INFLUENCE OF INTERFACIAL REACTIONS ON THE FIBER-MATRIX INTERFACIAL SHEAR-STRENGTH IN SAPPHIRE FIBER-REINFORCED NIAL(YB) COMPOSITES

The influence of microstructure of the fiber-matrix interface on the i nterfacial shear strength, measured using a fiber-pushout technique, h as been examined in a sapphire-fiber-reinforced NiAl(Yb) matrix compos ite under the following conditions: (1) as-fabricated powder metallurg y (PM) composites, (2) PM composites after solid-state heat treatment (HT), and (3) PM composites after directional solidification (DS). The fiber-pushout stress-displacement behavior consisted of an initial '' pseudoelastic'' region, wherein the stress increased linearly with dis placement, followed by an ''inelastic'' region, where the slope of the stress-displacement plot decreased until a maximum stress was reached , and the subsequent gradual stress decreased to a ''frictional'' stre ss. Energy-dispersive spectroscopy (EDS) and X-ray analyses showed tha t the interfacial region in the PM NiAl(Yb) composites was comprised o f Yb2O3, O-rich NiAl and some spinel oxide (Yb3Al5O12), whereas the in terfacial region in the HT and DS composites was comprised mainly of Y b3Al5O12. A reaction mechanism has been proposed to explain the presen ce of interfacial species observed in the sapphire-NiAl(Yb) composite. The extent of inter facial chemical reactions and severity of fiber s urface degradation increased progressively in this order: PM < HT < DS . Chemical interactions between the fiber and the NiAl(Yb) matrix resu lted in chemical bonding and higher interfacial shear strength compare d to sapphire-NiAl composites without Yb. Unlike the sapphire-NiAl sys tem, the frictional shear stress in the sapphire-NiAl(Yb) composites w as strongly dependent on the processing conditions.