This study examines the formation of different phases of Al-6 mass% Mg–xCu (x = 1 and 3 mass%) alloys in as-cast condition. Further, it investigates the dissolution of these phases upon solution heat treatment (SHT) and studies the precipitation behavior of these ternary alloys. Scanning electron microscopy with energy-dispersive spectrometry and high resolution X-ray diffraction analyses show the presence of the second phases of Al3Mg2 (β), Al6CuMg4 (T), and Al2CuMg (S) in Alloy I (Al–6Mg–1Cu), whereas Alloy II (Al–6Mg–3Cu) had only T and S second phases (with a much higher number of S phases). Upon SHT, a significant number of eutectic phases were dissolved in Alloy I, whereas in Alloy II, the number of undissolved S phases was relatively high. A differential scanning calorimetry (DSC) analysis of experimental alloys in as-quenched states reveals two exothermic peaks related to the formation of nanoclusters and S″ or S′ metastable phases. Both alloys undergo a rapid hardening stage during the aging process, in which approximately 50%–60% of total hardness was achieved. This is attributed to the formation of nanoclusters. The maximum yield strength achieved at the peak hardness condition was approximately 200 MPa for Alloy I, whereas it was approximately 160 MPa for alloy II. Alloy I took a long time to reach peak hardness, which is correlated with the stability of nanoclusters for a longer time. Earlier peak hardness in Alloy II, despite having nanoclusters, is correlated with undissolved eutectic phases acting as heterogeneous nucleation sites for the formation of S″ or S′ metastable phases.
This study investigated the heat treatment response and tensile properties of Al–6 mass%Mg–xSi (x = 1, 3, 5, and 7 mass%) ternary alloys. Further, the fracture behavior of these alloys in response to heat treatment for different temper conditions was also examined. Scanning electron microscopy–energy dispersive X-ray spectrometry (SEM–EDS) analysis of the as-cast alloys revealed, in all of them, the presence of iron-bearing phases (in a size range of 10˜60 μm) that did not dissolve or become refined upon heat treatment. Additionally, eutectic Mg2Si and Al3Mg2 phases were found in Alloy I (Al–6Mg–1Si), while eutectic Mg2Si and Si phases were found in the rest of the alloys. In the as-cast condition, the tensile properties of the examined alloys decreased in relation to increasing Si content. Nonetheless, after heat treatment, the yield strength of the alloys with high Si content (>3 mass%) increased significantly compared with that in the as-cast condition. A yield strength greater than 300 MPa was achieved in both Alloy III (Al–6Mg–5Si) and Alloy IV (Al–6Mg–7Si), although this was achieved at the expense of ductility. According to the fractography of the tensile-fractured surfaces undertaken using optical and scanning electron microscopy, fractures of the iron-bearing phases were found to be the source of cracking in alloys with high Si content. In the case of those with low Si content (≤3 mass%), cracks were believed to have been caused by the debonding of iron-bearing phases from the aluminum matrix.
In this study, the oxidation behavior of Cu alloys containing two alkaline earth metals (i.e., Mg and Ca) at 500 °C was investigated. The Mg+Mg2Ca master alloy was used for the simultaneous addition of Mg and Ca into Cu. As a result of the oxidation test, all examined samples showed weight gains that followed parabolic laws. Mg addition in Cu considerably slowed down the oxidation rate, while the use of the Mg+Mg2Ca master alloy as an alloying element for Mg led to an even further reduction in the oxidation rates at the testing temperature. The phase diagrams with the oxygen partial pressure showed that the Ca and Mg-containing alloy resulted in the formation of CaO as the primary oxide and MgO as the secondary oxide. The improved oxidation resistance can be attributed to the mixed surface layer of CaO and MgO, which control the growth rate of Cu2O.
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