The effect of scandium on the phase stability of Al sub 3 Nb and Al sub 3 Zr

A series of Al-25 (Nb,Sc), at. %, and Al-25 (Zr,Sc), at. %, alloy buttons were arc-cast in argon and homogenized for 2 h in vacuum at temperatures ranging from 1473 to 1573K. X-ray powder-diffraction indicated that almost all the Nb in Al-25 Nb (DO{sub 22}) had to be replaced by Sc in order to obtain the L1{sub 2} structure. In the case of Al{sub 3}Zr much less Sc was required -- the single phase L1{sub 2} composition is approximately Al-16Zr-8Sc. Recent calculations show that once the tetragonality is properly included in the phase stability of the Tri-Aluminide transition-metal binary alloys can be understood in terms of their electronic density of states. The dominating feature is a deep minimum in the density of states just below the major transition metal d-band peak. The exact position of the minimum changes with structure type (i.e., L1{sub 2}, DO{sub 22}, or DO{sub 23}, and with c/a). The alignment of the Fermi energy with the minimum appears to determine the equilibrium structure. The results of linearized-muffin-tin-orbital (LMTO) electronic structure calculations are compared to the rigid band model and checked against the experimentally determined phase boundaries. 5 refs., 5 figs., 1 tab