Pressure casting of a zirconia-toughened alumina fiber-reinforced NiAl composite

A pressure-cast NiAl composite reinforced with polycrystalline alumina (PRD-166) fibers containing 0.2 weight fraction of partially stabilized zirconia was examined by optical and transmission electron microscopy (TEM). Fibers in the preform used for casting were forced into contact, and fiber bonding occurred in a number of instances. Fiber volume fraction was increased from an initial value of 0.4 to 0.6 as a consequence of the applied pressure. An explanation is offered for the interaction of applied pressure, wetting angle, and the rigidity of the fiber preform on the final volume fraction of the fibers in the composite. At the fiber/matrix interface, the alumina was free of zirconia particles. It is proposed that alumina grain growth forced the zirconia into the molten NiAl, where it dissolved. As solidification took place, the concentration of zirconium in the molten NiAl increased to a point where zirconium reacted with alumina to form zirconia again.