First Principle Calculation And Experimental Study On Grain Refinement Mechanism Of Carbon Inoculation For Mg-Al Alloy

Although many theories have been performed to explore the refinement mechanism and the potential of heterogeneous nucleation nucleus of carbon inoculation for Mg-Al alloy, due to the lack of study on the interface of a-Mg and heterogeneous nucleus, there are contradictions between the explanation offered by different theories with experimental phenomenon. The approach of studying the properties of the interface of a-Mg and some heterogeneous nucleation nucleus by the first principle calculation is one of the feasible methods to analyze the mechanism for Mg-Al alloy which refined by carbon. This research will provide some theoretical guides for developing efficient grain refiner and advanced grain refining theory.First-principles calculations on the interface of Mg/Al4C3and Mg/Al2CO reveal that the interfacial Mg atoms in Mg/Al4C3interface prefer to directly stack onto the interfacial C atoms, and the interfacial Mg atoms in Mg/Al3CO interface prefer to directly stack onto the interfacial O atoms. The interfacial electronic redistribution is locally confined to interfacial region. It exist the mixed polar covalent/ionic C-Mg bond and adhesion energy is1.968J/m2for the interface of Mg/Al4C3. But it only exists the ionic O-Mg bond and adhesion energy is0.427J/m2for Mg/Al2CO interface, which results in weaker interfacial adhesion than that for the Mg/Al4C3interface.The interfacial energy for Mg(0002)/Al4C3(0001) interface does not depend on the chemical potential. The Mg/AL4C3solid-to-solid interfacial energy is lower than that of solid-to-liquid interfacial energy between Al4C3and magnesium melt, which is responsible for the good heterogeneous nucleation of α-Mg on Al4C3surface. But the interfacial energy for Mg(0002)/Al2CO(0001) interface depend on the chemical potential. However, the Mg/Al2CO interfacial energy is lower than that of between α-Mg and magnesium melt only when the chemical composition of the system is held on a certain level. The above analysis results theoretically support the hypothesis of Al4C3and Al2CO can be the heterogeneous nucleation nucleus for α-Mg in Mg-Al alloy refined by carbon.The SEM and EDS results show that, when Mn content is low, it is easy to form effective heterogeneous nucleation nucleus for α-Mg such as Al4C3and Al-Mn-C-Fe phase which has a good refined effect on Mg alloy grain. But, with the increase of the content of Mn, the phase of Al8Mn5or Al8(Fe,Mn)5interact with carbide and continue to grow up to form flower-like intermediate phases. The new phases can not act as the heterogeneous nucleus for α-Mg and refine the grain size of Mg-Al alloys because these large size phases were formed from the interaction between Al4C3and Al-Mn-C-Fe phase, which result in the reduction of Al4C3particles.