Dislocation loop-free zones around grain boundaries in quenched aluminium and aluminium alloys

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Abstract
Measurements have been made of the width of the dislocation loop-free zones formed around 〈100〉 tilt boundaries in quench-aged pure aluminium and some dilute aluminium solid solutions as a result of vacancy-loss to the boundary during cooling. The width is found to increase as the quench-rate decreases except for the slowest quench-rate used. The data are analysed in terms of the vacancy-concentration profile derived for simple monovacancy diffusion and a linear quench rate, and assuming that a critical vacancy concentration is required for loop nucleation. This critical concentration is derived from nucleation theory. Reasonable agreement between measured and calculated zones is obtained. The model suggests a linear relationship between the square of the zone width and the reciprocal of the (linear) quench rate. The zone width decreases rapidly as the boundary tilt angle decreases below ∼ 3° and this is interpreted in terms of a loss of sink efficiency at small misorientations. The model indicates that a 2° tilt boundary maintains ∼ 25% vacancy supersaturation relative to an ideal sink. The widths of the loop-free zones in the dilute alloys are interpreted on the assumption that the major effect of solute atoms is on vacancy–diffusivity because of solute–vacancy interaction.