First-Principles Study Of The Elastic Properties Of Al₄Re And Al₂ReZn₂（Re=La, Ce）

Magnesium alloys have recently attained an increasing interest in weight reduction for the automotive industry because of their low density, high specific strength and stiffniess. However, the low strength, poor formability and limited ductility at room temperature restrict its wider application. Until now, a series of heat resistant Mg-Al based alloys have been explored. The recent developments of heat resistance Mg alloys are mainly concentrated on the improvement of conventional Mg-Al based alloys by reducing the Al content to relatively low level and alloying with third element, such as Si, rare earth (RE), and alkaline earth metals. The aims of these attempts are based on the idea of suppressing the formation of P-Mg17Al12phase in alloys, combining with the formation of high thermal stability Mg alloy and Al alloy particles, thus the heat resistance of the alloys is improved. For the ZAE674alloy, except for the formation of Al2REZn2and Al4RE phases, τ-Mg32(Al,Zn)49phase can also be found in the alloy. As important and typical phases, the structural and mechanical properties of Al2REZn2and A14RE can provide fundamental data on the development and design of Al-based alloys. Ce is vital and critical RE elements, so it is very necessary to further study the elastic properties of Al4Ce and Al2CeZn2.Elastic properties of Al4Ce and Al2CeZn2phases have been studied within density functional theory framework. With substitution of Zn for part Al, the lattice constant a of Al2CeZn2is reduced while c is increased. Al2CeZn2is thermodynamically more stable due to the lower formation enthalpy. The obtained elastic constants show that both Al4Ce and Al2CeZn2are mechanically stable. With the VRH approximation, the elastic modulus B, G and E are obtained. The elastic constants are also obtained. With exception of C66, Cij of Al2CeZn2is smaller than that of Al4Ce. Due to the lower bulk and shear moduli, Al2CeZn2has the lower resistance to volume change under applied pressure and the smaller resistance to shear deformation against external forces. Al2CeZn2is more ductile than Al4Ce although both two phases behave as brittle materials. Furthermore, the elastic anisotropy of Al2CeZn2is smaller than that of Al4Ce. Also, the Debye temperature has been calculated. The electronic structures, including density of states and charge density distributions are further investigated to uncover the underlying mechanism for elastic properties of Al2CeZn2and Al4Ce.Elastic properties of Al4La and Al2LaZn2phases have also been studied within density functional theory framework. With substitution of Zn for part Al, the lattice constant a of Al2LaZn2is reduced while c is increased. Al2LaZn2is thermodynamically more stable due to the lower formation enthalpy. The obtained elastic constants show that both Al4La and Al2LaZn2are mechanically stable. With the VRH approximation, the elastic modulus B, G and E are obtained. The elastic constants are also obtained. With exception of C12and C13, Cij of Al2LaZn2is larger than that of Al4La. Due to the higher bulk and shear moduli, Al2LaZn2has the higher resistance to volume change under applied pressure and the smaller resistance to shear deformation against external forces. Al2LaZn2is more ductile than Al4La while only Al2LaZn2behaves as brittle material. Furthermore, the elastic anisotropy of Al2LaZn2is smaller than that of Al4La. Also, the Debye temperature has been calculated. The electronic structures, including density of states and charge density distributions are further investigated to uncover the underlying mechanism for elastic properties of Al2LaZn2and Al4La.