Grain refinement of Mg-Zn and Mg-Mn alloys by SiC inoculation

Abstract

The addition of a grain refiner to molten metal prior to casting is one of the most important approaches to reduce the final grain size of both cast and wrought Mg alloys. Although it has been investigated for decades, the practical grain refinement methods of Mg alloys are still limited, and the grain refining mechanisms are still not fully understood. For Al-free Mg alloys, Zr is an extremely effective nucleant but an expensive one. Reducing the consumption of Zr and hence the cost of grain refinement is of great interest. Moreover, Zr will lose its grain refinement effect in Al-free Mg alloys if it encounters Mn, Si, or Fe due to the formation of stable intermetallic compounds. Thus, the development of a compatible and potent grain refiner to substitute Zr to be used in Al-free Mg alloys that contain some amount of Mn, Si, or Fe is necessary. In the present work, SiC particles were validated as an effective grain refiner for both as-cast binary Mg-Zn and Mg-Mn alloys. The effects of grain-refining parameters on the grain sizes have been investigated. Comparative tests using high purity Mg ingots were carried out to clarify the grain refining mechanisms. The effects of micro-alloying element Al in Mg-Zn-SiC system were also investigated, and the poisoning grain refining mechanism was proposed. The experimental results show that the added SiC particles cannot directly act as the nucleation sites of α-Mg grains. Mn3Si or (Mn,Fe)3Si particles formed in-situ prior to the formation of α-Mg grains from the reaction between SiC particles and Mn (or Fe) are the actual nucleation sites, which depends on whether Fe atoms can substitute some of the Mn atoms during reaction. This work also found that the grain refining efficiency of SiC inoculation is not only dominated by the presence of in-situ formed heterogeneous nucleation sites, but also by the solute element of Mn or Zn. The co-existence of potent nucleants and solutes with high growth restriction factor (Q) values is necessary to obtain the highest grain refining efficiency. The grain coarsening effect with Al addition into Mg-Zn-SiC system is attributed to a decrease in nucleation potency where the lower potent Al8(Mn,Fe)5 intermetallic particles formed.