Uniaxial creep response of Alloy 800H in impure helium and in low oxygen potential environments for nuclear reactor applications

Studies were conducted on the creep behavior of Alloy 800H in impure helium and in a 1%CO–CO2 environment. At relatively low applied stresses and at low temperatures, the presence of methane in helium reduced the rupture strain significantly while increasing the rupture life relative to the behavior in pure helium. The degradation in rupture strain is due to the occurrence of cleavage fracture in the He + CH4 environment; this explanation is also supported by high activation energy (Q = 723 kJ/mol) for creep in He + CH4. At higher applied stresses and also at higher temperatures, creep-rupture behavior in He and He + CH4 was similar. Creep response in pure He and in CO–CO2 follows a dislocation climb-controlled power-law behavior whereas that in He + CH4 has a different behavior as indicated by the high stress exponent (n = 9.8). The activation energy for creep in pure He was 391 kJ/mol and in CO–CO2 was 398 kJ/mol, and appeared to be independent of stress in both environments. On the other hand, in He + CH4, the activation energy (Q = 723 kJ/mol) seems to be dependent on stress.