Oxidation and mechanical behaviors of solid-solution hardening nickel-base superalloys in high temperature helium environments

Abstract

Solid-solution hardening nickel-base superalloys Alloy 617 and Haynes 230 were examined for VHTR intermediate heat exchanger applications. This study focused on high temperature oxidation and aging degradation and how they affected low temperature embrittlement and creep behaviors at high temperatures. Oxidation tests and aging heat treatment were carried out at $900\degC$ and $1000\degC$ in impure helium environments that contained $H_2$, $H_2O$, CO, $CO_2$, and $CH_4$ in order to simulate VHTR coolant chemistry. The oxidation kinetics of the alloys followed the parabolic law of oxidation in all cases. In impure helium environments that had very low oxygen activity, a $Cr_2O_3$ layer, $TiO_2$ ridges on the grain boundaries, and isolated $MnCr_2O_4$ grains on top of the $Cr_2O_3$ layer were formed for Alloy 617, while a $Cr_2O_3$ inner layer and a NiO outer layer were formed in air with high oxygen activity due to the different thermodynamics. For Haynes 230, a $Cr_2O_3$ inner layer and a protective $MnCr_2O_4$ outer layer formed, which increased the oxidation resistance. The oxidation kinetics of Haynes 230 was changed by Mn depletion at the initial stage of oxidation. A dense $MnCr_2O_4$ oxide layer grew on top of a $Cr_2O_3$ layer in impure helium environments for Haynes 230, while quasi-columnar $MnCr_2O_4$ grains grew in air, which resulted in a better oxidation resistance for the impure helium environments. Cellular $Cr_2O_3$ oxides for Alloy 617 and $MnCr_2O_4$ whiskers/platelets for Haynes 230 were formed in impure helium at $900\degC$ due to the more rapid dissociation of water vapor and $CO_2$ molecules compared to the oxygen molecules. The formation of irregular oxides are believed to have been due to a VLS mechanism based on the appearance of droplets on the tips of the whiskers, the formation of which was more preferential on the vertex of $MnCr_2O_4$ crystals. The effect of a small amount of $CH_4$ and $H_2$ on the oxidation kinetics of the all...