Influence of trace addition of Be on the oxidation behaviour of Mg alloys

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 Be 2+ cations along the MgO grain boundaries inhibits the outward diffusion of Mg 2+ 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 XI My special thanks also go to Prof. Fusheng Pan for his insightful advices on my projec...

Enantioselective Total Syntheses of (+)-Fendleridine and (+)-Acetylaspidoalbidine

“…A few syntheses of 1-acetylaspidoalbidine were carried out by the oxidation and cyclization of 8. [18][19][20] The first total synthesis of fendleridine (2) and 1-acetylaspidoalbidine (3) was reported by Ban andco-workers in 1975 and1976. 19,21 Overman and co-workers in 1991 also reported a formal synthesis of 1acetylaspidoalbidine (3) in racemic form, using the aza-Cope-Mannich reaction as the key strategy.…”