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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mrow><mml:mn>1</mml:mn> <mml:mo>/</mml:mo> <mml:mn>6</mml:mn> <mml:mo>〈</mml:mo> <mml:mover><mml:mn>1</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mover><mml:mn>1</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mover><mml:mn>1</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mo>〉</mml:mo></mml:mrow> </mml:mrow> </mml: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.