Structure of ordered and disorderedα-brass

Alloys of copper and zinc (brass) have been widely used since Neolithic times due to the discovery that unlike regular copper this alloy can be worked ``cold'' around a 3:1 copper-to-zinc ratio. While it is now known that the as-grown system is a disordered fcc solid solution, no 3:1 ordered phase has yet been directly observed even though the negative mixing enthalpy of the disordered alloy suggests ordering tendencies. Moreover, neutron scattering experiments have been deduced that this disordered alloy contains peculiar chains of Zn atoms. We have expressed the first-principles calculated total energy of general Cu-Zn fcc-lattice configurations using a mixed-space cluster expansion. Application of Monte Carlo--simulated annealing to this generalized Ising-like Hamiltonian produces the predicted low-temperature ground state as well as finite-temperature phase diagram and short-range order. We find (i) that at low temperature the disordered fcc alloy will order into the ${\mathrm{DO}}_{23}$ structure, (ii) the high-temperature short-range order in close agreement with experiment, and (iii) chains of Zn atoms in the [001] direction, as seen experimentally. Furthermore, our model allows a detailed study of the influence and importance of strain on the phase stability.