Creep-behavior modeling of the single-crystal superalloy CMSX-4
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I. INTRODUCTION
SINGLE-CRYSTAL Ni-based superalloys are commonly employed for use as high-temperature creep- and oxidation-resistant blade alloys in the early stages of modern gas turbine aeroengines. Their excellent high-temperature creep resistance is a result of a combination of solid-solution strengthening, the absence of deleterious grain boundaries, and a high volume fraction (approximately 70 pct) of regular cuboidal g 8-phase precipitates. These act as high-temperature barriers to dislocation motion. In service, turbine blades are subjected to severe conditions of creep and thermo-mechanical fatigue (TMF). Superimposed on the TMF cycles are additional high-frequency load fluctuations known as high-cycle fatigue. To extend blade life and improve oxidation resistance, the blades are coated in service and in some cases have an additional insulating layer of a ceramic thermal barrier coating on their outer surface, to allow operation at higher gas-stream temperatures. The blade life can be limited for one of several reasons. In particular, creep may limit the blade life in one of two ways: first, the blades may creep and deform to such an extent that they are no longer within the tolerances required for service; or second, they may reach their predicted failure life. For design and lifing purposes, it is, therefore, imperative that accurate constitutive equations and lifing techniques are formulated for these materials over a sufficient range of stresses and temperatures. Due to the high temperature of operation of blade alloys, the fatigue behavior must be analyzed in conjunction with creep damage and deformation. D.W. MacLACHLAN, Research Associate, and D.M. KNOWLES, Lecturer, are with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB2 3QZ, United Kingdom. Manuscript submitted June 8, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
To ensure consistency and to facilitate full component analysis, the equations must be formulated such that they interpolate accurately over wide ranges of test data and extrapolate sensibly outside the fitted range. One important aspect of creep in single crystals, which is commonly overlooked during modeling, is the region in which transient sigmoidal creep occurs. Transient sigmoidal creep consists of an initial increase in strain rate followed by hardening and the attainment of a new minimum creep rate. This is particularly significant at relatively low temperatures (1023 to 1123 K) and can give rise to strains of a few percent in relatively short periods of time. While many of the outer regions of turbine blades experience temperatures up to 1323 K, in cooled blades, the major load is borne by the central webbing, where temperatures are more commonly in the lower regime of 1023 K. For accurate prediction of the shaken-down blade stresses, it is essential that the initial creep transients be effectively modeled by creep equations which allow for this rapid plastic deformation and the resultant stress redistribution. A theory has been de
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