Phase relation and superlattice formation in the AgZn-AgMg quasibinary alloys

Abstract The phase relation in AgZnAgMg quasibinary alloys (Ag 50 Zn 50− x Mg x ternary alloys) was studied by high-temperature X-ray diffractometry and differential thermal analysis. The ζ-phase (complex hexagonal structure) which exists at low temperature in a terminal binary alloy AgZn disappears when Zn atoms of a few at. % are replaced by Mg atoms. On the other hand, the β'-phase (CsCl-type ordered structure) which is a stable phase of another terminal alloy AgMg extends to all the remaining composition. The transition temperature from the β'-phase to a β-phase (b.c.c. disordered structure) rises up rapidly with the increase of Mg content and the β'-phase becomes stable up to melting point in a composition range of more than about 25 at, % Mg. Further, a Heusler-type ordered phase appears at low temperature around a stoichiometric composition of Ag 2 ZnMg. The critical temperature between the Heusler- and β'-phases has a maximum (523 K) near the stoichiometric composition. The superlattice formation of Ag 2 ZnMg was discussed on a basis of the pseudopotential method. For this purpose, an expression of the structure-dependent energy was given for the ternary alloy Ag 2 ZnMg. The numerical result explained satisfactorily the existence of the two types of ordered phase.