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Dissolution of grain boundary allotriomorphs in the Al-Cu and Al-Ag systems Pasparakis, Aristedes

Abstract

The dissolution behaviour of grain boundary allotriomorphs has been studied in the systems Al-Cu and Al-Ag. Using electron probe microanalysis, isoconcentration contours around dissolving allotriomorphs in an effectively infinite matrix have been determined. The influences of allotriomorph shape, grain boundary misorientation, solute super-saturation in the matrix, volume and grain boundary diffusion, and homologous temperature on the shape of these contours have been examined. It was established that the shape of the isoconcentration contours is dependent only on homologous temperature (T[sub H]) for both systems studied. Above T[sub H] = 0.92, no grain boundary diffusion contribution to the dissolution process is observed. The grain boundary diffusion contribution was found to increase with decreasing T[sub H] and to completely dominate the dissolution process below T[sub H] = 0.72. Using the back scattered electron image on the electron probe microanalyzer, the dissolution rate of individual grain boundary allotriomorphs was determined for various values of T[sub H], under conditions in which there was impingement of diffusion fields from adjacent precipitates. An exponential relationship between axial length or width and dissolution time was found to adequately describe the observed dissolution kinetics. A change in axial ratio accompanied the dissolution of the grain boundary allotriomorphs. At high T[sub H] an increase in axial ratio with dissolution time was observed, whereas at low T[sub H], a decrease in axial ratio (i.e., spheroidization) was observed. A model has been proposed to account for this behaviour.

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