INFLUENCE OF FABRICATION TECHNIQUE ON THE FIBER PUSHOUT BEHAVIOR IN ASAPPHIRE-REINFORCED NIAL MATRIX COMPOSITE

Directional solidification (DS) of ''powder-cloth'' (PC) processed sap phire-NiAl composites was carried out to examine the influence of fabr ication technique on the fiber-matrix interfacial shear strength, meas ured using a fiber-pushout technique. The DS process replaced the fine , equiaxed NiAl grain structure of the PC composites with an oriented grain structure comprised of large columnar NiAl grains aligned parall el to the fiber axis, with fibers either completely engulfed within th e NiAl grains or anchored at one to three grain boundaries. The load-d isplacement behavior during the pushout test exhibited an initial ''ps eudoelastic'' response, followed by an ''inelastic'' response, and fin ally a ''frictional'' sliding response. The fiber-matrix interfacial s hear strength and the fracture behavior during fiber pushout were inve stigated using an interrupted pushout test and fractography, as functi ons of specimen thickness (240 to 730 mu m) and fabrication technique. The composites fabricated using the PC and the DS techniques had diff erent matrix and interface structures and appreciably different interf acial shear strengths. In the DS composites, where the fiber-matrix in terfaces were identical for all the fibers, the interfacial debond she ar stresses were larger for the fibers embedded completely within the NiAl grains and smaller for the fibers anchored at a few grain boundar ies. The matrix grain boundaries coincident on sapphire fibers were ob served to be the preferred sites for crack formation and propagation. While the frictional sliding stress appeared to be independent of the fabrication technique, the interfacial debond shear stresses were larg er for the DS composites compared to the PC composites. The study high lights the potential of the DS technique to grow single-crystal NiAl m atrix composites reinforced with sapphire fibers, with fiber-matrix in terfacial shear strength appreciably greater than that attainable by t he current solid-state fabrication techniques.