Environmental effects on the creep behavior of a nickel-base superalloy

Environmental effects on the 760 °C creep behavior of a nickel-base superalloy are isolated by testing at varying stress levels, in laboratory air and vacuum, specimens of different gage diameters and grain sizes, and in a few cases, different grain boundary microstructures. For all specimens receiving a standard heat treatment that results in grain boundaries free of discrete carbides, the minimum creep rate is greater in air than in vacuum for a given specimen and grain size. In contrast, for specimens aged to precipitate carbides along the grain boundaries, the creep rate is lower in air than in vacuum. The minimum creep rate and the sensitivity of this rate to applied stress also are found to be functions of grain size, specimen size, and grain boundary microstructure to extents dependent on the test environments. Rationalizations of these environmental creep behaviors are suggested in terms of the apparently competing phenomena of the relative weakening of the alloy grain boundaries in the air environment, and the alloy creep strengthening in air due to the external surface oxide scale. Formerly Graduate Research Assistant, Henry Krumb School of Mines, Columbia University