Phase stability of XZn (X = Cu, Ag, and Au) B2 phase alloys
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While CuZn and AgZn have a disordered bcc structure at a high temperature and stoichiometric concentration, this is not the case for AuZn. Moreover, when the temperature is reduced, only CuZn alloys transform to B2 phase, while the AgZn alloy transforms instead to a hexagonal structure, known as the ζ phase. Above 265 ºC, the B2 phase in the AgZn system may be retained as a metastable phase on quenching of the disordered phase. The estimated ordered bcc phase transition temperatures for CuZn, AgZn, and AuZn are 763, 518, and 1056 K respectively. Surprisingly, compared to CuZn and AuZn, AgZn has a lower stability temperature. In the present work, the first-principles method has been used to study the formation of the bcc phases of the aforementioned XZn (X = Cu, Ag, and Au) systems and to determine the physical reasons behind the temperature anomaly and metastability of the ordered B2 phase of the AgZn alloy. It is shown that the stability/instability of the B2 structure of XZn systems can be explained in terms of the strength of the bond between X and Zn atoms. The charge density studies and pair potential modeling of XZn alloys show that the Ag-Zn bond is significantly weaker than the Cu-Zn and Au-Zn bonds. The lattice parameters, bulk modules, elastic constants, Debye temperature, and heat of formation for the bcc phases of the three XZn alloys are calculated and compared with available experimental values in order to interpret and validate the study findings.