Effect of oxygen on the degradation of alloy 908 in gaseous environments

TitleEffect of oxygen on the degradation of alloy 908 in gaseous environments
Publication TypeJournal Article
Year of Publication2008
AuthorsMin, SK, Ballinger, R
JournalCorrosion
Volume64
Issue9
Pagination733 - 743
Date Published2008/09//
Abstract

Alloy 908 (UNS N09908), a Ni-based, low-thermal expansion superalloy, has been developed as a structural material for high-field, Nb(3)Sn, 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 degrees C to 750 degrees C. In this study oxygen-induced embrittlement (OIE) in Alloy 908 has been investigated. Constant Delta K fatigue crack growth testing was used as a toot 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, Delta K, and load ratio over the temperature range from 350 degrees C to 750 degrees 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.