Bonding in MgSi and AlMgSi Compounds Relevant to AlMgSi alloys

The bonding and stability of MgSi compounds relevant to AlMgSi alloys is investigated with the use of APW+lo DFT calculations. We show that the beta and beta'' phases found in the precipitation sequence are characterised by the presense of covalent bonds between SI-Si nearest neighbour pairs and covalent/ionic bonds between Mg-Si nearest neighbour pairs. We thus argue that a proposed model for the beta' phase is not correct due to the absence of this type of bonding. Furthermore, we propose that the metallurgical rule of thumb that successfull precipitation hardening alloys contain secondary(A) and ternary(B) elements that are smaller and larger than Al can be made more precise by requiring that the bond length of stable A-B, A-A, and B-B bonds should be comparable to the Al-Al bond length.