Strengthening effect of Cr2O3 thermally grown on alloy 617 foils at high temperature

Alloy 617 has been selected for the intermediate heat exchanger (IHX) of the very high temperature gas-cooled reactor (VHTR) for the economic production of electricity and hydrogen. In this work, the strengthening effects of Cr2O3 thermally grown on alloy 617 foils at 800 and 900 °C were investigated. A micro-tensile test system was used for in situ measurement of tensile strain in the foils and superficial thermally-grown Cr2O3. Each foil was heated until the thermally-grown Cr2O3 reached a predetermined thickness; then, a load was applied to measure the tensile response. As the Cr2O3 layer thickened on the surface of the metal foils, the strengths and stiffnesses of the foils were enhanced. We assumed that there was no interaction between the substrate and the superficial chromia, and the strength of Cr2O3 itself was measured. At 800 °C, the Cr2O3 was brittle and the strength was governed by crack initiation. At 900 °C, the Cr2O3 was much more ductile, and strain hardening was observed for even the smallest thickness. The strength was maintained even after crack initiation was observed on the surface.