Abstract
It has been reported that native MgO particles in Mg alloy melts can act as heterogeneous nucleation substrates such that grain refinement of Mg alloys is achieved. A recent study showed the addition of Ca, combined with the native MgO particles, significantly improves grain refinement of Mg and its alloys. However, the mechanism underlying the grain refining phenomenon is not well understood due to the lack of direct experimental evidence. In this work, we investigated the segregation of Ca atoms at the Mg/MgO interface and its effect on grain refinement in Mg-0.5Ca alloys by utilizing advanced analytical electron microscopy. The experimental results focus on the chemical and structural information at the interface between MgO and the Ca solute. Adsorption layers rich in Al, N and Ca have been detected on {1 1 1} facets of MgO particles, with the lattice structure resembling the structure of MgO. It is suggested that the significant grain refinement improvement can be attributed not only to the growth restriction due to the presence of Ca addition but also to the specific chemistry and structure of the adsorption layers.
Highlights
As the lightest structural metal, magnesium and its alloys have found a wide variety of applications in the industry due to their high specific strength and excellent castability [1,2]
We investigated the segregation of Ca atoms at the Mg/MgO interface and its effect on grain refinement in Mg-0.5Ca alloys by utilizing advanced analytical electron microscopy
Either the addition of 0.5% Ca or the application of HS leads to the columnar-to-equiaxed transition (CET) and reduces the grain size to 296.6±34.1μm in Mg-0.5Ca and 235±18.1μm in CP-Mg-HS, respectively
Summary
As the lightest structural metal, magnesium and its alloys have found a wide variety of applications in the industry due to their high specific strength and excellent castability [1,2]. The grain refinement of Mg alloys has received much attention over the past few decades since fine grain size is one of the factors improving the alloys’ mechanical properties [3,4]. Chemical and physical methods have been extensively studied from both experimental and simulation perspectives [57]. More than the improvement of mechanical properties, the addition of solute elements such as Al, Zr, Si and Ca is a common and simple way to achieve grain refinement [8,9,10]. Ca produces a strong grain refining effect in pure magnesium [9], whose mechanism has been interpreted by the interdependence theory where Ca has the second highest grow restriction factor (Q) [10]. In Albearing Mg alloys, the intermetallic compound Al2Ca was believed to act as a potent substrate for Mg nucleation, supported by the strong geometric misfit calculated in the edge-to-edge matching model [11]
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