The Nature of Native MgO in Mg and Its Alloys
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TO improve fuel economy and reduce CO2 emissions, it has been reported that Mg-alloys have been regarded as one of the most promising structural materials for automotive applications because of their high specific strength and good castability.[1] However, due to the high affinity of Mg with oxygen, oxidation happens throughout the life cycle of Mg-alloys from the foundry to practical service and recycling.[2,3] Oxidation brings up many problems, such as the presence of oxide impurities in the as-cast products,[4,5] non-protective oxide films for corrosion[6] and a high tendency to ignite and burn when exposed to air at high temperatures.[7] These together with other issues inhibit the widespread application of Mg-alloys.[8–10] Therefore, significant efforts have been directed to understand oxidation mechanisms in Mg-alloys and to develop practical approaches to minimize their negative effects,[11–15] as has been reviewed recently by Czerwinski[16] and Tan et al.[17] Extensive practical methods were proposed to either retard or eliminate oxidation. These include the prevention of ignition and burning by applying a flux or
SHIHAO WANG, YUN WANG, and ZHONGYUN FAN are with the BCAST, Brunel University London, Uxbridge, Middlesex, UB8 3PH, UK. QUENTIN RAMASSE is with the SuperSTEM Laboratory, SciTech Daresbury Campus, Daresbury WA4 4AD, UK and also with the School of Chemical and Process Engineering and School of Physics, University of Leeds, Leeds LS2 9JT, UK. Contact e-mail: [email protected] Manuscript submitted November 28, 2019. Article published online April 12, 2020 METALLURGICAL AND MATERIALS TRANSACTIONS A
protective gas during casting,[18,19] the addition of reactive elements to enhance the oxidation resistance through the formation of protective oxide films,[16, 20] the application of melt cleaning techniques and innovative die filling to ensure the casting quality and mechanical properties of Mg-alloys.[9,21] Nevertheless, different approaches have also been tried recently to explore potential benefits from the native oxide particles. For example, the oxide particles were found to be exploitable to produce MgO particle-reinforced metal matrix composites,[22] and to serve as substrates for heterogeneous nucleation to refine the as-cast grain structures of Mg alloys.[23–27] Decades ago, MgO was already proposed as a potential nucleating substrate for heterogeneous nucleation of Mg and its alloys.[23] Lee demonstrated that the addition of exogenous MgO particles results in moderate grain refinement in binary Mg-Al alloys.[24] Recently, significant grain refinement of AZ91D alloy[25] and commercial purity Mg (CP Mg)[27] was achieved through manipulating the native MgO particles by means of intensive melt shearing,[28] which was capable of breaking up MgO films into dispersedly distributed particles. The heterogeneous nucleation of a-Mg on MgO particles has been confirmed by the identification of well-defined orientation relationships (ORs) between the faceted MgO particles and a-Mg grains in AZ91D and CP Mg:[25,27,29]
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