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 1700 °C of these alloys are described. We observe that a new eutectic of L → Nbss (Nb solid solution) + βNb5Si3 (W5Si3-type, D8m) 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.