The dependence of carbide morphology on grain boundary character in the highly twinned Alloy 690

The dependence of morphology of grain boundary carbides on grain boundary character in Alloy 690 (Ni–30Cr–10Fe, mass fraction, %) with high fraction of low Σ coincidence site lattice (CSL) grain boundaries was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Some of the surface grains were removed by means of deep etching. It was observed that carbides grow dendritically at grain boundaries. The carbide bars observed near incoherent twin boundaries and twin related Σ9 grain boundaries are actually secondary dendrites of the carbides on these boundaries. Higher order dendrites could be observed on random grain boundaries, however, no bar-like dendrites were observed near Σ27 grain boundaries and random grain boundaries. The morphology difference of carbides precipitated at grain boundaries with different characters is discussed based on the experimental results in this paper.