Mg_2Si的声子谱与力热性能的第一性原理计算

First principle calculation for phonon spectrum and thermodynamic properties of Mg_2Si

针对6xxx系铝合金的主要强化相Mg2Si的力热性能,采用密度泛函理论、第一性原理以及CASTEP软件进行了理论计算,得到了Mg2Si的平衡晶格常数、弹性系数、声子谱、声子态密度以及相关热力学参量,并对计算曲线进行了理论分析.结果表明:合金强化相Mg2Si的刚性较好,脆性较强,在合金中能起到强化作用.通过声子谱及声子态密度曲线可知,存在9条色散关系曲线,其中3条为声学波,6条为光学波,且频率在8.51 THz附近的格波振动较强.随着温度的增加,熵、焓均增加,而自由能减少.当温度低于200 K时,热容量随着温度的增加而明显增加;当温度高于400K后,热容量接近于常量,其计算结果与物理规律一致.

Aiming at the thermodynamic properties of main strengthening phase Mg2Si in 6xxx series aluminum alloys,the theoretical calculation was performed with the density functional theory,first principle and CASTEP software. The balance lattice constant,elastic coefficient,phonon spectrum,phonon density of states( DOS) and related thermodynamic parameters of Mg2Si were obtained,and the theoretical analysis was performed for the calculated curves. The results showthat the strengthening phase Mg2Si has good rigidity and large brittleness,and can play a role of reinforcement in the alloy. From the phonon spectrum and phonon DOS curves,it can be noted that there are nine dispersion relation curves,where three curves are acoustic waves and six curves are optical waves,and the vibration of lattice wave is stronger around8. 51 THz. With increasing the temperature,both entropy and enthalpy increase,while the free energy reduces. When the temperature is below200 K,the heat capacity significantly increases with increasing the temperature. When the temperature is above 400 K,the heat capacity is close to a constant. Furthermore,the calculated results are consistent with the physics laws.