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Mechanical and Ignition-proof Properties of Cubic Mg-based Alloys

Author: WangNa
Tutor: TangBiYu
School: Xiangtan University
Course: Condensed Matter Physics
Keywords: Mg-based alloys First-principles calculation Electronic structure Mechanical property Ignition-proof property
CLC: TG146.22
Type: Master's thesis
Year: 2009
Downloads: 66
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Abstract


Magnesium alloys as lightweight structural materials have received increasing attention in the microelectronic, automotive and aerospace industries. However, the application of Mg-based alloys is still limited. Its poorer mechanical properties than those of aluminum alloy are the one reason, and the other important factor is that magnesium alloys are easily burnt especially at high temperatures. Therefore, much effort has been recently devoted to improve the properties of Mg-based alloys. Recent experiments show that it is possible to get excellent mechanical properties, as well as good ignition-proof performance, by addition of rare earth (RE) elements, which have high solid solubility in Mg-based alloys. However, research on RE magnesium alloys has so far been focused only on experiments, and the mechanism as to why the earth elements can improve the mechanical and ignition behavior of Mg-based alloys is still not clear.First-principles calculations have become an important tool for the accurate study of the electronic structures and mechanical properties of solids, as well as the study of oxidation in metal surfaces. Firstly, we report a systematic investigation of the structural, electronic and elastic properties of typical cubic precipitates of Mg-based alloys (Mg3Gd, Mg3Gd0.5Y0.5, Mg3Zn3Y2, Mg17Al12 and Mg24Y5) by first-principles calculations. Secondly, we carry out a systematic first-principles study on the structural and electronic properties of oxygen adsorption on the Mg3Nd (001) surface. The main contents of our work are:(1) First-principles calculations were performed to study structural, elastic and electronic properties of typical face-centered cubic (fcc) precipitates of Mg-based alloys (Mg3Gd, Mg3Gd0.5Y0.5 and Mg3Zn3Y2). The calculated results show that the substitution of part of the Gd with Y in Mg3Gd leads to a slight decrease in the cell volume (0.35%). The calculated negative formation enthalpies and the cohesive energies show that these typical fcc precipitates of Mg-based alloys have good alloying ability and structural stability. According to the calculated density of states of these phases, it is found that the highest structural stability of Mg3Zn3Y2 is attributed to an increase in the bonding electron numbers below the Fermi level. In addition, the elastic constants Cij of these phases were also calculated, and the bulk modulus B, shear modulus G, Young’s modulus Y, Poisson’s ratioνand anisotropy value A of polycrystalline materials were derived from the elastic constants. The mechanical properties are further discussed.(2) Using First-principles calculations, the structural and elastic properties of two important phases (Mg17Al12 and Mg24Y5) in Mg-based alloys have been studied. The obtained equilibrium structural parameters for both phases agree very well with experimental data. The calculated negative cohesive energy and formation energy show that both cubic precipitates have strong structural stability as well as good alloying ability. Three independent single-crystal elastic constants (C11, C12 and C44) at zero pressure as well as polycrystalline mechanical parameters such as bulk modulus B, shear modulus G, Young’s modulus Y, Poisson’s ratioνand anisotropy value A for both phases have been calculated. The mechanical properties of the cubic phases such as ductility and tenacity are further analyzed and discussed.(3) Based on the density functional theory, the stability of three possible Mg3Nd (001) surfaces is investigated, and the structural and electronic properties of O adsorption on the most stable Mg3Nd (001) surface are studied. The calculated results show that the most favorable adsorption site is the (2Nd+Mg) hollow site, and the adsorption energy decreases as the coverage increases. At low coverage O atoms prefer Nd atoms to Mg atoms as nearest neighbors. When the coverage increases to a full monolayer, some O atoms sink into the alloy surface and stay between Nd and Mg atoms. The electronic structures of the adsorption system show that for all coverage the interaction between O and the alloy surface mainly arises from the hybridization of O 2p states and Nd 5d states, while the interaction between Mg 3s states and O 2p states becomes noticeable at the high coverage. The present study reveals that during the initial oxidation stage of the alloy, Nd atoms can get a priority of oxidation, followed by Nd rich oxide film.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Non - ferrous metals and their alloys > Light non-ferrous metals and their alloys > Magnesium
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