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Transition from source- to stress-controlled plasticity in nanotwinned materials below a softening temperature (低于软化温度时纳米孪晶材料塑性的源-控向应力-控转变)
发布时间:2019-03-05

Transition from source- to stress-controlled plasticity in nanotwinned materials below a softening temperature (低于软化温度时纳米孪晶材料塑性的源-控向应力-控转变)
Seyedeh Mohadeseh, Taheri Mousavi, Haofei Zhou, Guijin Zou & Huajian Gao 
npj Computational Materials 5:2 (2019)
doi:s41524-018-0140-5
Published online:04 January 2019
Abstract| Full Text | PDF OPEN

摘要:纳米孪晶材料是一种具有高强度、良好延展性、断裂韧性大、抗疲劳性能好、蠕变稳定性好等优异性能的纳米结构材料。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。当孪晶厚度降低到临界值以下时,纳米孪晶Cu发生了从硬化到软化的转变,而在陶瓷和金刚石中则发生了连续硬化。本研究通过原子模拟和纳米孪晶PdCu系统的理论模型构建,发现存在一个软化温度,当低于该软化温度时,材料随孪晶厚度减小而不断硬化(如纳米孪晶陶瓷和金刚石),而高于该软化温度时,其强度先增加后降低,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(如纳米孪晶Cu)。这一重要现象归因于在软化温度以下,塑性从“源-控”向“应力-控”的转变。同时,这一现象表明,即使在相同的纳米孪晶材料中,也可能存在不同的硬化行为,且在一定的温度下,不同的材料在不同的软化温度下也会表现出不同的硬化行为   

Abstract: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. 

Editorial Summary

Nanotwinned materials: Plasticity below a softening temperature 纳米孪晶材料:软化温度下的塑性转变(自拟) 

该研究证明了纳米孪晶(nanotwined materials, nt)材料存在一个软化温度Ts,温度低于Ts时材料随着孪晶厚度的减小而持续硬化,而温度高于Ts时,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。来自美国 Brown UniversityHuajian Gao教授领导的团队,使用分子动力学(MD)对多晶nt-Pdnt-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。研究结果表明,在非常小的孪晶厚度(<λcrit)下,变形受孪晶晶界(twin boundaries, TBs)的迁移控制,这些TB与在TB-晶界(grain boundaries, GBs)交叉点成核的孪晶部分位错有关。虽然孪晶部分的成核受限于高于Ts的位错源的数量,但相同的成核过程在低于Ts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。该理论模型提示,原子键合越强,软化温度越高。他们所观察到的规律可适用于所有nt材料。

There exists a softening temperature, Ts, for nano-twinned (nt) materials, below which the material hardens continuously as the twin thickness is reduced, 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. A team led by Prof. Huajian Gao from Brown University in the United States established the basic phenomenon thorough molecular dynamics (MD) simulations of polycrystalline nt-Pd and nt-Cu samples, and by theoretical modeling that is not subjected to the usual limitations of MD in size and time scale. The change in hardness when the thickness of the nt-materials is reduced below the critical value. Their simulation and modeling results reveal that at very small twin thicknesses (<λcrit), the deformation is governed by the migration of twin boundaries (TBs) associated with twinning partial dislocations nucleated at TB–grain boundaries (TBs) intersections. While the nucleation of twinning partials is limited by the number of dislocation sources above Ts, below Ts the same nucleation process becomes limited by local stress concentration at the TB–GB intersections, whose peak stress level, decreases with reduced TB spacing, leading to continuous hardening. Thus, the softening temperature Ts demarcates a transition from source- to stress-controlled TBs migration. The theoretical model suggests that the stronger the atomic bonding, the higher the softening temperature, and that the observed behavior could be generic to all nt-materials.

 
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