First-principles Study On The Interfaces Of Crystal-nucleus/Matrix Inculated By Carbon In Mg-Al Melt

Carbon-inoculation grain refinement method is considered to be the most potential processto reduce grain sizes because it can be conducted at low temperature with little fading. The mostwidely accepted mechanism of the method is heterogeneous nucleation mechanism, however thespecific nucleus still remains unclear. Thereinto, Al4C3is accepted to be the most effective crystalnucleus, and Al2MgC2is considered likely to be one effective crystal nucleus in Mg-Al meltinoculated by carbon. To explore the mechanism deeply at a theoretical level, we appliedfirst-principles method to investigate the interface properties among different phases, in order todecide the crystal nucleu, and finally reveal the grain refining mechanism.In this paper, we mainly discuss about the issues as following:1.The bulk properties ofmatrix phase Mg，carbonaceous phases Al4C3and Al2MgC2;2. Surface properties of Mg(0002),Al4C3(0001) and Al2MgC2(0001);3. The models and study of the properties of matrix/crystalnucleus interfaces;4. Effects of alloying elements and impurity elements on Mg/Al2MgC2interface. The results of our study are listed as following:1. The calculated cohesive energy of Mg bulk is1.51eV, and the formation energies of Al4C3and Al2MgC2are-0.68eV and-0.34eV; both Al4C3and Al2MgC2possess of the semiconductingproperties.2. There are2and5different surface structures for Al4C3(0001) and Al2MgC2(0001)surfaces respectively. The Al-terminated (0001) is the most stable for Al4C3, while the most stableAl2MgC2(0001) surface could be Mg-terminated or Al-terminated (C) surfaces under Mg-richenvironment. Thus, these three surface models would be considered in the interface models withMg(0002) surface.3. Al4C3(0001) and Al2MgC2(0001) surfaces were chosen to built interface models withMg(0002) surface and calculate related surface properties. For Mg/Al-terminated Al4C3interface,HCP stacking sequence is most stable. And covalent bond are formed at interface, however, thereis little charge translation, and charge interaction is very weak. For Mg/Al2MgC2interface, HCP and MT stacking sequence are most stable for Mg/Al-terminated(C) Al2MgC2interface andMg/Mg-terminated Al2MgC2interface, respectively. And, charge translation ofMg/Mg-terminated Al2MgC2interface is much more and intensive, covering the whole interface.While, charge translation of Mg/Al-terminated(C) Al2MgC2interface is less and the interaction isweaker.4.The interface energy of Mg/Mg-terminated Al2MgC2interface is much less than that ofMg/Al4C3interface. Thus, when Al4C3and Al2MgC2exist simultaneously in the Mg-Al meltinoculated by carbon, Al2MgC2is likely to be the crystal nucleus.5. The stability study of Ca(Sr)/Fe(Mn)-doped interfaces shows that Ca addition enhancesthe stability of Mg/Al2MgC2interface, while Sr/Fe(Mn) addition lowers the stability ofMg/Al2MgC2interface. This indicates that Ca is helpful to the effects of carbon inoculation, whileSr/Fe(Mn) would inhibit the refining effects of carbon inoculation. Besides, it is found that thestability of Mg/Al2MgC2interface with Ca(Sr) and Fe(Mn) co-doping is higher than that ofFe(Mn)-doped Mg/Al2MgC2interface, but lower thant that of Ca(Sr)-doped one. This indicatesthat，to some extent, Ca(Sr) addition could limit the harm caused by Fe(Mn) addition to thecarbon inoculation of Mg-Al alloy.6. The doping of Al, Zn and Si is found to reduce the interface stability of Mg/Al2MgC2andrefrain the refining effects carbon inoculation.