The influence of grain boundary character on the precipitation of intergranular M23 C6 carbides in alloy 690TT

The grain boundaries characterized by means of coincidence site lattice (CSL) model, and the related precipitation of intergranular carbides in alloy 690 thermally treated at 72 deg C for 10 hours were studied through transmission electron microscopy. The precipitation of intergranular chromium carbides was suppressed 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 hardly found on the coherent twin boundaries, however, needle-like ones were evolved on the incoherent twin boundaries. The chromium carbides were also suppressed on the nearly exact CSL boundaries such as Σ11 and Σ15, for which the Brandon criterion was fulfilled. The precipitation behaviors of chromium carbides are explained by the grain boundary energy, which could be a variable for certain grain boundary types. (author)