Anomalous hardening in magnesium driven by a size-dependent transition in deformation modes
Anomalous hardening in magnesium driven by a size-dependent transition in deformation modes
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DOI：
10.1016/j.actamat.2017.10.033
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发表时间：
2016-12
期刊：
影响因子：
9.4
通讯作者：
G. Sim;Gyuseok Kim;Steven Lavenstein;M. H. Hamza;H. Fan;J. El-Awady
中科院分区：
文献类型：
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作者：
G. Sim;Gyuseok Kim;Steven Lavenstein;M. H. Hamza;H. Fan;J. El-Awady
在这里，我们报告了一项综合研究，结合原位扫描电镜实验和原子模拟来量化晶体尺寸对室温下a轴取向Mg单晶变形模式转变的影响。实验结果表明，变形主要由拉伸孪晶的形核和扩展主导。孪晶扩展所需的应力随试样尺寸的减小而增大，表现出典型的“越小越强”的行为。此外，首次报道了直径大于~ 18 μm的微晶的应变硬化异常增加，这是由孪晶和位错孪晶相互作用引起的。随着微晶尺寸的增大，硬化速率逐渐向体响应方向降低。在18 μm以下，变形以单拉伸孪晶的形核扩展为主，孪晶中存在基底滑移活动，未发生明显的硬化。此外，分子动力学模拟表明，在几百纳米以下的晶体尺寸中，由孪晶介导的塑性向位错介导的塑性转变。在模拟和实验的基础上，提出了双取向镁单晶从纳米尺度到体尺度的变形机理图。目前预测的孪晶取向Mg单晶的尺寸影响变形机制有助于更好地理解位错塑性和孪晶塑性之间的竞争。
Here, we report a comprehensive study that combinesin situscanning electron microscopy experiments and atomistic simulations to quantify the effect of crystal size on the transformation in deformation modes in a-axis oriented Mg single crystals at room temperature. The experimental results indicate that the deformation is dominated by the nucleation and propagation of tensile twins. The stress required for twin propagation was found to increase with decreasing sample size, showing a typical “smaller is stronger” behavior. Furthermore, an anomalous increase in strain hardening is first reported for microcrystals having diameters larger than ∼18 μm, which is induced by twin-twin and dislocation-twin interactions. The hardening rate gradually decreases toward the bulk response as the microcrystal size increases. Below 18 μm, deformation is dominated by the nucleation and propagation of a single tensile twin followed by basal slip activity in the twinned crystal, leading to no apparent hardening. In addition, molecular dynamics simulations indicate a transition from twinning mediated plasticity to dislocation mediated plasticity for crystal sizes below a few hundred nanometers in size. A deformation mechanism map for twin oriented Mg single crystals, ranging from the nano-scale to bulk scale is proposed based on the current simulations and experiments. The current predicted size-affected deformation mechanism of twin oriented Mg single crystals can lead to better understanding of the competition between dislocations plasticity and twinning plasticity.