A calorimetric study of precipitation in an Al-Cu alloy with silicon particles

Abstract

The precipitation of copper and silicon from the Al-rich matrix in an AI-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, 0', to the state with the equilibrium copper-containing phase, 0. 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-<20 K/min, copper precipitated as the 0' phase, while for heating rates->40 K/min, copper precipitated mainly as the 0 phase. In SQ specimens, Guinier-Preston (GP)zone formation occurred during aging at room temperature with a rate approximately 104 times slower than in the corresponding binary A1-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 0' phase.

Interaction between precipitating phases in quenched Al-Cu-Si alloys

“…These three phases are the only ones reported to exist above 600 K [1,14]. Correspondingly, the three sub-effects are interpreted to be due to the dissolution of these three phases [1]. From this identification of the dissolution effects it is derived that the precipitation effect contains the formation of Si and 0' phases.…”

“…The end temperatures of these three effects are about 680, 730 and 790 K. These temperatures correspond with the semi-equilibrium solvus of 0' and the equilibrium solvi of 0 and Si phase, respectively [12, !3]. These three phases are the only ones reported to exist above 600 K [1,14]. Correspondingly, the three sub-effects are interpreted to be due to the dissolution of these three phases [1].…”

“…Specimens were solution-treated for 5 rain at 793 K in a vertical tube furnace and subsequently quenched in water at room temperature. After this solution treatment, the Si-particle size in the alloy studied is ~ 1-2/am [1]. A part of the specimens was stored at room temperature awaiting further experiments; another part was stored in liquid nitrogen.…”

“…Recently, precipitation in the solution-treated and subsequently quenched Al-l.3 at % Cu-19.1 at % Si alloy was studied by differential scanning calorimetry (DSC) [1]. It was observed that for heating rates smaller than 20 K min-t the precipitation of the Si and 0' phases from the supersaturated Al-rich phase occurs simultaneously.…”

“…These dislocations can act as extra nucleation sites (enhancing precipitation) or they can annihilate excess vacancies (retarding precipitation). The elastic stresses resulting from misfit accommodation can also influence precipitation phenomena [8,9].…”