1ST-PRINCIPLES ALL-ELECTRON THEORY OF ATOMIC SHORT-RANGE ORDERING IN METALLIC ALLOYS - D022-VERSUS L12-LIKE CORRELATIONS

We use a ''first-principles'' concentration-wave approach based on a f inite-temperature, electronic density-functional, mean-field, grand po tential of the random alloy to investigate the high-temperature atomic short-range order (ASRO) in Ni75V25 and Pd75V25 solid solutions. Expe rimentally, these similar alloys both develop D0(22)-type long-range o rder at low temperatures but different ASRO at high temperatures. Our calculations describe the measured ASRO well. We compare these results with those found for a hypothetical Co75Ti25 solid solution. Since th is alloy orders directly from the melt into the L1(2) phase, it should exhibit strong L1(2)-like ASRO, as we find in our calculations. We an alyze the features in the calculated diffuse intensities in terms of v arious factors in each alloy's electronic structure. Because we have a ssumed that the atoms are fixed to the Bravais lattice, we discuss two additional examples, Al75Ti25 and Ni50Pt50, to show the limitations o f neglecting atomic displacements. Notably, the Onsager cavity fields have been incorporated into the theory to conserve the diffuse scatter ing intensity over the Brillouin zone and to provide a better descript ion of the long-ranged, electrostatic screening effects.