Creep damage of Ni-base superalloys in helium gas at very high temperatures

The present paper investigates creep damage of two different types of advanced Ni-base superalloys, i.e., Hastelloy XR and Inconel MA 754 at very high temperatures of up to 1,273 K in impure helium gas, simulated environment of Japanese high-temperature gas-cooled reactors. Creep strength was much higher in MA 754 than in XR at 1,273 K. MA 754 fabricated via mechanical alloying, however, showed drastic decrease in creep strength in longer life region. MA 754 also showed anisotropic creep rupture behavior with longer lives in the longitudinal or extruding direction than in the long-transversal direction. This anisotropy is attributed to grains elongated in the extruding direction. Statistical evaluation of the creep damage was made on ruptured specimens by using the A-parameter. Although this parameter is usually treated as a deterministic one, its values showed wide variations and followed two-parameter Weibull distributions in both alloys, reflecting spatial randomness of creep damage. The mean of the parameter Am remained almost constant in XR alloy. It showed different behavior in the two types of MA 754 specimens; i.e., it was increased in the longitudinal type of specimens whereas decreased in the long-transversal type ones with increasing applied stress level. This tendency was elucidated to arise from the anisotropy of elongated grains in MA 754. It is also found that maximum percent damaged grains reached as high as 20% in as-received MA 754 material