Environmental effects on the creep behavior of a nickel-base superalloy
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has long been recognized that external environments can significantly influence, either beneficially or adversely, the ambient temperature mechanical properties of metals and alloysJ ~ More recently, similar environmental sensitivity of mechanical properties has been demonstrated to occur at high temperatures where time dependent, thermally activated deformation, or creep, is expected. 5-~8 Such environmental creep effects have been shown to be significant not only in pure metals and single phase alloys, 14 but also in the more microstructurally complex, highly creep and corrosion resistant alloys such as the nickel-base superalloys. 5-~7 The past investigations focused mainly on the minimum creep rate as well as stress rupture properties. In most cases, comparisons have been made between the creep properties in oxidizing environments such as air, and in relatively inert environments such as helium, argon or vacuum. The results of these past studies still do not seem to have settled the issue of the exact roles played by the environment in creep deformation. Air strengthening, manifested by a lower creep rate and a longer rupture life for specimens tested in air as compared to a relatively inert environment, has been widely reported. 5,7,9,12,13A6-18On the other hand, results indicating the existence of the opposite phenomenon, i.e. air weakening, are also quite common2 ,7,1~ Two of the more in depth investigations on environmental creep studied the air vs v a c u u m ( 1 0 -3 to 10 -4 Pa) creep behavior of as-cast and wrought versions of a high volume fraction 3" strengthened nickel-base superalloy. .5-8 Results from tests on specimens with constant * Udimet-700, a trademark alloy of Special Metals Corporation.
gage size, but varying grain size, suggest that a test environment of air may strengthen the alloy in creep compared to vacuum for specimens with a small J. M. DAVIDSON, formerly Graduate Research Assistant, Henry Krumb School of Mines, Columbia University, is now Metallurgist, Inco Research & Development Center, Inc., Sterling Forest, Suffern, NY 10901. J. K. TIEN is Professor, Henry Krumb School of Mines, Columbia University, NY, NY 10027. Manuscript submitted May 13, 1980. METALLURGICAL TRANSACTIONS A
number of grains per cross section2 ,7 However, a transition to an air weakening mode may occur as the number of grains per cross section increases to greater than about 15.5,7 Furthermore, in tests on specimens with a single grain size for which air strengthening was observed, it was found that environment had little effect on the temperature sensitivity, or activation energy, of creep rate. On the other hand, much greater stress sensitivities of creep rate were observed for creep in air. 8 The purpose of this investigation is to explore in further detail microstructural and macrostructural parameters that may affect environmental creep behavior of complex, high temperature alloys. Accordingly, an experimental matrix was designed to test in both air and vacuum, specimens of a single alloy with specim
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