Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA.
Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA.
Nat Mater. 2015 Jun;14(6):594-600. doi: 10.1038/nmat4228. Epub 2015 Mar 9.
Twinning is a fundamental deformation mode that competes against dislocation slip in crystalline solids. In metallic nanostructures, plastic deformation requires higher stresses than those needed in their bulk counterparts, resulting in the 'smaller is stronger' phenomenon. Such high stresses are thought to favour twinning over dislocation slip. Deformation twinning has been well documented in face-centred cubic (FCC) nanoscale crystals. However, it remains unexplored in body-centred cubic (BCC) nanoscale crystals. Here, by using in situ high-resolution transmission electron microscopy and atomistic simulations, we show that twinning is the dominant deformation mechanism in nanoscale crystals of BCC tungsten. Such deformation twinning is pseudoelastic, manifested through reversible detwinning during unloading. We find that the competition between twinning and dislocation slip can be mediated by loading orientation, which is attributed to the competing nucleation mechanism of defects in nanoscale BCC crystals. Our work provides direct observations of deformation twinning as well as new insights into the deformation mechanism in BCC nanostructures.
孪生是一种基本的变形模式，与晶体固体中的位错滑移竞争。在金属纳米结构中，塑性变形需要比其体相对应物更高的应力，从而导致“越小越强”的现象。这种高应力被认为有利于孪生而不是位错滑移。面心立方 (FCC) 纳米晶体中的孪生已得到充分证明。然而，体心立方 (BCC) 纳米晶体中的孪生仍然没有被探索。在这里，通过使用原位高分辨率透射电子显微镜和原子模拟，我们表明孪生是 BCC 钨纳米晶体的主要变形机制。这种变形孪生是赝弹性的，表现在卸载过程中可逆的退孪生。我们发现孪生和位错滑移之间的竞争可以通过加载方向来调节，这归因于纳米体 BCC 晶体中缺陷的竞争形核机制。我们的工作提供了对变形孪生的直接观察，并为 BCC 纳米结构的变形机制提供了新的见解。
