The extrinsic stacking-fault energy of aluminium has been determined from the isothermal annealing of double-faulted dislocation loops in the electron microscope and found to be 30% greater than the intrinsic energy. The observed rarity of extrinsic faulting in thin foils is accounted for in terms of a nucleation barrier to the formation of an extrinsic fault. Several previous observations of extrinsic faults, some of which have been taken to imply a higher extrinsic stacking-fault energy, are re-examined in the light of this barrier.
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