Anti-twinning in nanoscale tungsten.

Abstract

Nanomaterials often surprise us with unexpected phenomena. Here, we report a discovery of the anti-twinning deformation, previously thought impossible, in nanoscale body-centered cubic (BCC) tungsten crystals. By conducting in situ transmission electron microscopy nanomechanical testing, we observed the nucleation and growth of anti-twins in tungsten nanowires with diameters less than about 20 nm. During anti-twinning, a shear displacement of 1/3〈111〉 occurs on every successive {112} plane, in contrast to an opposite shear displacement of <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><mrow><mn>1</mn> <mo>/</mo> <mn>6</mn> <mo>〈</mo> <mover><mn>1</mn> <mo>¯</mo></mover> <mover><mn>1</mn> <mo>¯</mo></mover> <mover><mn>1</mn> <mo>¯</mo></mover> <mo>〉</mo></mrow> </mrow> </math> by ordinary twinning. This asymmetry in the atomic-scale shear pathway leads to a much higher resistance to anti-twinning than ordinary twinning. However, anti-twinning can become active in nanosized BCC crystals under ultrahigh stresses, due to the limited number of plastic shear carriers in small crystal volumes. Our finding of the anti-twinning phenomenon has implications for harnessing unconventional deformation mechanisms to achieve high mechanical preformation by nanomaterials.