Effects of grain refinement on internal oxidation of Alloy 617

The effects of grain size refinement on the internal oxidation behavior of Alloy 617 were investigated by exposure at 950 °C up to 2000 h in He. As-received (AR) and grain-refined (GR) Alloy 617 were oxidized forming Cr2O3 as an external oxide scale and Al2O3 as an internal oxide. The average depth of the internal oxide formed along the grain boundary in the GR sample was approximately half of that in the AR. The internal oxide in the GR sample was densely stretched by grain refinement because internal oxidation took place mainly along grain boundaries. These differences in internal oxidation behavior affected crack propagation under tensile stress. The longer internal oxides in AR induced rapid crack propagation by a relatively higher stress concentration resulting in larger cracks along the grain boundary, while a number of short cracks located near the surface were observed in GR. Thus, crack propagation was restricted in GR by the even distribution of stress and reduction of stress concentration because of the densely stretched internal oxide.