npj: 纳米孪晶的软硬变化—受控于一种还是两种临界点？

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纳米孪晶材料作为一类有前景的纳米结构材料已被广泛研究，它具有强度高、延展性好、断裂韧性大、抗疲劳性能强、蠕变稳定性好等优异性能。最近出现了一个明显的争议，即关于纳米孪晶材料的强度如何随着孪晶厚度的减小而变化。虽然孪晶厚度降低到临界值以下时，在纳米孪晶Cu中观察到从硬化到软化的转变，但在纳米孪晶陶瓷和纳米孪晶金刚石中却是持续地硬化。如何揭示这种矛盾尚没有细致的研究报道。

来自美国布朗大学的高华健教授领导的团队，使用分子动力学（MD）对多晶nt-Pd和nt-Cu样品进行了模拟，并建立了不受MD尺寸和时间尺度限制的基本理论模型，研究了孪晶厚度降低到临界值以下时硬度的变化。他们的MD模拟和模型构建结果表明，在非常小的孪晶厚度(<λ_crit)下，变形受孪晶晶界（twin boundarys, TBs）的迁移控制，这些TB与在TB-晶界(grain boundary, GB)交叉点成核的孪晶部分位错有关。虽然孪晶偏区（twinning partials）成核受到高于Ts的位错源数量的限制，但相同的成核过程在低于T_s时，则会受到TB-GB交叉点局部应力集中的限制，其峰值应力水平随着TB间距的减小而减小，导致连续硬化。因此，软化温度Ts划分了从位错源数-控制（源-控）向位错应力值-控制（应力-控）的TB迁移转变。对于具有低T_s如nt-Cu的材料，“源-控”的TB迁移在很宽的温度范围内占优势，其材料强度先增加后减小，在临界孪晶厚度下，材料强度达到最大值，材料由硬化过渡到软化。对于具有高Ts的材料，在较宽的温度范围内存在“应力-控”的TB迁移，TBs间距减小时会出现连续硬化。他们的理论模型表明，原子键合越强，软化温度Ts越高；这一规律可应用于所有nt材料。

该文近期发表于npj Computational Materials 5: 2 (2019)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。

Transition from source- to stress-controlled plasticity in nanotwinned materials below a softening temperature 

Seyedeh Mohadeseh, Taheri Mousavi, Haofei Zhou, Guijin Zou & Huajian Gao 

Nanotwinned materials have been widely studied as a promising class of nanostructured materials that exhibit an exceptional combination of high strength, good ductility, large fracture toughness, remarkable fatigue resistance, and creep stability. Recently, an apparent controversy has emerged with respect to how the strength of nanotwinned materials varies as the twin thickness is reduced. While a transition from hardening to softening was observed in nanotwinned Cu when the twin thickness is reduced below a critical value, continuous hardening was reported in nanotwinned ceramics and nanotwinned diamond. Here, by conducting atomistic simulations and developing a theoretical modeling of nanotwinned Pd and Cu systems, we discovered that there exists a softening temperature, below which the material hardens continuously as the twin thickness is reduced (as in nanotwinned ceramics and diamond), while above which the strength first increases and then decreases, exhibiting a maximum strength and a hardening to softening transition at a critical twin thickness (as in nanotwinned Cu). This important phenomenon has been attributed to a transition from source- to stress-controlled plasticity below the softening temperature, and suggests that different hardening behaviors may exist even in the same nanotwinned material depending on the temperature and that at a given temperature, different materials could exhibit different hardening behaviors depending on their softening temperature. 

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