Sn合金化MgZn_2相及Mg_2Sn相结构稳定性的第一原理研究

First-principle study on structural stability of Sn alloying MgZn_2 phase and Mg_2Sn phase

采用基于密度泛函理论CASTEP和DMol程序软件包,从合金形成热、结合能、热力学性能和电子结构等方面,研究Sn合金化MgZn2相及Mg2Sn相的结构稳定性,探讨Sn合金化改善ZA62镁合金抗蠕变性能的机理。结果表明:当Sn和Al分别置换ZA62镁合金中MgZn2相的Zn(Ⅰ)和Zn(II)原子时,仅Sn与Al置换的MgZn2相中Zn(Ⅰ)原子能形成稳定的MgZn2固溶体结构,而Sn在MgZn2相中的固溶量有限;与合金化形成的固溶体结构相比,其稳定性比未合金化时的弱,而析出的第二相金属间化合物Mg2Sn的结构比MgZn2的更稳定。而不同温度下热力学性能的计算结果表明:合金体系中形成了结构稳定性强的Mg2Sn,其结构稳定性在温度373～473K的范围内并不因温度的升高而消失,仍比MgZn2的高;由于ZA62镁合金体系中形成了高热稳定性的Mg2Sn相,Sn合金化有利于ZA62镁合金抗蠕变性能的提高。电子态密度和Mulliken电子占据数的分析结果表明:与MgZn2、Mg2AlZn3及Mg2SnZn3固溶体相比,热稳定性强的Mg2Sn相形成的主要原因在于Mg2Sn体系中存在强烈的离子键与共价键的共同作用。

By CASTEP and DMol program based on the density functional theory,the heat of formation,the cohesive energy,the thermodynamic properties and the electronic structure of the alloying system were investigated to study the structural stability of Sn alloying MgZn2 phase and Mg2Sn phase and explain the mechanism of the influence of Sn alloying on improving the creep resistance properties of ZA62 magnesium alloy.The results show that the structure of these phases can exist and be stable when the Zn atoms at the I positions of the MgZn2 phase are substituted with Sn and Al,whereas,it is also found that Sn is little solved in MgZn2 phase.By comparing with the stable MgZn2 phase,it is found that the stability of MgZn2 phase is reduced with Sn addition,and the structure of intermetallic compound Mg2Sn is more stable than that of MgZn2 phase.By calculating the thermodynamic properties of different phases,it is found that the improved creep resistance properties of ZA62 magnesium alloy are caused by forming intermetallic compound Mg2Sn with higher structural stability which is not changed with the elevated temperature in the range of 373-473 K.The calculations of the density of states（DOS） and Mulliken electronic populations of the alloying system show that the form of Mg2Sn with the highest structural stability in ZA62 magnesium alloy with Sn addition attributes to the ionic bond and covalent bond in the bonding electron numbers compared with those of MgZn2 phase,Mg2AlZn3 and Mg2SnZn3 solid solutions.