  Phase field crystal methodology is applied, for the first time, to study the
effect of alloy composition on the clustering behavior of a quenched/aged
supersaturated ternary Al alloy system. An analysis of the work of formation is
built upon the methodology developed in Fallah {\it et al.} to describe the
dislocation-mediated formation mechanisms of early clusters in binary alloys
[Phys. Rev. B., DOI: 10.1103/PhysRevB.00.004100]. Consistent with the
experiments, we demonstrate that the addition of Mg to an Al-1.1Cu alloy
increases the nucleation rate of clusters in the quenched/aged state by
increasing the effective driving force for nucleation, enhancing the
dislocation stress relaxation and decreasing the surface energy associated with
the Cu-rich Cu-Mg co-clusters. Furthermore, we show that it is
thermodynamically favourable for small sub-critical clusters to have higher
affinity for Mg than larger overcritical Cu-rich clusters, particularly
depicting a two-stage clustering phenomenon.
