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中国物理学会期刊

难熔金属钒熔化行为的局域原子结构模拟与分析

CSTR: 32037.14.aps.69.20200185

Simulation and analysis of melting behavior of local atomic structure of refractory metals vanadium

CSTR: 32037.14.aps.69.20200185
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  • 采用经典分子动力学(molecular dynamics, MD)方法, 模拟了16000个钒原子在5种不同熔化速率(γ1 = 1 × 1011 K/s, γ2 = 1 × 1012 K/s, γ3 = 1 × 1013 K/s, γ4 = 1 × 1014 K/s 与γ5 = 1 × 1015 K/s)下原子结构的熔化行为. 结果表明: 不同熔化速率对难熔金属钒的熔点影响明显, 不过随着温度升高, 体系特征原子结构诸如体心立方(BCC)、六角密堆(HCP)、面心立方(FCC)、简单立方(SC)以及二十面体(ICO)的相对分布次序并不随熔化速率的改变而改变, 温度仍然是影响原子结构分布的主要因素. 通过从头算分子动力学(ab initio MD)与热力学分析发现, ICO能够在液态金属区域稳定存在, 一方面是因为其孤立团簇的相对稳定性和团簇寿命要优于晶体型原子团簇, 另一方面是因为其拥有相对较高的团簇熵与相对较低的自由能.

     

    By using large-scale atomic/molecular massively parallel simulator (LAMMPS) code, a molecular dynamics simulation is performed in the NPT ensemble at zero pressure to investigate the influence of melting rates γ on the evolutional characteristics of vanadium atomic structure such as body-centered cubic (BCC), hexagonal close-packed structure (HCP), face centered cubic (FCC), simple cubic (SC) and icosahedra (ICO) during the rapid melting of solid vanadium crystal at five different melting rates (γ1 = 1 × 1011 K/s, γ2 = 1 × 1012 K/s, γ3 = 1 × 1013 K/s, γ4 = 1 × 1014 K/s , γ5 = 1 × 1015 K/s), in which 16000 atoms in a cubic box under the periodic boundary condition are considered, and their motion equations are solved by Verlet’s algorithm in the velocity form in time steps of 1 fs. Constant pressure P and temperature T are imposed by a modified Nose-Hoover method for both P and T variables, and an embedded-atom model (EAM) potential is utilized. For identifying the local atomic structures of liquid and solid vanadium at different temperatures, a polyhedral template matching method (PTMM) is used by measuring the root-mean square deviation (RMSD), in which clusters are classified as the topology of the local atomic environment without any ambiguity in the classification. Subsequently, the variation of the potential energy, entropy and Gibbs free energy of FCC, HCP, BCC and ICO vanadium clusters are calculated through ab initio MD simulation in the canonical ensemble (NVT) at selected temperatures, and the lowest-energy dynamic structure and its corresponding static heating structure are also shown in this paper. Based on the above calculated results, it is found that the melting point of refractory metal vanadium increases obviously with the increase of heating rate, but the heating rate only presents a limited effect on the population of atomic structure for each of BCC, HCP, FCC, SC and ICO. Namely, the temperature still plays a dominant role in the rapid melting process of V rather than heating rate. Moreover, the ab initio MD simulation and thermodynamics analysis further reveal that lots of ICO clusters of vanadium can exist stably in the liquid region rather than in solid crystal, which is not only due to its higher stability and longer lifetime than those of crystalline atomic clusters, but also because ICO possesses higher entropy and lower Gibbs free energy in high temperature liquid region.

     

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