Mechanical properties of lightweight metals from first principles orbital-free density functional theory

Abstract

Accurate quantum mechanics theory and a fast linear-scaling algorithm that OFDFT adopts can create a great synergy to understand underlying atomic-scale physics of material properties and to provide accurate predictions of mesoscale properties for novel materials. We employ OFDFT simulations to study mechanical properties of lightweight metals: FCC Al, HCP Mg, and BCC Mg-Li alloys. The accuracy of OFDFT is mainly governed by two approximations: an electron kinetic energy density functional (KEDF) and a local electron-ion pseudopotential (LPS). We propose and validate a new KEDF for semiconductors and a new LPS for Mg-Li alloys. First, we investigate dislocation structures in Al. OFDFT-optimized dislocation structures are consistent with an experimental estimation. We then calculate the Peierls stress (σp) of Al dislocations. We discover two possible screw dislocation structures (dissociated and undissociated), whose σps differ by two orders of magnitude. This result may resolve the decades-long mystery in FCC metals regarding the two orders of magnitude discrepancy in σp measurements. Next, we investigate plastic properties of various slip systems in Mg. We propose that strong anisotropies in stacking fault energy surfaces, cross-slip of screw dislocations to basal planes, and the compact nature of edge dislocations on non-basal planes are responsible for Mg's limited ductility. We then explicitly calculate the σp of Mg dislocations on the basal and prismatic slip planes. OFDFT-calculated σps are in excellent agreement with experiments. We predict a basal edge dislocation can move 59 times more readily than a prismatic one, which gives rise to the characteristically large anisotropy in Mg's plasticity. Next, we study plasticity of novel BCC Mg-Li alloys as potential lightweight metals. We propose alloys with 31-50 at.% Li can maximize potential strength while increasing ductility compared to Mg, with their σps predicted to be ~0.3 GPa. Finally, we propose a new KEDF for semiconductors via enhancing the semilocal vonWeizsäcker functional in the Wang-Govind-Carter KEDF. The enhancement factor is strongly correlated with the extent to which electron density is localized. Our new KEDF shows a clear improvement in accuracy, transferability, and efficiency compared to previous OF KEDFs. This result holds great promise for large-scale OFDFT simulations for semiconductors.