Microstructure and properties of Nb-Mo-Zr based refractory alloys

Microstructure and mechanical properties of five Nb alloys containing 8 to 17 at.% Mo, 8 or 35 at.% Zr, up to 7 at.% Ti, up to 2 at.% Al and up to 2 at.% Cr are reported. These alloys have been developed to replace heavy, expensive and difficult to process commercial Nb alloys, such as C-3009, for use at temperatures up to 1600 degrees C. The density of the alloys is in the range from 7.6 to 8.6 g/cm(3). The alloys have a BCC matrix phase, and they also contain small amounts of secondary phases, which are rich in Zr and have BCC, FCC, hexagonal or monoclinic crystal structures depending on the concentration of other alloying elements, including oxygen and nitrogen. In the temperature range from 25 degrees C to 1600 degrees C, the alloys with a smaller amount of Zr are ductile and have higher specific strength than C-3009. The alloy containing 35 at.% Zr is stronger, but less ductile, than other alloys at temperatures up to 600 degrees C; however, it loses the strength rapidly at higher temperatures and becomes softer than other alloys at T > 1000 degrees C. The possible strengthening mechanisms responsible for the observed temperature dependence of the yield stress of the alloys are also discussed.