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 1/6〈1¯1¯1¯〉 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.

A systematic study on the deformation mechanisms of molybdenum (Mo) nanowires (NWs) was conducted using molecular dynamics simulations. Both axial orientation and wire thickness were found to play important roles in determining the deformation pathways. In the NWs with orientation 〈110〉/{111}, full dislocation plasticity is referentially activated on {110} planes. For both 〈100〉/{110} and 〈100〉/{100} NWs, twinning is the dominant mechanism with {112} being the coherent twin boundaries. A progressive slip process leads to a uniform elongation of 41% and the 〈100〉 wire axis reorients to 〈110〉. For 〈100〉/{100} NWs, the reorientation mechanism ceases to operate when the diameter d < 1 nm or d > 8 nm. The atomic chains are energetically preferred for ultrathin NWs after yielding due to the resemblance of the surface to the close-packed bcc planes, while multiple slip systems tend to be activated for larger NWs. Finally, a theoretical model is proposed to explain the underlying mechanism of size dependence of the yield stress.

The interest in promoting deformation twinning for plasticity is mounting for advanced materials. In contrast to disordered grain boundaries, highly organized twin boundaries are beneficial to promoting strength-ductility combination. Twinning deformation typically involves the kinetics of stacking faults, its interplay with dislocations, as well as the interactions between dislocations and twin boundaries. While the latter has been intensively studied, the dynamics of stacking faults has been rarely touched upon. In this work, we report new physical insights on the stacking fault dynamics in twin induced plasticity (TWIP) steels. The atomistic simulation is made possible by a newly introduced approach: meta-atom molecular dynamics simulation. The simulation suggests that the stacking fault interactions are dominated by dislocation reactions that take place spontaneously, different from the existing mechanisms. Whether to generate a single stacking fault, or a twinning partial and a trailing partial dislocation, depends upon a unique parameter, namely the stacking fault energy. The latter in turn determines the deformation twinning characteristics. The complex twin-slip and twin-dislocation interactions demonstrate the dual role of deformation twins as both the dislocation barrier and dislocation storage. This duality contributes to the high strength and high ductility of TWIP steels. 2016 Elsevier Ltd