Microstructure and Fatigue Behavior of Three Nickel Base Eutectic Composites.

摘要

The high cycle fatigue behavior of three advanced unidirectionally solidified nickel-base eutectic alloys has been studied. AG-170, strengthened by Alpha-Mo fibers, and brittle carbide reinforced Nitac 14B and Cotac 744 were tested in vacuum under axial tension-tension, sinusoidal loading at 20 Hz. Fatigue properties of the as D.S. and heat treated conditions were examined at both 25 deg C and 825 deg C. In addition, the effects of reduced frequency on the high temperature fatigue life of aged Nitac 14B have been determined. Fatigue resistance of the as D.S. material decreased between 25 deg C and 825 deg C. This has been related to a variation in the mode of cracking. Room temperature failures were surface initiated and propagation proceeded along crystallographic planes, wheras Stage II crack propagation was observed at 825 deg C. Nitac 14B was shown to be superior at both temperatures owing to its high matrix strength and large Gamma' volume fraction. The elevated temperature fatigue life of aged Nitac 14B was substantially reduced at low test frequencies in vacuum. The decrease in frequency caused a shift from surface initiated failure to an almost featureless fracture surface where the initiation, fatigue, and overload zones were not easily identified. Reduced specimen life was postulated to be the result of enhanced creep.