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

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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.