Microstructure and room temperature fracture toughness of Nb ss/Nb 5Si 3 in situ composites

Abstract

The microstructure and room temperature fracture toughness of binary Nb ss/Nb 5Si 3 and ternary Nb ss/Nb 5Si 3 in situ composites alloyed with Mo are investigated at hypo- and hypereutectic compositions, where Nb ss denotes the niobium solid solution. The binary and ternary alloys consist of coarse primary Nb ss particles and fine eutectic at a hypoeutectic composition, while they are composed of fine eutectic at near-eutectic compositions. The room temperature 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 increases the fracture toughness up to 9–15 MPa m 1/2 depending on Si content. The fracture toughness of the arc-melted alloys with eutectic microstructure 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 observed for the DS alloys. Scanning electron micrographic observations confirm that the fracture toughness is increased by large scale bridging of thick primary Nb ss particles in the hypoeutectic composition for the binary alloys, and by complicated bridging of Nb ss 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 decohesion.