On the primary creep of CMSX-4 superalloy single crystals
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I. INTRODUCTION
THE introduction of single-crystal turbine blades has been one of the most significant developments in the materials technology of advanced turbines in the last 20 years and has significantly raised operating temperatures and efficiency.[1,2] While the benefits of removing grain boundaries and the elements added to strengthen them have been considerable, the use of single-crystal turbine blades continues to present new challenges to the materials community. For example, single-crystal nickel alloys are inherently anisotropic; the materials engineer can exploit this situation, provided the foundryman can control the orientation of the blade. It is fortunate that the preferred growth direction for the single crystals ([001]) coincides with the one of the best orientations for creep resistance. However, a further complication is that the creep life varies rapidly, not only with misorientation from the [001] pole, but also with the direction of the deviation.[3] This article is concerned with the primary-creep deformation of CMSX-4 superalloy single crystals and, in particular, its dependence on crystallographic orientation in the vicinity of the [001] axis. At low temperatures (up to 850 8C in CMSX-4), the creep behavior of this kind of material has distinct characteristics: rapid initial deformation (primary creep), followed by a decrease in the creep rate, which may then remain constant for a period of time (secondary or steady-state creep), and, finally, rupture. It has been found[4–6] that the primary-creep strain is a sensitive function of the specimen orientation and can be as high as 15 pct; where this occurs, it constitutes a considerable proportion of the lifetime strain of the specimen. Indeed, failure can C.M.F. RAE and M.A. RIST, Senior Research Associates, D.C. COX, Research Student, and R.C. REED, Assistant Director of Research, are with the Department of Materials Science and Metallurgy, Cambridge University, Cambridge, United Kingdom CB2 3QZ. N. MATAN, formerly a research student at the Department of Materials Science and Metallurgy, Cambridge University, is with the Institute of Science, Walailak University, NakornSi-Thammarat 80160, Thailand. Manuscript submitted October 19, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
occur within a few hours where there is no transition to secondary creep. When secondary creep is established, a high primary-creep strain is associated with a high secondary creep rate[5] and again leads to a rapid failure. In contrast, creep curves observed at higher temperatures, typically 950 8C, are relatively insensitive to orientation and exhibit only small amounts of primary-creep strain, with the tertiarycreep strain rate increasing monotonically with creep strain. The work reported here is part of a wider study[5,7] aimed at elucidating the deformation mechanisms operating in singlecrystal superalloys under various conditions of stress, temperature, and orientation. A particular aim has been to clarify the relative contributions of ^112&{111} and ^011&{1
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