Investigation on atomic mechanism of initial process of α-Mg heterogeneously nucleated on AlN in Mg–Al alloy

The valence electron structure, bond energy and cohesive energy of AlN, AlN doped x%Mg, Mg3N2, Mg3N2 doped x%Al and Mg doped x%Al crystals were calculated using the empirical electron theory of solids and molecules (EET). The calculation results show that AlN consists of two kinds of Al–N bonds and the weak Al–N bond on the outermost surface will break under high temperature in the Mg–Al melt, and liquid Mg atoms are attracted by the exposed N atoms to preferably form a single-layer of Mg3N2 before α-Mg crystallization. Besides, this single Mg layer also has a similar crystal structure to that of the (0001) plane in α-Mg, both of which make AlN an effective heterogeneous nucleant. The increase in the Al content reduces the cohesive energy of Mg3N2; when the Al content exceeds a certain value, the cohesive energy of Mg3N2 is lower than that of α-Mg, resulting in a single Mg layer in Mg3N2 not formed prior to α-Mg. Simultaneously, Al–N bonds might be formed again on AlN surface to increase the Al content, which finally contributes to AlN losing its refining effect. Based on the calculation results, a novel atomic model of AlN heterogeneously nucleated to α-Mg is proposed.