Co-clusters in ternary or higher order metallic alloys are metastable structures involving two or more distinct alloying atoms that retain the structure of the host lattice. A thermodynamic model based on a single interaction energy of dissimilar nearest neighbour alloying elements is presented, and a model for the strengthening due to these co-cluster dimers is derived. The model includes a new treatment of (short-) order strengthening relevant to these co-clusters and further encompasses modulus hardening and chemical hardening. The models are tested against data on a wide range of Al-Cu-Mg alloys treated at temperatures between 20 and 220ºC. Both quantitative calorimetry data on the enthalpy change due to co-cluster formation and strengthening due to co-clusters is predicted well. It is shown that in general (short-range) order strengthening will be the main strengthening mechanism
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