纳米压痕下非晶难熔金属的晶体形成能力：分子动力学研究
Crystal forming ability of amorphous refractory metals under nanoindentation: a molecular dynamics study
AI总结：
该研究通过分子动力学模拟纳米压痕，揭示了非晶难熔金属V、Nb、Mo、Ta、W的晶体形成能力规律，发现其与内聚键强度相关，转变速率几乎与压头速度无关。
AI中文摘要：
非晶难熔金属涂层兼具高硬度与化学惰性，但其亚稳性使其在接触载荷下易发生机械诱导结晶（失透），且这类玻璃态物质重组为体心立方（bcc）母相的难易程度在整个难熔金属系列中尚不明确。我们通过熔体淬火法制备了非晶V、Nb、Mo、Ta和W，将各原子间势与从头算液体径向分布函数进行了验证，并通过大规模分子动力学纳米压痕对其进行了探测。压痕通过体相成核、生长与合并驱动局域非晶向bcc的转变。晶体形成能力（CFA，即bcc分数随深度变化的S型曲线的最大斜率）跨度约4倍，且按V>Mo>Nb>Ta>W的顺序递减；CFA随压头速度的变化满足CFA ∝v⁻ᵐ，平均指数为1.08，因此CFA·v几乎为常数，转变速率几乎与速度无关。发生转变的原子携带过量非仿射位移与局域剪切应变，而非静水压力，表明这是一条与剪切相关的位移型路径；转变的体相驱动力在最具抗性的元素W中最大，因此抗性与内聚键强度相关，而非热力学驱动力。早期类bcc序决定了转变的尖锐程度与深度，而半转变的机械功决定了持久晶核密度。晶粒总数平衡将成核、生长与合并关联至最终微观结构，其完整性并不遵循CFA的顺序。
英文摘要：
Amorphous refractory metal coatings combine high hardness with chemical inertness, yet their metastability makes them prone to mechanically induced crystallisation (devitrification) under contact loading, and how readily such a glass re-orders to its parent body centred cubic (bcc) crystal is unknown across the refractory series. We prepared amorphous V, Nb, Mo, Ta and W by melt quenching to 300 K, validated each interatomic potential against ab initio liquid radial distribution functions, and probed them by large scale molecular dynamics nanoindentation. Indentation drives a localised amorphous to bcc transformation by bulk nucleation, growth and coalescence. A crystal forming ability (CFA), the maximum slope of the sigmoidal bcc fraction versus depth curve, spans about a factor of four and decreases as V > Mo > Nb > Ta > W; it falls with indenter velocity as CFA $\propto v^{-m}$ with a mean exponent of 1.08, so CFA$\cdot v$ is nearly constant and the transformation rate is almost velocity independent. Transforming atoms carry excess non affine displacement and local shear strain, not hydrostatic pressure, marking a shear associated displacive pathway; the bulk driving force is largest for the most resistant element, W, so resistance tracks cohesive bond strength rather than the thermodynamic driving force. Early bcc like order sets the sharpness and depth of the transition, whereas the mechanical work to half transformation sets the persistent nucleus density. A grain population balance links nucleation, growth and coalescence to a terminal microstructure whose completeness does not follow the CFA order.
