Atomic arrangements in the gamma brasses

The structural details of the ternary gamma brass phase in the Cu-Al-Zn alloy system have been determined at three chemical compositions intermediate to the two binary phases Cu₉A1₄ (primitive cubic) and Cu₅Zn₈ (body-centred cubic). In addition, the preferred sites for nonstoichiometric aluminium atoms in aluminium rich Cu-Al gamma brass alloys has been found. At higher aluminium concentrations, a rhombohedral derivative structure has been studied. The ordering of the atoms and the location of the vacant lattice sites has been discovered. Rietveld refinement of high resolution neutron powder diffraction data, and to a lesser degree X-ray powder diffraction data, have been the major source of structural information. Both diffraction types were necessary for the ternary alloys since it has been shown to be impossible to extract a unique structure from just one type of data in the general case. This work has led to a new outlook on gamma brass structures. If only the ordering within the individual clusters is considered and not the Bravais lattice type, the number of primary ordering types can be reduced to four. An Algorithm is developed, in the form of a flow chart, for predicting the existence of a gamma brass phase in a new binary alloy system, its approximate structural characteristics and ordering. Thermal parameters derived from the refinement of solid solution structures are shown to be influenced by several factors. It is shown that data collected from alloys of different chemical composition can be used to extract semi-quantitative information about the substituting atoms directly from the thermal parameters. In addition, data collected at two widely spaced temperatures can give information about atom displacements due to the differing sizes of host and substituting atoms.