The present study has been undertaken to investigate the effect of alloying additions and high temperature T5-treatment consisting of high temperature direct aging after casting, on the microstructural characteristics of some Al-Si-based eutectic and near-eutectic alloys, typically used in the automotive industry. For comparison purposes and in order to estimate the effect of Mg, three sets of samples consisting of Al-11.9wt%Si (AS13, B413), Al-11.67wt%Si-0.22wt%Mg (AS10G) and Al-7.2wt%Si-0.44wt% Mg (AS7G06, A356) alloys have been first studied in the as-cast state. In order to examine the effect of heat treatments, three groups of alloys were investigated in two different states, namely: (i) after T6-treatment consisting of solutionizing at 540 °C for 10 hours, water quenching and then aging at 160 °C for 4 hours; and (ii) after high temperature T5-treatment consisting of annealing at 300 °C, 450 °C and 500 °C for different aging times in order to locate a possible peak hardness. Microstructural examination was carried out using optical microscopy, scanning electron microscopy, energy Dispersive Spectrometry (EDS), X-Ray Diffraction and Brinell-hardness measurements. Our as-cast alloys’ microstructure revealed that all the different intermetallic phases, such as the Q-Al₅Cu₂Mg₈Si₆ phase, the α-Al₁₅(MnFe)₃Si₂, β-Al₅-Fe-Si, π-Al₈FeMg₃Si₆ and Al₇Cu₂Fe iron-rich compounds, as well as the Al₂Cu and Mg₂Si hardening precipitates, have been formed during solidification. Besides, X-rays diffraction patterns revealed that while some Al₂Cu hardening precipitates have exhibited total dissolution, after solutionizing, the undesirable iron-rich phases have remained at least partially insoluble after the different heat treatments. A semi-quantitative X-Rays diffraction analysis, performed after the different heat treatments, has shed light on the interplay between the Fe-rich intermetallic compounds, denoting the transformation of the π-Al₈FeMg₃Si₆ into the β-Al₅FeSi phase on the one hand and between π-Al₈FeMg₃Si₆ and Q-Al₅Cu₂Mg₈Si₆ phases in the precipitation and dissolution of Al₂Cu and Mg₂Si phases on the other hand. Both of the two processes, marking a certain interaction between the different intermetallic compounds, are observed to give rise to a significant release of Mg and Si solute atoms and to enhance the hardening precipitates volume fraction after heat treatments. It turns out that high temperature T5-treatment is more appropriate and even more efficient than the usual T6 one, in the sense that it yields a more significant amount of hardening precipitates giving rise to higher hardness peaks with just one step treatment.

本研究旨在研究合金添加物和由铸造后高温直接时效组成的高温 T5 处理对某些 Al-Si 基共晶和近共晶合金的显微组织特征的影响，这些合金通常用于汽车行业。为了比较和估计 Mg 的影响，由 Al-11.9wt%Si（AS13、B413）、Al-11.67wt%Si-0.22wt%Mg（AS10G）和 Al-7.2wt 组成的三组样品%Si-0.44wt% Mg (AS7G06, A356) 合金首先在铸态下进行研究。为了检查热处理的效果，在两种不同状态下研究了三组合金，即：(i) 在 T6 处理后，包括在 540 °C 下固溶 10 小时，水淬，然后在 160 °C 下时效4小时；(ii) 在高温 T5 处理后，包括在 300°C、450°C 和 500°C 下退火不同的时效时间，以定位可能的峰值硬度。使用光学显微镜、扫描电子显微镜、能量色散谱 (EDS)、X 射线衍射和布氏硬度测量进行微观结构检查。我们铸态合金的微观结构表明，所有不同的金属间相，例如Q- Al ₅ Cu ₂ Mg ₈ Si ₆相、α-Al ₁₅ (MnFe) ₃ Si ₂、β-Al ₅ -Fe-Si、π-Al ₈ FeMg ₃ Si ₆和Al ₇ Cu ₂ Fe富铁在凝固过程中形成了 Al ₂ Cu 和 Mg ₂ Si 硬化沉淀物。此外，X 射线衍射图表明，虽然一些 Al ₂Cu硬化沉淀物表现出完全溶解，在固溶化之后，在不同的热处理之后不希望的富铁相至少部分保持不溶。在不同热处理后进行的半定量 X 射线衍射分析揭示了富铁金属间化合物之间的相互作用，表明 π-Al ₈ FeMg ₃ Si ₆转化为 β-Al ₅一方面是 FeSi 相和介于 π-Al ₈ FeMg ₃ Si ₆和 Q-Al ₅ Cu ₂ Mg ₈ Si ₆相之间的 Al 的析出和溶解另一方面，₂ Cu和Mg ₂ Si相。这两个过程都标志着不同金属间化合物之间的某种相互作用，观察到导致 Mg 和 Si 溶质原子的显着释放，并在热处理后提高硬化沉淀物的体积分数。事实证明，高温 T5 处理比通常的 T6 处理更合适，甚至更有效，因为它会产生更大量的硬化沉淀物，只需一步处理即可产生更高的硬度峰值。