= Solution
Choose an objective aperture around a precipitate <superlattice reflection>, such as $100$, which is absent from the disordered aluminium matrix. The selected beam then makes precipitates bright in <dark-field transmission electron microscopy>, while the matrix gives little intensity in that beam. Tilt the grain to place the chosen reflection close to its <Bragg diffraction> condition and preferably approach a two-beam geometry. This reduces excitation error and can increase precipitate brightness; the exact maximum also depends on foil thickness, <strain> and dynamical scattering. Selecting a shared fundamental reflection would give much poorer chemical-order selectivity.
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