Study on evolution of grain boundary composition in nickel based alloy 690

The solute and impurity atoms segregation at grain boundaries before carbide precipitation in Alloy 690 were examined by atom probe tomography. The evolution of grain boundary chemistry after carbide precipitation in Alloy 690 was examined by X-Ray energy dispersive spectrum adapted at high resolution transmission electron microscopy. C, B and Si atoms segregation, C and Cr co-segregation, and Cr enrichment at grain boundaries were observed before carbide precipitation at the grain boundaries. The chromium depletion zone formed at the grain boundaries after carbide precipitation. The chromium concentration at grain boundaries decreased with the aging time prolonging. The chromium concentration at grain boundaries reaches the minimum value after aging at 715°C for 15 h. And then, the chromium concentration at grain boundaries increased with the aging time prolonging. After the carbides were fully covering the grain boundaries, the enrichment of chromium at grain boundaries can be observed. The chromium depletion zone, especially the chromium content gradient in the chromium depletion zone was the main factor that influenced the corrosion resistance of the grain boundary in alloys with high chromium content.