Rationally-oriented, plate-shaped precipitates of transition phases are often key strengthening constituents in high strength and ultra-high strength aluminium alloys. These precipitate phases include {100}_(alpha) plates of theta' (Al_2Cu) and {111}_(alpha) plates of OMEGA (Al_2Cu) and T_1 (Al_2CuLi). The identity and distribution of these precipitate phases are often controlled by pre-precipitate clusters of microalloying elements. It is demonstrated that the transformation strain defining the change in structure associated with formation of such precipitate phases involves a large shear component and that the manner in which the shear strain energy is accommodated is a critical factor in controlling the formation of the precipitate plates. One way in which this shear strain energy might be minimised during nucleation is if the nucleus forms in association with rationally-oriented clusters combining relatively large solute atoms with a local excess of vacancies. For a given volume fraction and number density of precipitate plates which are either shearable or shear-resistant, plate-shaped precipitates formed on {111}_(alpha) planes of the aluminium-rich matrix solid solution are most effective in strengthening, and the yield strength increment produced by {111}_(alpha) precipitate plates increases with increasing plate aspect ratio.
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