Strong strain hardening in ultrafast melt-quenched nanocrystalline Cu: The role of fivefold twins

Abstract

Nanocrystalline and nanotwinned metals exhibit ultrahigh strength but suffer from low ductility due to the absence of the strain hardening effect. Here, we report sustained strong strain hardening up to 20% compressive strain in a melt-quenched nanocrystalline Cu structure, which contains numerous fivefold twins, stacking faults, and twin boundaries. Our molecular dynamics simulations reveal that the strong strain hardening results from the synergistic effect of constant nucleation and impedance of dislocations, restricted twin boundary migration, and abundant dislocation reactions in fivefold twin networks. Specifically, we find that fivefold twins both nucleate and impede dislocations, and the migration of fivefold twin boundary is restricted by the core of fivefold twins. Moreover, we observe a new migration mechanism, in which fivefold twin boundary migrates by two atomic planes directly, enabled by the gliding of two different Shockley partial dislocations in the opposite directions. Finally, dislocation transmission, which is adverse to strain hardening, occurs very scarcely in fivefold twins. This is caused by the large misfit strains in fivefold twins and abundant immobile dislocations generated by frequent dislocation reactions in fivefold twin networks. This work reveals the advantage of fivefold twins over nanotwins to overcome the strength-ductility trade-off.

Dispersion of Defects in TiO2 Semiconductor: Oxygen Vacancies in the Bulk and Surface of Rutile and Anatase

“…Nonstoichiometric titanium dioxide (TiO 2 ) has drawn broad interest due to its important role in photocatalysis, solar cells, and environmental procedures [1][2][3][4][5][6][7][8][9]. Under various experimental conditions, a broad range of O-vacancies can be formed in TiO 2 (bulk and surface), i.e., 5-10% [10].…”

Interactions between Dislocations and Penta-Twins in Metallic Nanocrystals

“…As such, twinning partials are able to cross-slip onto different constituent TBs of a penta-twin. Previous studies have shown that penta-twin core can often impede dislocations, inducing restricted migration of a TB of the penta-twin with only slight oscillation around its original location [16]. On the other hand, some simulation studies reported layer-by-layer TB migration induced by twinning partial propagation during annealing [9] or stress-relaxed [38] evolutions of penta-twins.…”

“…penta-twin with only slight oscillation around its original location [16]. On the other hand, some simulation studies reported layer-by-layer TB migration induced by twinning partial propagation during annealing [9] or stress-relaxed [38] evolutions of pentatwins.…”

“…Cross-slip across constituent TBs and the penta-twin core should be partly enabled by the intrinsic disclination stress field generated by the 7.33 • gap of five twin units (Figure 3). Complex and changeable routes of dislocation cross-slip suggest that dislocation motions in penta-twins should be more difficult than that in coplanar nanotwins, thereby resulting in a stronger strain hardening effect [16]. On the other hand, vacancy generation may play a role in adjusting the dislocation motions given that vacancies are commonly observed in the vicinity of the penta-twin core in penta-twinned nanomaterials [22,39], which could facilitate stress relaxation and enhance diffusion-related dislocation motions, e.g., dislocation climb (Figure 4).…”

“…As an extreme of crystal twinning, penta-twin emerges when five TBs simultaneously converge at a line parallel to their common {110} axis. Penta-twins have been widely observed in various of FCC metals and alloys in the form of nanowires, nanoparticles and thin films [9][10][11][12][13][14][15], which were found to be capable of effectively enhancing Young's modulus, yield strength and strain hardening capability of metallic nanomaterials [14,[16][17][18][19]. Such enhanced mechanical properties of penta-twinned nanomaterials were often ascribed to dislocation nucleation and impedance, as well as dynamic dislocation reactions in penta-twin networks, distinct from dislocation activities in conventional single or coplanar nanotwins [20,21] that are mostly restricted to the free paths parallel to TBs.…”