Deformation twinning in nanocrystalline materials

Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target

“…The strong quadratic dependence of the elastic strain energy of a disclination on the size of the crystal precludes the five-fold twinned structures from appearing in bulk crystals and largely limits their habitat to nanoparticles and nanowires with sizes up to several tens of nanometers [79,[80][81][82][83]. The pentagonal twinned structures have also been reported for nanocrystalline metals synthesized by severe plastic deformation techniques [84][85][86], where the formation of the fivefold deformation twins is attributed to a sequential twinning occurring under conditions of changing orientation of external stresses [87,88]. Alternatively, the generation of high internal local stresses during annealing of nanocrystalline Cu has been predicted to lead to the formation of fivefold twins in MD simulations [89].…”

The role of Ta on twinnability in nanocrystalline Cu–Ta alloys

“…For example, Figure 3(a) shows that the ratio β is 1.11, 1.18, and 1.60 for Cu with no Ta, Cu with 2 nm radius semi-coherent Ta particle, and Cu with 5.4 nm radius Ta particle size, respectively. This suggests that similar to grain size as discussed in [14,15], Ta particles can be used to tailor twinnability, with coherent particles increasing the density of deformation twinning and incoherent particles promoting dislocation-based plasticity. This is also true for the dissolved Ta content in solid solution.…”

“…[12] Generally, these unique deviations in behavior are solely attributed to a continual reduction in grain size and an increase in the fraction of grain boundaries and triple junctions, which leads to experimentally reported mechanisms of deformation twinning, GB rotation/sliding and viscous flow. [13][14][15] Plastic instability [2,3,16,17] due to loss in the strain hardening behavior and grain growth (under both monotonic and cyclic loading) [18][19][20] have also been observed in various pure NC materials at small grain sizes. The ability to utilize such unique deformation responses advantageously depends heavily on our ability to recognize and to engineer them within NC metals.…”

Catch twin nucleation in action at atomic scale

“…1c, with further straining, one more SF formed in between 1-3 SFs, leading to the nucleation of a three-layer twin. This process of twin nucleation is intrinsically different from the previous twinning modes in which a twin embryo forms through the successive emission of partial dislocations layer-by-layer on consecutive {111} planes [6][7][8][9]. It also offers a possible explanation on the nucleation of a type of twins without macroscopic strain with the cooperative slip of three partials with the sum of their Burgers vectors equaling zero [10].…”

“…It has been believed that a deformation twin nucleates through the layer-by-layer slip of partial dislocations on consecutive close-packed atomic planes and that deformation twins should be suppressed in many face-centered cubic (FCC) metals with high unstable twin-fault energy, such as Al, Pt and Pd, in which the formation of twin embryos is difficult [6]. However, this is at odds with extensive experimental observations of deformation twins in these FCC nc metals [7][8][9]. Recently, Han and his colleagues presented atomicresolution in situ evidence of a new route of deformation twinning in nc Pt having a high twin-fault energy [5].…”