Isothermal fatigue of an aluminide-coated single-crystal superalloy: Part II. effects of brittle precracking
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I.
INTRODUCTION
THIS article, the second of two parts, reports on an extension of a study on the isothermal fatigue of an aluminide-coated nickel-base superalloy,m In the present work, the effect of brittle monotonic precracking of the coating prior to fatigue testing was investigated. Precracking was examined both as a means of further exploring and verifying the mechanisms of fatigue in the non-precracked condition and as a simulation of possible cracking of the coating due to the application of high strains at low temperatures, as may occur in an out-of-phase thermomechanical fatigue (TMF) cycle. Thermomechanical fatigue provides a closer simulation of the actual strain-temperature cycle that the coating and substrate experience in an engine environment than does isothermal fatigue. Unfortunately, testing of this type is more difficult to perform, and the results are harder to interpret. Coatings have shown the most detrimental effect on substrate TMF behavior when out-of-phase strain-temperature loops are used, i.e., when the tensile strain is at a maximum when the temperature is at a minimum. In this ease, brittle coating fracture at the lowest temperature leads to early fatigue crack initiation in the substrate and reduced life. As described by Strangman and Hopkins,t2] coating cracking can be assisted by compressive creep of the coating at high temperatures of the cycle. More recently, attempts have been made to relate isothermal and thermomechanical fatigue by using a "bithermal" cycle in which T.C. TOTEMEIER, formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge, is Metallurgical Engineer, Argonne National Laboratory, Idaho Falls, ID 83403. W.F. GALE, Assistant Professor, is with the Materials Engineering Program, Auburn University, Auburn, AL 36849. J.E. KING, formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge CB2 3QZ, United Kingdom, is Head of Materials, Rolls-Royce Aerospace Group, Derby DE24 8BJ, United Kingdom. Manuscript submitted June 24, 1994. METALLURGICALAND MATERIALSTRANSACTIONSA
strains were applied isothermally at two different temperatures in each cycle,t31 Again, a detrimental coating effect was observed when a tensile strain was applied at the lower temperature. In this study, isothermal fatigue tests at 600 ~ 800 ~ and 1000 ~ were performed on high-activity aluminidecoated nickel-base superalloy single crystals. These temperatures were chosen to be below, close to, and well above the coating ductile-brittle transition temperature (DBTT). The coatings were precracked by monotonic loading prior to fatigue testing. Three different types of precrack were examined: narrow precracks formed at room temperature by loading to just beyond the coating fracture strain, wide precracks formed at room temperature by loading to a substantial plastic strain, and narrow precracks formed at an elevated temperature. The precracks formed at temperature were intended to simulate possible coating cracking in an out-of-phase
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