Effect of crystallographic orientation on sub-critical grain boundary cracking in a nickel based superalloy

Kameshwaran Swaminathan, Purdue University

Abstract

Sub-critical intergranular crack growth is an important creep fracture mechanism in polycrystalline superalloys. The root cause of this fracture is not well understood. The role of grain orientation on the formation of sub-critical grain boundary cracks during creep of a conventionally cast nickel-based superalloy has been analyzed in this study. The crystallographic orientations of grain pairs adjacent to grain boundaries normal to the tensile axis were measured using Electron Back-Scattered Diffraction (EBSD). The difference in the Schmid factor for the {111} <112> slip system between the grains was compared to the occurrence of sub-critical grain boundary cracking. In addition, the difference in the amount of potential primary creep strain between the grains (for unconstrained state) was analyzed. The uncracked grain boundaries were found to have small Schmid factor mismatch, as well as small potential primary creep strain mismatch, than those of the cracked grain boundaries. Larger mismatches in Schmid factor and potential primary creep strain indicate possible development of large enough stress concentration at the grain boundary sufficient to initiate sub-critical cracks. The experimental results suggest correlations between crystallographic orientation of the grains and sub-critical cracking at the grain boundary with a critical value for Schmid factor mismatch and potential primary creep strain mismatch in the creep tested alloy. Assumptions and limitations are considered in proposing future experiments for further refinement of the analysis.

Degree

M.S.M.S.E.

Advisors

Trumble, Purdue University.

Subject Area

Materials science

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