A CALORIMETRIC STUDY OF PRECIPITATION IN AN AL-CU ALLOY WITH SILICON PARTICLES

The precipitation of copper and silicon from the Al-rich matrix in an Al-1.3 at. pct Cu-19.1 at. pct Si alloy was investigated by differential scanning calorimetry (DSC). Both as-extruded (AE) and extruded and solution treated and quenched (solid-quenched, SQ) specimens were studied. The DSC curves of the SQ specimens showed two exothermic effects, A and B. Effect A corresponded to the simultaneous precipitation of silicon and copper, whereas effect B was caused by the transition from the state with the intermediate copper-containing phase, theta', to the state with the equilibrium copper-containing phase, theta. The heat contents of effect A and B could quantitatively be described in terms of solid solubilities before and after precipitation and the heats of precipitation of the phases involved. From this description, it was derived that for heating rates less-than-or-equal-to 20 K/min, copper precipitated as the theta' phase, while for heating rates greater-than-or-equal-to 40 K/min, copper precipitated mainly as the theta phase. In SQ specimens, Guinier-Preston (GP)-zone formation occurred during aging at room temperature with a rate approximately 10(4) times slower than in the corresponding binary Al-Cu alloy. For the AE specimens, it was found that during extrusion, precipitation of copper and silicon proceeded to a large extent. However, from DSC experiments and from hardness measurements as a function of aging time at 453 K, it was deduced that copper precipitation had not finished during extrusion. The hardness increase as observed during aging directly after extrusion was interpreted to be due to formation of the semicoherent intermediate theta' phase.