Effect of trace impurities in helium on the creep behavior of Alloy 617 for very high temperature reactor applications

The effect of trace impurities, methane and oxygen, in helium on the creep behavior of Alloy 617, has been investigated. The creep rupture life at relatively low applied stresses was shortest in a helium environment containing 500 vppm oxygen (He + O2), while it was the longest in helium containing 675 vppm methane (He + CH4). However, the rupture strain was significantly lower in the He + CH4 environment compared to that in pure helium (He) and He + O2. The low rupture strain in the He + CH4 is caused by cleavage fracture. In the He + CH4 environment, the fracture mode was cleavage at lower applied stresses and ductile at higher applied stresses while in the He and He + O2, a ductile fracture was observed at all stress levels. The apparent activation energy for creep was determined in all three environments, and it appears to be independent of stress in the He, dependent in the He + CH4, while in the He + O2 environment the stress dependence could not be conclusively established.