Effect of Oxygen on the Degradation of Alloy 908 in Gaseous Environments Available to Purchase

Alloy 908 (UNS N09908), a Ni-based, low-thermal expansion superalloy, has been developed as a structural material for high-field, Nb₃Sn, superconducting magnets. Alloy 908 is characterized by high strength and toughness at cryogenic temperatures while, at the same time, exhibiting a thermal expansion coefficient close to that of Nb₃Sn. While ideally suited for service at cryogenic temperatures, Alloy 908 is susceptible to embrittlement if exposed to an oxygen-containing environment in the temperature range from 350°C to 750°C. In this study oxygen-induced embrittlement (OIE) in Alloy 908 has been investigated. Constant ΔK fatigue crack growth testing was used as a tool to explore the effect of oxygen on embrittling behavior. Crack growth rate, da/dN, was evaluated as a function of loading frequency, oxygen partial pressure, ΔK, and load ratio over the temperature range from 350°C to 750°C. The effect of oxygen manifested itself as a change in crack growth rate and crack path. Transgranular (TG) and intergranular (IG) crack paths were observed depending on loading frequency and oxygen partial pressure. A transition to IG cracking resulted in an increase in da/dN by a factor of up to 100 compared to growth rates when TG cracking was dominant. For a loading frequency of 0.1 Hz, the da/dN enhancement saturated at higher oxygen partial pressure. Saturation was not observed for 2 Hz loading. The apparent activation energy for IG OIE was estimated to be 162 kJ/mole, in good agreement with that for grain boundary oxygen diffusion in Ni. A grain boundary decohesion mechanism, in combination with grain boundary diffusion and repeated damage zone embrittlement followed by crack growth, is proposed as an explanation of the OIE of Alloy 908.