Improving Intergranular Corrosion Resistance in Inconel 625 via Grain Boundary Character Distribution Optimization

Abstract

The feasibility of applying the grain boundary character distribution (GBCD) optimization to Inconel 625 for improving the intergranular corrosion (IGC) resistance was studied. The GBCD was obtained and characterized by electron backscatter diffraction (EBSD) analysis, and its optimization was mainly attributed to annealing twins (Σ3) and twins related to boundaries formed during thermal-mechanical processing (TMP). Through TMP of 5% cold rolling and subsequent annealing at 1150 °C for 5 min, the proportion of low Σ coincidence site lattice (CSL) grain boundaries of the Inconel 625 can be enhanced to about 35.8% which mainly were of Σ3 ⁿ (n=1, 2, 3) type. There is an increase of 24.8% compared with the solution-treated sample, and simultaneously the large-size highly-twinned grain-cluster microstructure is formed. The grain-cluster is mainly composed of Σ3-Σ3-Σ9 or Σ3-Σ9-Σ27 triple junctions, which is mainly caused by boundary reactions during grain growth. Among them, the IGC resistance of Σ3 grain boundaries, Σ9 grain boundaries and random grain boundaries is sequentially weakened. With the increase of the low ΣCSL grain boundary fraction, the IGC resistance of Inconel 625 improves. The essential reason is the amount of Σ3 boundaries interrupting the random boundary network increases and the large grain-cluster arrests the penetration of IGC.

Keywords

nickel-based superalloys / grain boundary characteristic distribution (GBCD) / corrosion behavior / low Σ coincidence site lattice (low ΣCSL) / EBSD

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