Strength degradation of sapphire fibers during pressure casting of a sapphire-reinforced Ni-base superalloy
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Strength Degradation of Sapphire Fibers during Pressure Casting of a Sapphire-Reinforced Ni-Base Superalloy R. ASTHANA, S.N. TEWARI and S.L. DRAPER
IT is well known that solid-state techniques[1–6] of fabricating high-temperature fiber-reinforced superalloys and intermetallics suffer from the problems of interface contamination from binder residues and oxides on metal powders. Liquid-phase fabrication using pressure casting of fiber-reinforced Fe- and Ni-base intermetallics[7,8] offers flexibility in designing matrix and interface structures. Improvement in wettability by alloying[9] or application of pressure[7,8] is required to manufacture structurally viable defect-free composite castings. Relatively little work has been reported in the literature on liquid-phase fabrication of ceramic fiberreinforced superalloys. In a recent study on pressure-cast alumina fiber-reinforced superalloy (INCONEL* 718) com-
(a)
*INCONEL is a trademark of INCO Alloys International, Inc., Huntington, WV.
posites, Nourbakhsh et al.[10] reported formation of a strong interfacial bond, with a bond strength in excess of 150 MPa, between single crystal sapphire fibers and the superalloy matrix. It was stated that the high ductility of the matrix, approximately 20 pct, together with a strong interfacial bond, would yield a strong composite material provided that the fibers are not degraded during composite fabrication. A detailed microscopic characterization of the composite was carried out. However, metallography of fiber-matrix interface may not reveal the extent of fiber degradation during fabrication. The purpose of this study was to prepare sapphire fiberreinforced superalloy matrix composites using pressure infiltration casting, extract the fibers from the composite, and measure the room-temperature strength to determine the extent of fiber damage due to pressure infiltration casting. The superalloy (Hastelloy) selected for the present study (47.5Ni, 21.5Cr, 17.8Fe, 8.3Mo, 1.7Co, 0.3 Mn, 0.4Si, 0.2Al, 0.1Cu, 0.4Nb, 0.06Ti, 0.08C, and 0.02 O, in wt pct.) is very similar to the INCONEL 718 superalloy used by Nourbakhsh et al.[10] Single crystal sapphire fibers (c-axis oriented, nominal diameter: 130 mm) from Saphikon Inc. (Milford, CT)[11] were used as the reinforcement. Initial tri-
R. ASTHANA, Assistant Professor, is with the Manufacturing Engineering Program, Technology Department, University of WisconsinStout, Menomonie, WI 54751. S.N. TEWARI, Professor, is with the Chemical Engineering Department, Cleveland State University, Cleveland, OH 44115. S.L. DRAPER, Materials Research Engineer, is with the Materials Division, NASA Lewis Research Center, Cleveland, OH 44135. Manuscript submitted September 10, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
(b) Fig. 1—Infiltration configurations used in pressure casting of sapphirehastealloy composites: (a) bottom-fill infiltration and (b) top-fill infiltration.
als to make sapphire-superalloy composites using vacuum infiltration in a countergravitational configuration were unsuccessfu
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