstract The precipitation characteristics of chromium carbides on various types of grain boundaries in Alloy 690 thermally treated at 720 °C for 10 h were studied through transmission electron microscopy. Precipitation of the intergranular chromium carbides, identified as Cr-rich M 23 C 6 , was retarded on the low angle grain boundaries, compared to that on the random high angle grain boundaries on which coarse and discrete ones were found. They were rarely found on the coherent twin boundaries, however, needle-like ones were evolved on the incoherent twin and twin related Σ 9 boundaries. Precipitation of the chromium carbides was also suppressed on the nearly exact coincidence site lattice boundaries such as Σ 11 and Σ 15, for which the Brandon criterion was fulfilled. The results of the intergranular M 23 C 6 carbide precipitation were explained in terms of the influence of the grain boundary energy.