Revealing the Nuclei Formation in Carbon-Inoculated Mg-3%Al Alloys Containing Trace Fe

In this study, Fe-bearing Mg-3%Al alloys were inoculated by combining carbon with or without Ca. Both processes can significantly refine the grain size of Mg-3%Al alloys. The highest refining efficiency can be obtained by carbon combined with Ca. The synergistic grain refining efficiency can be attributed to the constitutional undercooling produced by the addition of Ca. Two kinds of carbon-containing nuclei with duplex-phase particles and cluster particles were observed in the carbon-inoculated alloys. A thermodynamic model was established to disclose the formation mechanisms of the duplex-phase particles and Al(4)C(3) cluster particles. This thermodynamic model is based on the change of Gibbs free energy for the formation of these two kinds of particles. The calculated results show that these two particles can form spontaneously, since the change of Gibbs free energy is negative. However, the Gibbs free change of the duplex-phase particle is more negative than the Al(4)C(3) cluster particle. This indicates that the adsorption process is more spontaneous than the cluster process, and tiny Al(4)C(3) particles are preferred to form duplex-phase particle, rather than gathering to form an Al(4)C(3) cluster particle. In addition, the addition of Ca can reduce the interfacial energy between the Al(4)C(3) phase and the Al–Fe phase and promote the formation of duplex-phase particles.