Chuang Deng

Enabling Ultrahigh Plastic Flow and Work Hardening in Twinned Gold Nanowires

By using molecular dynamics simulations, we show that significant strain hardening and ultrahigh flow stresses are enabled in gold nanowires containing coherent (111) growth twins when balancing nanowire diameter and twin boundary spacing at the nanoscale. A fundamental transition in mechanical behavior occurs when the ratio of diameter to twin boundary spacing is larger than 2.14. A model based on site-specific dislocation nucleation and cross-slip mechanisms is proposed to explain the size dependence of flow behavior in twinned nanowires under tensile loading.

Atomistic mechanisms of cyclic hardening in metallic glass

Molecular dynamics with an embedded-atom method potential is used to simulate the nanoindentation of Cu63.5Zr36.5 metallic glasses. In particular, the effects of cyclic loading within the nominal elastic range on the overall strength and plasticity of metallic glass are studied. The simulated results are in line with the characteristics of experimentally observed hardening effects. In addition, analysis based on local von Mises strain suggests that the hardening is induced by confined microplasticity and stiffening in regions of the originally preferred yielding path, requiring a higher applied load to trigger a secondary one.

Comparison of molecular dynamics simulation methods for the study of grain boundary migration

In the present study, grain boundary (GB) mobility was determined by molecular dynamics (MD) simulations using two different techniques: the applied strain method and the adapted interface random walk method. The first method involves a driving force while the second method does not. Nevertheless, both methods led to essentially the same values of the GB mobility. This shows that the GB mobility is independent of the nature of the driving force, provided that it is low enough that the linear velocity?driving force relationship is properly sampled. The case studied here can be viewed as a validated reference case that can be used in future studies to test new techniques to determine the GB mobility. For this purpose we provide the full information about the interatomic potential we employed and the initial atomic configurations. Finally, we use the obtained results to discuss whether any existing MD simulation data agree with experimental data on pure metals.

Strong Hall–Petch Type Behavior in the Elastic Strain Limit of Nanotwinned Gold Nanowires

Pushing the limits of elastic deformation in nanowires subjected to stress is important for the design and performance of nanoscale devices from elastic strain engineering. Particularly, introducing nanoscale twins has proved effective in rising the tensile strength of metals. However, attaining ideal elastic strains in nanotwinned materials remains challenging, because nonuniform twin sizes locally affect the yielding behavior. Here, using in situ high-resolution transmission electron microscopy tensile testing of nanotwinned [111]-oriented gold nanowires, we report direct lattice-strain measurements that demonstrate a strong Hall-Petch type relationship in the elastic strain limit up to 5.3%, or near the ideal theoretical limit, as the twin size is decreased below 3 nm. It is found that the largest twin in nanowires with irregular twin sizes controls the slip nucleation and yielding processes in pure tension, which is in agreement with earlier atomistic simulations. Continuous hardening behavior without loss of strength or softening is observed in nanotwinned single-crystalline gold nanowires, which differs from the behaviors of bulk nanocrystalline and nanotwinned-nanocrystalline metals. These findings are of practical value for the use of nanotwinned metallic and semiconductor nanowires in strain-engineered functional microdevices.

Influences of triple junctions on stress-assisted grain boundary motion in nanocrystalline materials

Stress-assisted grain boundary motion is among the most studied modes of microstructural evolution in crystalline materials. In this study, molecular dynamics simulations were used to systematically investigate the influences of triple junctions on the stress-assisted motion of symmetric tilt grain boundaries in Cu by considering a honeycomb nanocrystalline model. It was found that the grain boundary motion in nanocrystalline models was highly sensitive to the loading mode, and a strong coupling effect which was prevalent in bicrystal models was only observed when simple shear was applied. In addition, the coupling factor extracted from the honeycomb model was found to be larger andmoresensitivetotemperaturechangethanthatfrombicrystalmodelsforthe same type of grain boundary under the same loading conditions. Furthermore, the triple junctions seemed to exhibit unusual asymmetric pinning effects to the migrating grain boundary and the constraints by the triple junctions and neighboring grains led to remarkable non-linear grain boundary motion in directions both parallel and normal to the applied shear, which was in stark contrasttothatobservedinbicrystalmodels. Inaddition,dislocationnucleation and propagation, which were absent in the bicrystal model, were found to play animportantroleonshear-inducedgrainboundarymotionwhentriplejunctions were present. In the end, a generalized model for shear-assisted grain boundary motion was proposed based on the findings from this research.

Interaction of shear-coupled grain boundary motion with crack: Crack healing, grain boundary decohesion, and sub-grain formation

Stress-driven grain boundary motion is one of the main mechanisms responsible for microstructural evolution in polycrystalline metals during deformation. In this research, the interaction of shear-coupled grain boundary motion (SCGBM) in face-centered cubic metals with crack, which is a common type of structural defects in engineering materials, has been studied by using molecular dynamics simulations in simple bicrystal models. The influences of different parameters such as metal type, temperature, grain boundary structure, and crack geometry have been examined systematically. Three types of microstructural evolution have been identified under different circumstances, namely, crack healing, grain boundary decohesion, and sub-grain formation. The underlying atomistic mechanisms for each type of SCGBM-crack interaction, particularly grain boundary decohesion and crack healing, have also been examined. It is found that crack healing is generally favoured during the SCGBM-crack interaction at relatively high...

Disclination mediated dynamic recrystallization in metals at low temperature.

Recrystallization is one of the most important physical phenomena in condensed matter that has been utilized for materials processing for thousands of years in human history. It is generally believed that recrystallization is thermally activated and a minimum temperature must be achieved for the necessary atomic mechanisms to occur. Here, using atomistic simulations, we report a new mechanism of dynamic recrystallization that can operate at temperature as low as T = 10 K in metals during deformation. In contrast to previously proposed dislocation-based models, this mechanism relies on the generation of disclination quadrupoles, which are special defects that form during deformation when the grain boundary migration is restricted by structural defects such as triple junctions, cracks or obstacles. This mechanism offers an alternative explanation for the grain refinement in metals during severe plastic deformation at cryogenic temperature and may suggest a new method to tailor the microstructure in general crystalline materials.

Near-ideal strength in metal nanotubes revealed by atomistic simulations

Here we report extraordinary mechanical properties revealed by atomistic simulations in metal nanotubes with hollow interior that have been long overlooked. Particularly, the yield strength in [1 1 1] Au nanotubes is found to be up to 60% higher than the corresponding solid Au nanowire, which approaches the theoretical ideal strength in Au. Furthermore, a remarkable transition from sharp to smooth yielding is observed in Au nanotubes with decreasing wall thickness. The ultrahigh tensile strength in [1 1 1] Au nanotube might originate from the repulsive image force exerted by the interior surface against dislocation nucleation from the outer surface.

Deformation twinning-mediated pseudoelasticity in metal–graphene nanolayered membrane

Abstract In this study, we investigated the deformation behaviour of metal–graphene nanolayered composites for five face-centred cubic metals under compression using molecular dynamics simulations. It was found that by increasing the thickness of the individual metal layers, the composite strength increased, while the deformation mechanism changed from buckling to deformation twining in Cu, Au and Ag, which was absent in the monolithic form of those metals of the same orientation and size. The deformation twinning was found to be enabled by the graphene layer, which introduced pseudoelasticity and shape memory effects in the nanolayered membrane with more than 15% recoverable compressive strain.

Stress-induced solid-state amorphization of nanocrystalline Ni and NiZr investigated by atomistic simulations

In this study, the feasibility of stress induced solid-state amorphization (SSA) of nanocrystalline (NC) Ni and NiZr alloys having ∼10 nm grain size has been investigated under constant tensile load (uniaxial and triaxial) via molecular dynamics simulations. In order to track the structural evaluation in both NC Ni and NiZr alloys during the SSA process, various types of analysis have been used, including simulated X-ray diffraction, centro-symmetry parameter, Voronoi cluster, common neighbor analysis, and radial distribution function. It is found that SSA in both NC Ni and NiZr alloys can only be achieved under triaxial loading conditions, and the hydrostatic tensile stress required for SSA is significantly lower when at. % Zr is increased in the NC NiZr alloy. Specifically, SSA in NC Ni and Ni-5 at. % Zr alloy was observed only when the temperature and hydrostatic tensile stress reached 800 K and 6 GPa, while SSA could occur in NC Ni-10 at. % Zr alloy under just 2 GPa of hydrostatic tensile stress at 300 K.