Microstructure and room temperature fracture toughness of Nb-ss/Nb5Si3 in situ composites

The microstructure and room temperature fracture toughness of binary Nb-ss/ Nb5Si3 and ternary Nb-ss/Nb5Si3 in situ composites alloyed with Mo are inve stigated at hypo- and hypereutectic compositions, where Nb-ss denotes the n iobium solid solution. The binary and ternary alloys consist of coarse prim ary Nb,, particles and fine eutectic at a hypoeutectic composition, while t hey are composed of fine eutectic at near-eutectic compositions. The room t emperature fracture toughness of binary arc-melted alloys is 12 MPa m(1/2) at the hypoeutectic composition and decreases rapidly to about 4.5 MPa m(1/ 2) at near-eutectic compositions. In the arc-melted alloys, 5%Mo addition i ncreases the fracture toughness up to 9-15 MPa m(1/2) depending on Si conte nt. The fracture toughness of the arc-melted alloys with eutectic microstru cture is higher than that of the directionally solidified (DS) alloys with fine microstructure mostly aligned perpendicular to the direction of crack propagation. No significant influence of Mo addition on the toughness is ob served for the DS alloys. Scanning electron micrographic observations confi rm that the fracture toughness is increased by large scale bridging of thic k primary Nb-ss particles in the hypoeutectic composition for the binary al loys, and by complicated bridging of Nb,, with maze-like structure at near- eutectic compositions for the ternary alloys. The low fracture toughness of the DS alloys is discussed on the basis of unfavorable interface decohesio n. (C) 2001 Elsevier Science Ltd. All rights reserved.