M23C6 precipitation behavior and grain boundary serration in Ni-based Alloy 690

A study on the Cr-rich M23C6 precipitation and grain boundary serration in Ni-based Alloy 690 was conducted through various heat treatments. Fine granular carbides and discontinuous precipitates were evolved during isothermal treatments at 720 °C for 0.1-100 h, and their precipitation morphologies and behaviors were considerably varied depending on the grain boundary characteristic. Carbide precipitation was retarded on the low angle grain boundaries, compared to that on the random high angle grain boundaries. Needle-like carbides were precipitated on the incoherent twin boundaries. The discontinuous precipitates were initiated even by a heat treatment of 0.1 h, and covered the entire grain boundary region within a heat treatment of 10 h. Grain boundaries were serrated by a slow cooling process, and it was revealed that the serration occurred in a limited temperature range during controlled cooling. As the average grain size with more carbon content increased and/or the cooling rate decreased, the degree of serration increased with a precipitation of coarse planar intergranular M23C6 carbides. The grain boundaries had a convex shape into the incoherent grain. It is suggested that the grain boundary migration during serration is closely correlated with the precipitation of intergranular Cr carbides in this alloy.