Fractographic Investigation of Grain-Boundary Precipitates in Al–Cu Alloys (1 and 4 wt.-% Cu) by Gallium Embrittlement

Abstract

In aluminium alloys, intergrantilar surfaces can be observed by optical microscopy or scanning electron microscopy after intergranular decohesion by liquid gallium. This experimental technique has been applied to aluminium–copper alloys (1 and 4 wt.-% copper). Two types of thermal treatment were carried out: water-quenching followed by reheating at different temperatures; air cooling followed by reheating at different temperatures. The results have shown that the following parameters control the morphology of the inter granular surfaces: cooling rate, solute content of the alloys, time and temperature of reheating, and grain boundary structure. After water-quenching no precipitates were observed, but after air-cooling, the interfaces were found to contain copper-rich precipitates. Reheating after air-cooling led to an increase in the number of copper precipitates. The density of the precipitates increased with increased copper content. Reheating after water-quenching revealed the presence of vacancy condensation pits containing copper. The density of the pits increased with copper content. The geometrical size of the pits decreased with increased copper content.

The results are discussed from the viewpoint of the competition between the different fluxes towards and away from the intergranular interfaces.