Microstructure and room temperature fracture toughness of Nbss/Nb5Si3 in situ composites

The microstructure and room temperature fracture toughness of binary Nbss/Nb5Si3 and ternary Nbss/Nb5Si3 in situ composites alloyed with Mo are investigated at hypo- and hypereutectic compositions, where Nbss denotes the niobium solid solution. The binary and ternary alloys consist of coarse primary Nbss 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 m1/2 at the hypoeutectic composition and decreases rapidly to about 4.5 MPa m1/2 at near-eutectic compositions. In the arc-melted alloys, 5%Mo addition increases the fracture toughness up to 9–15 MPa m1/2 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 Nbss particles in the hypoeutectic composition for the binary alloys, and by complicated bridging of Nbss 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.