Observations of dislocations and precipitates in aluminium alloys

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Abstract
Electron transmission micrographs of quenched aluminium alloys show that the specimens contain many complex dislocations and large dislocation loops with diameters up to 2000 Å. These dislocations could not be moved by heating the foils in the electron beam and their immobility is thought to be due to substitutional atmospheres of solute atoms which form at, and lock, the dislocations by vacancy-aided diffusion. This has been confirmed by ageing the foils, which causes precipitation to occur on the dislocations. These precipitates also lock the dislocations by a pinning action because stresses of the order of μ/50 are required to free the dislocations. The observations are discussed in relation to the hardening effect of quenching and of precipitation after ageing. The movement of glissile dislocations in quenched alloys is typical of a material of high stacking fault energy, i.e. there are no partial dislocations and cross slip is frequently observed.