Microstructural aspects of the dissolution and melting of Al2Cu phase in Al-Si alloys during solution heat treatment |
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Authors: | A M Samuel J Gauthier F H Samuel |
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Affiliation: | (1) Département des Sciences Appliquées, Université du Québec à Chicoutimi, G7H 2B1 Chicoutimi, PQ, Canada |
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Abstract: | The dissolution and melting of Al2Cu phase in solution heat-treated samples of unmodified Al-Si 319.2 alloy solidified at ≈10 °C were studied using optical
microscopy, image analysis, electron probe microanalysis (EPMA), and differential scanning calorimetry (DSC). The solution
heat treat-ment was carried out in the temperature range 480 °C to 545 °C for solution times of up to 24 hours. Of the two
forms of Al2Cu found to exist,i.e., blocky and eutectic-like, the latter type is more pronounced in the unmodified alloy (at ≈10 °C) and was observed either
as separate eutectic pockets or precipitated on preexisting Si particles, β-iron phase needles, or the blocky Al2Cu phase. Dissolution of the (Al + Al2Cu) eutectic takes place at temperatures close to 480 °C through frag-mentation of the phase and its dissolution into the
surrounding Al matrix. The dissolution is seen to accelerate with increasing solution temperature (505 °C to 515 °C). The
ultimate tensile strength (UTS) and fracture elongation (EL) show a linear increase when plotted against the amount of dissolved
copper in the matrix, whereas the yield strength (YS) is not affected by the dissolution of the Al2Cu phase. Melting of the copper phase is observed at 540 °C solution temperature; the molten copper-phase particles transform
to a shiny, structureless phase upon quenching. Coarsening of the copper eutectic can occur prior to melting and give rise
to massive eutectic regions of (Al + Al2Cu). Unlike the eutectic, fragments of the blocky Al2Cu phase are still observed in the matrix, even after 24 hours at 540 °C. |
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