Fatigue of Ni-Ai-Mo aligned eutectics at elevated temperatures
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THE
Ni-A1-Mo ('YI3"-a) eutectic system has been the subject of considerable interest because o f the ductile nature of the o~-molybdenum reinforcing fibers. In contrast, most investigations of mechanical properties of aligned eutectics have been concerned with systems that contain brittle carbides or intermetallic compounds as reinforcing phases. The phase relationships and other physical metallurgical characteristics of the .y/y'-e~ system have been described in detail. 1,2,3 In addition, room temperature fatigue properties have been reportedA 4 The purpose of this investigation was to characterize the tensile, creep, and fatigue properties of two Ni-A1-Mo alloys at high temperatures. Recently, frequency effects on high temperature fatigue properties have been observed in several aligned eutectic systems. 5-8 The results of these investigations suggest that a creep-fatigue interaction is responsible for fatigue life reductions with decreasing test frequency. Therefore, frequency was chosen as a principal test variable for one of the alloys. EXPERIMENTAL PROCEDURE Specimen Preparation Master ingots of AG15* and AG34 with respective * " A G " is a General Electric Company designation.
compositions of 65.5 wt pct Ni, 8.1 wt pct AI, 26.4 wt pct Mo and 62.5 wt pct Ni, 6.3 wt pct A1 and 31.2 wt pct Mo were purchased from the General Electric Company. These ingots, 1.9 cm in diam and 16.5 cm in length, were directionally solidified (D.S.) in highpurity A1203 crucibles in a Bridgeman-type induction furnace, under argon. Solidification rates of 1.9 c m / h for AG15 and 0.76 c m / h for AG34 were chosen to produce approximately equal inter-fiber spacings of J. M. TARTAGLIA, formerly Graduate Assistant in the Department of Materials Engineering, Rensselaer Polytechnic Institute, is now Senior Research Associate, Climax Molybdenum Company of Michigan, P.O. Box 1568, Ann Arbor, MI 48106, a subsidiary of AMAX, Inc. N. S. STOLOFF is Professor of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY 12181. Manuscript submitted December 8, 1980.
about 0.5 to 1.5/zm. A longitudinal flat was polished along each ingot to metallographically determine the degree of alignment. Depending on the quality of the fibrous structure, four or eight specimen blanks (0.76 cm in diam and 3.8 cm in length) were electrodischarge machined from the aligned sections o f each ingot. The blanks were ground with a water cooled abrasive wheel and mechanically polished through 600 grit SiC paper on a high speed lathe. The final specimen configuration is shown in Fig. 1. After electropolishing the sample with an applied potential of 20 V in a solution o f 20 ml HF, 30 ml HNO3, 50 ml lactic acid (85 pct conc.), and 20 ml H 2 5 0 4 , cooled with an ice water bath, the specimen diameter was measured with a microscope. All specimens were tested in the as-D.S. condition. Mechanical Testing Tensile testing was performed on a screw-driven machine operated at a constant crosshead velocity of 0.0216 mm s-1. A laboratory air atmosphere was utilized duri
