Contrasting the nature of plastic fluctuations in small-sized systems of BCC and FCC materials

Jérôme Weiss, Peng Zhang, Pengming Cheng, Gang Liu
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Abstract

It is now well established that, upon decreasing system sizes down to a few \(\upmu\)m or below, the nature of plasticity of metallic materials is changing. Two important features of this small-sizes plasticity are two size effects, which can be summed up as “smaller is stronger” and “smaller is wilder”, this last observation meaning that the jerkiness of plastic deformation becomes prominent at small enough system sizes. In FCC and HCP materials, this is now rather well understood within the framework of obstacle-controlled plasticity, from the key role of a scaling ratio between the system size L and an internal scale l mainly dictated by dislocation patterning in pure materials, or by the nature of extrinsic disorder in alloys. The situation is more complex in BCC materials, for which screw dislocation motion becomes lattice-controlled, i.e. is thermally activated, below a transition temperature \(T_a\). Therefore, in small-sized BCC systems, temperature, size and strain-rate effects combine to give rise to a complex landscape. We show, from an analysis of the literature as well as micropillar compression tests on Molybdenum performed with different sample sizes, under different temperatures and different applied strain-rates, that (i) near or above \(T_a\), the plasticity of pure BCC metals is athermal and obstacle-controlled, much like at bulk scales, therefore mimicking that of pure FCC metals; (ii) below \(T_a\) and for sample sizes larger than \(\sim\)1 \(\upmu\)m, BCC plasticity becomes lattice-controlled, this damping dislocation avalanches and thus reducing wildness; but (iii) for very small systems, still below \(T_a\), the role of screw dislocations on plasticity vanishes, i.e. is no more lattice-controlled, opening again the door for wild plastic fluctuations and jerkiness.

对比 BCC 和 FCC 材料小尺寸系统中塑性波动的性质
目前已经确定的是,当系统尺寸减小到几(\upmu\)米或以下时,金属材料的塑性性质正在发生变化。这种小尺寸塑性的两个重要特征是两种尺寸效应,可以概括为 "越小越强 "和 "越小越狂野",最后一个观察结果意味着,在足够小的系统尺寸下,塑性变形的突变性会变得非常突出。在 FCC 和 HCP 材料中,这种情况在障碍物控制塑性的框架内已经得到了很好的理解,因为在纯材料中,系统尺寸 L 与内部尺度 l 之间的比例关系主要是由位错图案决定的,在合金中则是由外在无序的性质决定的。在BCC材料中,情况更为复杂,因为在低于转变温度(T_a)时,螺旋位错运动变得由晶格控制,即被热激活。因此,在小尺寸 BCC 体系中,温度、尺寸和应变速率效应共同作用产生了复杂的景观。我们通过分析文献以及在不同温度和不同应用应变速率下用不同尺寸的样品对钼进行的微柱压缩测试表明:(i) 接近或高于 \(T_a\) 时,纯 BCC 金属的塑性是受热膨胀和障碍物控制的,这与块体尺度非常相似,因此模仿了纯 FCC 金属的塑性;(ii) 低于\(T_a\)且样品尺寸大于\(\sim\)1 \(\upmu\)m时,BCC塑性变为晶格控制,这将抑制位错雪崩,从而降低野性;但(iii) 对于非常小的系统,仍然低于\(T_a\),螺位错对塑性的作用消失,即不再是晶格控制。即不再由晶格控制,从而再次为剧烈的塑性波动和颠簸敞开大门。
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期刊介绍: Journal of Materials Science: Materials Theory publishes all areas of theoretical materials science and related computational methods. The scope covers mechanical, physical and chemical problems in metals and alloys, ceramics, polymers, functional and biological materials at all scales and addresses the structure, synthesis and properties of materials. Proposing novel theoretical concepts, models, and/or mathematical and computational formalisms to advance state-of-the-art technology is critical for submission to the Journal of Materials Science: Materials Theory. The journal highly encourages contributions focusing on data-driven research, materials informatics, and the integration of theory and data analysis as new ways to predict, design, and conceptualize materials behavior.
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