Microstructure and mechanical properties of Nb/Nb5Si3 in situ composites in Nb-Mo-Si and Nb-W-Si systems

This paper presents the microstructure and mechanical properties of Nb/Nb5Si3 in situ composites prepared from Nb-Mo-Si and Nb-W-Si systems by casting and heat-treatment. The solidification behavior and the phase relationships at 1700degreesC of these alloys are described. We observe that a new eutectic of L --> Nb-ss (Nb solid solution) + betaNb(5)Si(3) (W5Si3-type, D8(m)) exists over a wide range of Mo or W content in the Nb-Mo-Si and Nb-W-Si ternary systems, which is absent in the Nb-Ti-Si and Nb-Hf-Si systems. A very fine two-phase structure of Nb and Nb-silicide can be obtained through this eutectic reaction. Mo and W produce strong solid- solution-hardening effect on the Nb phase, and this effect persists to very high temperature. In contrast, the solid-solution-strengthening of Si operates effectively only at relatively low temperatures. (C) 2004 Elsevier B.V. All rights reserved.