First-principles Study On The Ideal Strengths Of Typical Hcp Metals And L1₂Type Al₃X （X=Mg,Sc,Zr） Phases

A good combination of lightweight, specific strength and electromagneticshielding is the focus of interest in the aerospace and3C （Communication, Computer,Consumer Electronic Product） area. Beside the above mentioned, Al–Mg alloys havebecome the focus of people’s interests due to their excellent mechanical properties,such as superior electric, thermal conductivity, processing of low cost and easyrecycling. In order to improve their mechanical properties, despite the research inexperiment has been sustained, the theoretical study should also be conducted toreveal the intrinsic mechanism, which can provide guidance for the design anddevelopment of new materials. These years, the theoretical research of mechanicalproperties has received great improvement. For example, the mechanical propertiesare recently evaluated by the calculations of complete stress–strain curves rather thanelastic constants, which will provide an important opportunity to explore the excellentmechanical properties and meet the practical needs for Al–Mg alloys.In this paper, the first-principles calculations based on density functional theoryhave been carried out to investigate the stress–strain curves and ideal strengths ofseveral typical Hcp metals. The results reveal that the ideal shear strengths of theseHcp metals occur mainly on basal plane{0001}or prismatic plane{?}. Particularly,for basal plane the peak shear stress in （?）direction is smaller than thatin （?） direction. The calculated tensile strengths and elongationsin＜0001＞direction are broadly consistent with the available theoretical results.Furthermore, both the ideal shear and tension strengths become stronger with thedecreasing of c/a for these simple metals or transition metals. The calculatedelectronic structure further reveals the inherent mechanism of Hcp metals.Then the same method was also used to systematically research the elasticproperties and plastic deformations of L1₂type Al₃X （X=Mg, Sc, Zr）. The latticeconstants optimized are in good agreement with the available experimental andtheoretical values, showing the high reliability of our calculations. The calculatedformation enthalpies are negative, suggesting that these phases are stable from theview point of energetics. The obtained elastic properties show that Al₃Sc is the hardestand intrinsically brittle, while Al₃Mg is the softest and exhibited ductility tendency.The stress–strain curves demonstrate that the mechanical properties at small strainscan be assessed accurately by elastic constant calculations, but it should be measured by the ideal strength calculations at large strains. The ideal tensile and shear strengthsoccur on [110] tensile direction and{111}[?] shear system, respectively. In addition,both of the ideal tensile and shear strengths of Al₃Zr are the strongest, followed byAl₃Sc and Al₃Mg, and Al₃Zr is also the most ductile among three phases. Theelectronic density of states and the charge density distribution during structuralevolution demonstrate that the covalent bonding is stronger from Al₃Mg to Al₃Sc andAl₃Zr, so the Al₃Zr phase possesses higher ideal strength and ductility.