EVOLUTION OF CARBIDE MORPHOLOGY PRECIPITATED AT GRAIN BOUNDARIES IN Ni-BASED ALLOY 690

The effects of grain boundary characters on the morphology evolution of the inter-granular carbide in Ni-based alloy 690 with high proportional low Sigma coincidence site lattice (CSL) boundaries aged at 715 degrees C for 2 200 h were investigated by SEM. The results show that the sizes of intergranular carbides decrease with Sigma value decreasing, and only fine carbides at coherent Sigma 3 are almost not changed after aging for 200 h. Plate like carbides precipitated near both incoherent Sigma 3 and Sigma 9 boundaries, and the carbide plates grow bigger with the aging time prolonging, in which the former precipitated near both sides of incoherent Sigma 3 boundary and the latter precipitated near only one side of Sigma 9 boundary. The morphology of carbide precipitated at Sigma 27 boundary is similar to that precipitated at the. general high angle grain boundary, and no plate like carbides are observed near these boundaries.