Bonding in MgSi and AlMgSi Compounds Relevant to AlMgSi Alloys

TL;DR

This study uses advanced DFT calculations to analyze the bonding and stability of MgSi and AlMgSi compounds, revealing covalent and ionic bonds in key phases relevant to AlMgSi alloys.

Contribution

It provides new insights into the bonding nature and stability of MgSi, AlMgSi, U1, and U2 phases, especially highlighting Al-Si networks and charge transfer mechanisms.

Findings

β and β'' phases have covalent Si-Si bonds and covalent/ionic Mg-Si bonds.

U1 and U2 phases feature tightly bound Al-Si networks.

Charge transfer from Mg stabilizes Al-Si bonding in U phases.

Abstract

The bonding and stability of MgSi and AlMgSi compounds relevant to AlMgSi alloys is investigated with the use of (L)APW+(lo) DFT calculations. We show that the $\beta$ and $\beta''$ phases found in the precipitation sequence are characterised by the presence of covalent bonds between Si-Si nearest neighbour pairs and covalent/ionic bonds between Mg-Si nearest neighbour pairs. We then investigate the stability of two recently discovered precipitate phases, U1 and U2, both containing Al in addition to Mg and Si. We show that both phases are characterised by tightly bound Al-Si networks, made possible by a transfer of charge from the Mg atoms.