A lattice parameter method for the investigation of solid state precipitation

Over strictly limited composition ranges, the relationship between lattice parameter and solid solution composition can often be taken as linear even in multicomponent solid solution systems. The constants in the assumed linear equations relating the lattice parameter to the atomic percentage of each component may be calculated from lattice parameters measured for solution heat-treated alloys of known compositions. The subsequent changes in lattice parameters which occur during ageing of these alloys are shown to yield useful information about composition changes and the precipitate phases which occur during ageing. In particular, if the composition of the precipitate is known, then the linear equations may be used to calculate the composition of the remaining solid solution alloys. Even if both the composition of the precipitate phase and the remaining solid solution are unknown, changes in lattice parameter, combined with known lattice parameter versus composition functions, may be used to determine what average precipitate compositions are not allowed. These methods are illustrated in the case of Al-Cu, Cu-Ni-Al and Cu-Zn-Ni-Al alloys.