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Abstract

The aim of the project is to model as cast micro-structures of Mg-Al alloy consisting of HCP-Mg phase dendrites surrounded by Mg17Al12 beta phase. The essential properties such as corrosion-resistance and mechanical properties are extremely sensitive to the scale of the HCP-Mg dendritic structure and the formation of closed shell of beta-phase around the dendrites.

Our first results reveal that while it is relatively straight-forward to simulate dendritic solidification of Mg-Al alloy at high cooling rates (~100 K/s), it is not so easy at lower cooling rates because of extremely long simulation times. The problem can be addressed to the diffusion coefficient of Mg-Al melt which is according to the Von Neumann stability condition is the controlling parameter for the time step of the simulation. In order to overcome this limitation we altered the diffusion coefficient of melt assuming that by cooling the melt by 200 K from the solidification start temperature down to the eutectic point the diffusion slows down by a factor of 10. This is justified because in the end of dendritic regime of solidification the amount of melt is significantly reduced and is confined to the thin channels between the dendrites. This allows effectively limit the necessary diffusion length in the melt.