MICROSTRUCTURE OF AL2O3 FIBER-REINFORCED SUPERALLOY (INCONEL-718) COMPOSITES

Composites of INCONEL 718 alloy reinforced with either single-crystal (SAPHIKON) or polycrystalline (Du Pont's FP) Al2O3 fiber were fabricat ed by pressure casting. Optical and transmission electron microscopy w ere used to characterize the microstructure of the composites and to d etermine the nature of the fiber/matrix reaction The widely dispersed fibers in the SAPHIKON-fiber-reinforced composite had no influence on the solidification of the matrix. Six phases, gamma-Ni3Al, gamma ''-Ni 3Nb, delta-Ni3Nb, TiC, NbC, and Laves, were present in the matrix of t he composite. The last three phases were formed during solidification and the others precipitated during subsequent cooling. The high densit y of fibers in the FP-fiber-reinforced composite led to a more uniform microstructure within the matrix. Only three phases, gamma ''-Ni3Nb, NbC, and Laves, were identified. Diffusion of Ti into the Al2O3 fiber resulted in preferential grain growth in the FP fiber in areas adjacen t to the fiber/matrix interface. The fiber/matrix bond strength in she ar in the SAPHIKON-fiber-reinforced composite was in excess of 150 MPa .