Laves phases for high temperatures - structure, stability and constitution

The effect of alloying with Al on phase stability and crystal structure of the Laves phases NbCo2 and TaFe2 was studied as part of extended work on Laves phases of transition metals. The ternary phases can be described as Nb(Co1-xAlx)2 and Ta(Fe1-xAlx)2 in view of the respective ternary phase diagrams. Melting temperatures and lattice constants have been measured. The phase equilibria between Ta(Fe1-xAlx)2 and bcc Fe-Al, Fe3Al with D03 structure and FeAl with B2 structure have been studied. The cubic C15 structure of NbCo2 transforms to the hexagonal C14 structure in the temperature range 1050 to 1350 C and is unstable for Al contents above 5 at.% Al. The stability of Laves phases with C15 structure is favoured by a high valence electron concentration. (orig.)