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• W2918442881 abstract "As the lightest structural metallic materials, magnesium (Mg) alloys have attracted increasing interest in the automotive and aerospace industries. However, low oxidation resistance of Mg alloys limits their wider applications at elevated temperatures. Beryllium (Be) has long been considered as one of the most effective alloying elements in improving the oxidation resistance of Mg alloys even at ppm concentration. However, a convincing mechanism of this unique effect of Be in Mg alloys still remains unclear. Two mechanisms have been proposed. The BeO hypothesis assumes that the BeO layer forms prior to the MgO, whereas the reactive element effect model considers that the segregation of Be2+ cations along the MgO grain boundaries inhibits the outward diffusion of Mg2+ cations. Both models have never been experimentally verified. The present work aims to comprehensively investigate the effects of trace additions of Be on the high temperature oxidation behaviour of Mg alloys and, therefore, to understand the mechanism of how Be improves the oxidation resistance of Mg alloys.Be-containing AZ91 alloys were produced with trace additions of Be (10 ppm-60 ppm) into the alloy melt. Thermogravimetric analysis (TGA) and long-term furnace oxidation (LTFO) were used to characterize the oxidation behaviours of the alloys. All results showed that trace additions of Be significantly improved the oxidation resistance of the AZ91 alloy at 400 C, even at 10 ppm. In order to investigate the effectiveness of Be in other Mg alloys, 60 ppm Be was added to Mg-2Zn, Mg-2Sn, Mg-2Y, AS21, AM60, ZK20 and ZC63. Similar to the AZ91 alloy, experimental results showed that microalloying with Be effectively lowered the oxidation rates at 500 C and increased the ignition temperatures of the Mg-2Zn, Mg-2Sn, AS21, and ZC63 alloys. But, Be addition marginally influenced the oxidation behaviours of the ZK20, AM60 and Mg-2Y alloys.In order to understand the mechanisms behind why Be improves the oxidation resistance of cast Mg alloys, the oxidation layers were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nanoindentation. No continuous BeO surface layers were observed on the surfaces of the alloys tested and no Be segregation was detected along the grain boundaries of MgO. However, TEM examination detected the accumulation of Be in the initially formed MgO layer during the oxidation process, which led to the formation of fine-grained (Mg,Be)O solid solutions. As a result, the initially formed surface oxide was effectively strengthened. The nanoindentation and nanoscratch analyses verified that the reinforced (Mg,Be)O layers on the Becontaining AZ91, Mg-2Zn, Mg-2Sn, AS21 and ZC63Be alloys exhibited a higher hardness and strength than the MgO layer on the alloys without Be. Hence, it is considered that during high temperature oxidation, the accumulation of Be on the surface of Mg alloys reinforced the initially formed (Mg,Be)O layer. This effectively delayed the cracking and debonding of the oxide layers from the substrates. As a result, the oxidation incubation period was extended through retarding the Mg vapour diffusion, resulting in high oxidation resistance. Because Be is expensive and toxic, another aim of the thesis was to reduce the usage of Be. Based on previous published results, Ca was chosen as a candidate. A series of AZ91 alloys containing both Be and Ca were produced, trying to partially replace the Be with Ca. The TGA, LTFO, and ignition test results verified that the synergistic effect of Be and Ca enabled the reduction in usage of Be in the oxidation-resistant Mg alloys. Addition of 20 ppm (wt) Be and 0.5 wt.% Ca was considered as an optimal combination to sufficiently improve the high temperature oxidation resistance of the AZ91 alloy. The differential thermal analysis (DTA) and FESEM indicated that the synergistic effect of Be and Ca allowed (i) suppression of Mg vaporisation along the grain boundaries of the alloy, and (ii) the formation of a compact duplex oxide layer consisting of the outer Be-reinforced MgO layer and the inner CaO/MgO composite layer. The duplex layer acted as barrier for both the outward evaporation of Mg and the inward penetration of oxygen and, therefore, achieved high oxidation resistance. In summary, the present PhD thesis has advanced the understanding of the roles of Be in improving the oxidation resistance of cast Mg alloys. Based on the results, it is concluded that the improved oxidation resistance of Mg alloys through trace addition of Be was attributed to the reinforcement effect of Be in the initially formed surface oxide layer. It has also been evidenced that the oxidation resistance of Mg alloys is closely related to the strength/hardness of the oxide layers formed on the alloy surface. In addition, the present thesis showed the synergistic effect of Be and Ca in improving the oxidation resistance for Mg alloys, which has technological and practical significance in the development of new oxidation resistant Mg alloys with lower Be content." @default.
• W2918442881 created "2019-03-11" @default.
• W2918442881 creator A5019267718 @default.
• W2918442881 date "2018-12-10" @default.
• W2918442881 modified "2023-09-27" @default.
• W2918442881 title "Influence of trace addition of Be on the oxidation behaviour of Mg alloys" @default.
• W2918442881 doi "https://doi.org/10.14264/uql.2018.827" @default.
• W2918442881 hasPublicationYear "2018" @default.
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