Creep-Fatigue Resistance of a Nickel-Aluminide
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CREEP-FATIGUE RESISTANCE OF A NICKEL-ALUMINIDE J. K.TIEN and R. S. BELLOWS Center for Strategic Materials, Henry Krumb School of Mines, Columbia University, New York, NY 10027. ABSTRACT A modified fatigue test matrix that provided both creep (high mean stress) and fatigue (alternating stress) damage was used to evaluate the creep-fatigue behavior of D.S. Ni3 AI(B,Hf). The three generic regimes of below, at, and above peak yield strength temperature were investigated. Itwas found that the creep-fatigue resistance of this intermetallic remains high through the three temperature regimes, allowing it to be further considered as a monolith or as the matrix of high-temperature fiber reinforced composite. INTRODUCTION This investigation determined the creep-fatigue behavior of Ni3 AI(B,Hf), an ordered L12 intermetallic, and evaluated this material as a potential structural monolith or as a potential composite matrix in terms of creep-fatigue resistance. The modified fatigue test matrix at each test temperature maintained the maximum stress constant but employed a different minimum stress for each test [1]. The procedure delineated an inverse relationship between the cyclic stress range and the mean stress and defined a spectrum of conditions between an R=O fatigue test and a static creep test. This testing protocol affords maximum creep-fatigue information and permits a fairly rapid evaluation of the candidate material. Directionally solidified (D.S.) specimens were used in the investigation to reduce the number of transverse grain boundaries and thus minimize associated grain boundary effects and problems. Testing of the D.S. specimens was performed at 7600C, near the peak yield strength temperature, and at 4500C 0 0 0 and 982 C. At 450 C and 982 C, the yield strength of Ni3 AI(B,Hf) has fallen appreciably with respect to 7600 C. Some of these results have been previously discussed [2 - 41. EXPERIMENTAL PROCEDURE
The nickel-aluminide used, IC-72, was developed at the Oak Ridge National Laboratory and has the nominal composition (in wt%) 11.7 Al, 0.51 B, 3.5 Hf,
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