The niobium-osmium constitution diagram

The Nb-Os alloy system has been investigated over the entire composition range using metallographic X-ray diffraction and electron microprobe techniques. The epsilon-Os solid solution extends to about 27 at.% Nb at high temperatures but this is reduced to about 20 at.% Nb at lower temperatures. Three intermediate phases occur in this system and all of them possess topologically close-packed structures of the Frank-Kasper type. These phases are chi (α-Mn type), sigma(sigma-phase type) and β (A15 or βW-type). The α-Nb solid solution extends to about 19 at.% Os at high temperatures but only to about 13 at.% Os at lower temperatures. Two eutectic reactions occur at about 21200C and 21750C. The chi-phase is formed by a peritectic reaction at about 22700C, the sigma-phase melts congruently at about 22000C and the β-phase forms by a peritectoid reaction at about 19750C. All the intermediate phases possess highly ordered structures with the larger (Nb) atoms preferring the sites having high coordination while the smaller (Os) atoms prefer the sites of lowest coordination. (Auth.)