First-principles Study Of The Generalized-stacking-fault Energy In Mg₁₃₉Al₄Sn₁and Ti_4.5Si₃Me_0.5 With Hexagonal Structure

The metal Mg is regarded as a kind of structural material which has widely appliedfuture due to low density, high strength-to-weight ratio, high elastic modulus and otherfeatures. However, its HCP structure leads to limited number of slip systems and poorformability at room temperature which restrict its application. Many researchers expect toimprove its properties by grain refinement, alloying and other methods. Because alloying notonly improves its plasticity but also increases strength, it is considered to be a good way toimprove the overall performance. A large number of experiments indicate that additions ofAl and Sn can dramatically strengthen their plasticity at room temperature. But themechanism is not clearly known. Generalized stacking fault energy, GSF energy, can reflectthe difficulty for activation of slip systems.Obviously, it is helpful to take the inherent correlation between elemental additions andthe GSF energy of various slip systems into account to better understand the mechanism.Consequently, it is important to investigate effects of elemental additions on the GSF energyof common slip systems for Mg alloys; moreover, although the intermetallic compound,Ti₅Si₃, has many excellent characteristics, its hexagonal structure like Mg results in lowroom-temperature brittleness, which restricts its application. In order to improve itsroom-temperature brittleness and fracture toughness, researchers try to employ refininggrains, composite, alloying and other methods, in which alloying is thought to be aneffective way to improve ductility. The prismatic slip system {11|-00}[0001] is regarded asthe easiest activation due to the lowest CRSS （Critical Resolved Shear Stress）. Therefore, itis very important to investigate effects of the addition of alloying elements on GSF energiesof the slip system for selection of alloying elements.People tend to use computer simulations to carry out scientific researches with the rapid development of computer technology. The first-principles calculations based on DFT areconsidered as an excellent tool for studying the properties and microcosmic electronstructure of solid materials. Moreover, it has achieved certain results in studing ceramics,semiconductors, metals and other materials.In this paper, the first-principles calculations are used to study the effect of additions ofAl and Sn to pure Mg on GSF energies of the basal slip system {0001}<112|-0>, prismaticslip systems {1010}<112|-0>and {112|-0}<101|-1>, and pyramidal slip systems{101|-1}<112|-0>,{1012}<101|-1>, and {1122}<1123>. What is more, this method is alsoemployed to investigate the site occupancy behaviors of alloying elements in Ti₅Si₃bycalculating the formation enthalpy; moreover, the GSF energy is calculated to study theeffect of the addition of different elements （Sc, Y, Zr, V, Cr, Cu, Mn, Fe, Co and Ni） onprismatic slip system {11|-00}[0001].The calculations of Mg alloys show that the unstable stacking fault enegy （γus） isreduced due to dopings of Al and Sn, resulting in improving activity of basal slip system{0001}<112|-0>and the activation of pyramidal slip system {101|-1}<112|-0>and{1122}<1123>; the lower γusof pyramidal slip system {101|-1}<112|-0>than that ofprismatic slip system {1010}<112|-0>leads to the replacement of the {1010}<112|-0>bythe {101|-1}<112|-0>to be the second primary slip system by Al and Sn addtions; decreasingγusin basal slip system {0001}<112|-0>promotes the extension of the dislocation andhinders the formation of the cross slips, which facilitate the formation of multipledeformation-twins.The results of Ti₅Si₃indicate that the site occupancy behaviors of alloying elements inTi₅Si₃are decided by atomic radius. That is to say that the substitutional atoms larger than Tiare likely to occupy the Ti6gsites, while they smaller than Ti tend to occupy Ti4dsites.Moreover, the site occupancy behaviors of alloying elements have nothing to do with magnetism. For the addition of elements with relatively larger radius than Ti （Sc, Y and Zr）,only the addition of Y decreases γusof prismatic slip system {11|-00}[0001]. Whereas theaddition of elements whose radius are smaller than Ti （V, Cr, Cu, Mn, Fe, Co and Ni）,especially smaller than1.25, can dramatically reduce γus. For the element withoutmagnetism （V, Cr and Cu）, the smller the atomic radius is, the more significantly the γusdecreases. But the law is not found in elements with magnetism （Mn, Fe, Co and Ni）.Consequently, the magnetism may have the certain effect on decreasing γus.