Effect of nature of grain boundaries on intergranular liquation during weld thermal cycling of nickel base alloy

Abstract

An Inconel 718 based alloy containing only trace amounts of C, S, and P was subjected to thermal cycling in a Gleeble 1500 system to induce microstructural features which are produced in the heat affected zone (HAZ) during welding. In the thermally cycled material many grain boundaries were observed to liquate; however, the liquation was heterogeneously distributed, that is, only some grain boundaries, and often only a few segments of the same grain boundary, liquated. Orientation imaging microscopy and electron backscatter diffraction analysis in a scanning electron microscope revealed that the liquated grain boundaries or segments of a grain boundary that liquated were always non-special geometry (or random) boundaries, whereas low reciprocal volume density Σ coincident site lattice and twin boundaries did not liquate. Therefore, it is concluded that the extent of B segregation, to which liquation of grain boundaries in the H AZ during welding has been attributed, is greater on random boundaries owing to their ability to accommodate a large number of B atoms in comparison with the low Σ boundaries leading to their liquation during Gleeble simulation.