Materials Science and Engineering Program, Department of Electrical and Computer Engineering, Texas A & M University , College Station, TX 77843-3128, USA.
Sci Rep. 2013;3:1061. doi: 10.1038/srep01061. Epub 2013 Jan 14.
Strain hardening capability is critical for metallic materials to achieve high ductility during plastic deformation. A majority of nanocrystalline metals, however, have inherently low work hardening capability with few exceptions. Interpretations on work hardening mechanisms in nanocrystalline metals are still controversial due to the lack of in situ experimental evidence. Here we report, by using an in situ transmission electron microscope nanoindentation tool, the direct observation of dynamic work hardening event in nanocrystalline nickel. During strain hardening stage, abundant Lomer-Cottrell (L-C) locks formed both within nanograins and against twin boundaries. Two major mechanisms were identified during interactions between L-C locks and twin boundaries. Quantitative nanoindentation experiments recorded show an increase of yield strength from 1.64 to 2.29 GPa during multiple loading-unloading cycles. This study provides both the evidence to explain the roots of work hardening at small length scales and the insight for future design of ductile nanocrystalline metals.
应变硬化能力对于金属材料在塑性变形过程中实现高延展性至关重要。然而，大多数纳米晶金属的应变硬化能力固有较低，仅有少数例外。由于缺乏原位实验证据，纳米晶金属中应变硬化机制的解释仍然存在争议。在这里，我们通过使用原位透射电子显微镜纳米压痕工具，直接观察到纳米晶镍中的动态应变硬化事件。在应变硬化阶段，在纳米晶粒内和孪晶界上都形成了丰富的 Lomer-Cottrell（L-C）位错锁。在 L-C 位错锁与孪晶界之间的相互作用中，确定了两种主要的机制。定量纳米压痕实验记录显示，在多次加载-卸载循环过程中，屈服强度从 1.64 增加到 2.29 GPa。这项研究不仅为解释小尺寸应变硬化的根源提供了证据，也为未来设计韧性纳米晶金属提供了思路。
