Microstructure tailoring for property improvements by grain boundary engineering

Grain boundary engineering (GBE) was employed to improve materials properties such as corrosion resistance and strength by optimizing the grain boundary character distribution. Two high-temperature alloys, designated Incoloy 800H and Inconel 617 were selected in this study due to their potential applications for the Generation IV nuclear power systems. The GBE treatments on the alloys 800H and 617 were accomplished by a series of thermomechanical processing. The effect of the GBE treatments on the corrosion resistance and mechanical properties of the materials were evaluated using supercritical water exposure tests, cyclic oxidation tests, impact tests, and tensile tests. The microstructures of the tested samples were analyzed by means of optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction, X-ray photoelectron spectroscopy, and grazing incidence X-ray diffraction. The results indicate that the GBE treatments greatly mitigated the oxide exfoliation of the alloy 800H and reduced the oxidation rate of the alloy 617. The GBE treatment also greatly enhanced the strength of alloy 800H at room temperature (e.g. impact tests) and high-temperatures (e.g. tensile tests after neutron irradiation), but did not significantly impair the material’s ductility.