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锆合金(如: 锆铌(Zr-Nb)合金)的辐照损伤问题是裂变堆结构材料和燃料棒包壳材料设计的关键, 而深入理解辐照损伤的物理机制, 往往需借助于原子尺度的计算模拟, 如: 分子动力学和第一性原理等. 针对随机置换固溶体合金的模拟, 首先需构建能反映合金元素随机分布特征的大尺寸超胞, 然而第一性原理计算量大, 不宜使用过大(如 ≥ 200原子)超胞. 通常第一性原理计算使用的是特殊准随机(SQS)超胞, SQS超胞可部分反映合金元素的随机分布特性, 但对于特定组分只对应一种构型, 这种模型是否能反映真实随机置换固溶体中多种局域构型的统计平均还有待进一步研究验证. 分子动力学可在更大的尺度上进行计算模拟, 能够通过随机取代(RSS)模型研究更多的合金构型, 因此, 本文基于RSS超胞模型及SQS扩展超胞模型, 运用分子动力学方法对Zr-Nb合金进行了研究. 首先通过构型误差分析确定了能真实反映固溶体合金性能统计性的RSS超胞的临界尺寸; 然后计算比较了Zr-Nb合金SQS扩展超胞和一系列RSS超胞的晶格常数、形成能和能量-体积关系. 研究表明, 利用SQS超胞模拟得到的固溶体的晶格常数、形成能和能量体积曲线与一系列RSS超胞的对应统计值接近, 因而SQS超胞可用于研究随机置换固溶体合金.Irradiation damage to zirconium alloys (e.g., zirconium niobium (Zr-Nb) alloy) is the key to the design of fission-reactor structural materials and fuel rod cladding materials. Atomic scale computational simulations such as molecular dynamics and first principles are often needed to understand the physical mechanism of irradiation damage. For the simulation of randomly substitutional solid solution, it is necessary to construct large-sized supercells that can reflect the random distribution characteristics of alloy elements. However, it is not suitable to use large-size supercells (such as ≥ 200 atoms) for first principle calculation, due to the large computational cost. Special quasirandom supercells (SQS) are usually used for first principles calculation. The SQS can partly reflect the random distribution characteristics of alloy elements, but it only corresponds to one configuration for specific components, hence whether this model can reflect the statistical average of multiple local configurations in a real randomly substitutional solid solution is still an open question, and needs further studying and verifying. Molecular dynamics (MD) simulation can be carried out on the randomly substitutional solid solution with a larger scale based on random substitution (RSS) method, these supercells include more local configurations. Therefore, the MD studies of Zr-Nb alloy are carried out for the RSS and SQS-extended supercells. The critical size of RSS supercell which can truly reflect the statistical properties of solid solution alloy is determined. Then the lattice constant, formation energy and energy-volume relationship of SQS-extended supercell of Zr-Nb alloy and a series of RSS supercells are calculated and compared. The results show that the lattice constants, the formation energy and energy volume curves of the solid solution obtained by SQS supercell simulation are close to a series of corresponding statistical values of the physical properties of RSS supercells, so the SQS supercells can be used to study the random substitution of solid solution alloys.
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Alloy a/Å c/Å Ec/(eV·atom–1) B0/GPa Zr0.75Nb0.25(bcc) 3.527 6.450 96 3.531 6.447 100 Zr0.75Nb0.25(hcp) 3.200 5.097 6.468 144 3.202 5.100 6.477 142 *L12-Zr3Nb 4.49 6.45 107 Zr0.5Nb0.5(bcc) 3.442 6.745 118 3.441 6.744 117 *B2-ZrNb 3.48 6.69 116 Zr0.25Nb0.75(bcc) 3.366 7.127 153 3.367 7.124 156 *L12-ZrNb3 4.34 6.96 100 
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