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In-Situ Investigation of Plasticity at Nano-Scale

机译:纳米级可塑性的原位研究

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摘要

Mechanical behavior of crystals is dictated by dislocation motion in response to applied force. While it is extremelyuddifficult to directly observe the motion of individual dislocations, several correlations can be made between theudmicroscopic stress-strain behavior and dislocation activity. Here, we present for the first time the differences observedudbetween mechanical behavior in two fundamental types of crystals: face-centered cubic, fcc (Au, Cu, AI, Ni, etc.) andudbody-centered cubic, bcc (W, Cr, Mo, Nb, etc.) with sub-micron dimensions subjected to in-situ micro-compression inudSEM chamber. In a striking deviation from classical mechanics, there is a significant increase in strength as crystal sizeudis reduced to 100nm; however in gold crystals (fcc) the highest strength achieved represents 44% of its theoreticaludstrength while in molybdenum crystals (bcc) it is only 7%. Moreover, unlike in bulk where plasticity commences in audsmooth fashion, both nano-crystals exhibit numerous discrete strain bursts during plastic deformation. These remarkableuddifferences in mechanical response of fcc and bcc crystals to uniaxial micro-compression challenge the applicability ofudconventional strain-hardening to nano-scale crystals. We postulate that they arise from significant differences inuddislocation behavior between fcc and bcc crystals at nanoscale and serve as the fundamental reason for the observeduddifferences in their plastic deformation. Namely, dislocation starvation is the predominant mechanism of plasticity inudnano-scale fcc crystals while junction formation and subsequent hardening characterize bcc plasticity, as confirmed byudthe microstructural electron microscopy. Experimentally obtained stress-strain curves together with video frames duringuddeformation and cross-sectional TEM analysis are presented, and a statistical analysis of avalanche-like strain bursts isudperformed for both crystals and compared with stochastic models.
机译:晶体的机械行为由响应于施加力的位错运动决定。虽然直接观察单个位错的运动极其困难,但微观应力应变行为与位错活动之间可以建立几种相关性。在这里,我们首次提出了两种基本类型的晶体的力学行为之间观察到的差异:面心立方晶体,fcc(Au,Cu,Al,Ni等)和体心立方晶体,bcc(W (Cr,Mo,Nb等)的亚微米尺寸在 udSEM室中进行原位微压缩。与经典力学截然不同的是,随着晶体尺寸减小至100nm,强度显着增加。但是,在金晶体(fcc)中,达到的最高强度代表其理论强度的44%,而在钼晶体(bcc)中,仅为7%。而且,与大量以可塑性开始的可塑性不同,两种纳米晶体在塑性变形期间均表现出大量离散的应变破裂。 fcc和bcc晶体对单轴微压缩的机械响应中的这些显着 ud差异,挑战了非常规应变硬化技术对纳米级晶体的适用性。我们假设它们是由纳米级的fcc和bcc晶体之间的 ud位错行为的显着差异引起的,并且是观察到的 ud塑性差异的根本原因。即,位错饥饿是 udnano级fcc晶体的可塑性的主要机理,而结结构的形成和随后的硬化则是bcc可塑性的特征,这已经通过微观结构电子显微镜得到了证实。呈现了实验获得的应力应变曲线以及未变形和横截面TEM分析期间的视频帧,并对两种晶体的雪崩样应变爆发进行了统计分析,并与随机模型进行了比较。

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