Mg1-xZnx合金的彈性和熱力學(xué)性質(zhì)的第一性原理研究

Mg1-xZnx合金的彈性和熱力學(xué)性質(zhì)的第一性原理研究FIRST PRINCIPLES STUDY ON ELASTIC AND THERMODYNAMIC PROPERTIES OF Mg1-xZnx ALLOYS

基于密度泛函理論及密度泛函微擾理論為基礎(chǔ)的第一性原理計算，采用虛晶近似的方法，研究了具有hcp結(jié)構(gòu)且Zn含量在2%（原子分?jǐn)?shù)）范圍內(nèi)的8種Mg1-xZnx合金的晶格常數(shù)、彈性性質(zhì)和熱力學(xué)性質(zhì)。通過優(yōu)化結(jié)構(gòu)計算Mg和Zn含量不同的Mg1-xZnx合金彈性常數(shù)，對Young’s模量、Poisson比、彈性各向異性等進行了詳細(xì)分析，給出了Mg及Mg1-xZnx合金的晶格振動Helmholtz自由能、內(nèi)能、熵和定容比熱容等隨溫度變化情況。結(jié)果表明，隨著Zn含量增加晶格常數(shù)a和c均相應(yīng)減小，彈性常數(shù)、自由能和內(nèi)能增大，熵及定容比熱容減??；另一方面，隨溫度升高，Zn含量對自由能和熵的影響程度增大，而對定容比熱容的影響程度先增大后減小。Mg1-xZnx合金中Zn含量的增加有利于提高材料硬度和韌性，但也增大了材料的各向異性。

As one of the lightest metal material in current industrial application, Mg alloys are being widely used in automotive, aircraft, aerospace and biomedical industries because of their super high strength-to-weight ratio and biodegradability. However, their limited ductility and workability at room temperature have become a bottleneck for many applications. Therefore, it has become critically important to obtain the Mg alloys with improved strength and ductility. On the other hand, Zn is a transition metal element, often applied to improve the mechanical properties. Also it has basic safety for biomedical applications. So the Mg-Zn alloys have attracted considerable attentions in recent years. Extensively investigated experiments indicated that the hardness of Mg-Zn alloys increases with increasing Zn content. However, there are hardly reported about their mechanical properties and not theoretically investigated on the thermodynamic properties of Mg-Zn alloys. In this work, first-principles investigations are performed on lattice parameters, elastic properties and thermodynamic properties of hexagonal-close-packed (hcp) Mg and eight kinds of Mg1-xZnx alloys with different content of Zn less than 2% (atom percent), using the virtual crystal approximation in the frame of the density functional theory and the density functional perturbation theory. The elastic constants of Mg and Mg1-xZnx alloys with different Zn content are investigated by using optimized lattice, and the Young's modulus, Poisson ratio and elastic anisotropy are analyzed in detail. Also, the thermodynamic properties including Helmholtz free energy, internal energy, entropy and constant volume heat capacity of these alloys system as functions of temperature are discussed. The results show that with increasing Zn content in Mg1-xZnx alloys, the lattice constants a and c, the entropy and constant volume heat capacity of Mg1-xZnx alloys decrease, while the elastic constants, Helmholtz free energy and internal energy of Mg1-xZnx alloys increase correspondingly. On the other hand, further discussions find that the effects of Zn content on free energy and entropy of Mg1-xZnx alloys enhance and the effects on heat capacity of these alloys at constant volume first increase, then decrease as the temperature rises. In summary, it can be given the conclusions that the high content of Zn in Mg1-xZnx alloys is beneficial to increasing the hardness and ductility of Mg1-xZnx alloys, but decreasing the isotropy of alloy materials.

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