The Refinement Effect of Alumina–Silica Sol‐Coated ZnO Particles for Cast Magnesium

Abstract

Grain refinement is a practical method to improve the mechanical properties of magnesium alloys, and relevant research demonstrates that inoculation is an effective method of melt treatment to achieve grain refinement. [1-5] Thus, the development of efficient and reliable refiners for Mg alloys is technologically important and necessary. According to the crystallographic perspective-based edge-to-edge matching model, Zhang and co-workers predicted and explained a series of currently available grain refiners for Al and Mg alloys. [2,6,7] Combining the crystallographic database and edge-to-edge matching model calculation, a series of potential grain refiners for Mg alloys have been predicted, such as Al 2 Ca, AlN, Al 2 Y, CaO, AlMn, and ZnO, and experimental results match well with the theoretical predictions. [8-14] Based on edge-to-edge crystallographic matching calculation, ZnO is predicted to be one of the most effective candidates for Mg refinement, as it has an hexagonal close-packed (HCP) structure and belongs to the P63mc space group. The lattice parameters of ZnO are a ¼ 0.3265 nm and c ¼ 0.5219 nm, which are very close to those of the Mg matrix, i.e., a ¼ 0.320936 nm, and c ¼ 0.521123 nm. This prediction was based on the lattice parameters of ZnO and Mg at room temperature, which are expected to vary slightly at higher temperatures. However, some previous work showed that a small variation in lattice parameters between room temperature and solidification temperature does not affect the refining results. [15-17] Therefore, based on the success of previous applications of the edge-to-edge crystallographic matching model to the crystallographic understanding of grain refinement, these small misfits suggest that ZnO is likely to be an efficient grain refiner for Mg alloys. This prediction has been supported by preliminary results from other researchers. [18,19] However, according to thermodynamic data, ZnO particle reduction in molten Mg could occur as follows [7] MgðlÞ þ ZnOðsÞ ! MgOðsÞ þ ZnðlÞ ΔG θ 1000 ¼ À236.7 kJ mol À1 (1) ZnO would be reduced into Zn solute in contact with molten Mg, which would damage the crystallographic structure of the ZnO particles, affecting its use as a refiner for Mg. As the separation of inclusion and casting processes usually takes time, it is difficult to completely prevent the reduction between ZnO and Mg. At the same time, a reduction that generates Zn solute and native MgO could also lead to grain refinement for Mg alloys. [20] Although there is a reduction reaction between the added ZnO particles and molten Mg, the grain refinement achieved by ZnO particles is still better than that achieved with the equivalent addition of pure Zn. Saha and Ravindran and Ohtomo et al. suggested that ZnO acts as a heterogeneous nucleation before the ZnO particle is totally reduced to Zn solute by the molten Mg. [21,22] However, the refinement effect of ZnO on Mg may still be compromised because of the reduction reaction. Alumina and silica sol have been applied to fabricate protective coat...