Microstructure and high temperature strength at 1773 K of Nbss/Nb5Si3 composites alloyed with molybdenum

Nbss/Nb5Si3 composites alloyed with and without Mo, where Nbss denotes Nb solid solution, are prepared by various processes such as arc-melting (AM), isothermal forging (IF), directional solidification (DS) and powder metallurgy (PM), followed by heat treatment at 1973 and 2123 K. The microstructures and mechanical properties at 1773 K of the composites are investigated under various processing and heat treatment conditions. In binary alloys the microstructures of AM composites are characterized by a dispersed structure of Nbss in Nb5Si3 matrix, while those of PM composites are composed of an aggregate structure. In ternary alloys microstructures consist of a maze-like structure for AM composites, an equiaxed structure for IF composites and lamella structure aligned to the growth direction for DS composites. The yield strength at 1773 K of AM binary alloys is higher than that of PM binary alloys. The increase in yield strength at 1773 K of the ternary composites by Mo addition is attributable to solid solution hardening in Nbss. The highest yield strength at 1773 K is obtained with an aligned lamella microstructure produced by DS, which may be associated with fine lamella spacing. It is suggested that the yield strength at 1773 K of α-Nb5Si3 is superior to that of β-Nb5Si3 alloyed with Mo.