Tracking the sliding of grain boundaries at the atomic scale Wang, Lihua; Zhang, Yin; Zeng, Zhi ...
Science (American Association for the Advancement of Science),
2022-Mar-18, 2022-03-18, 20220318, Letnik:
375, Številka:
6586
Journal Article
Recenzirano
Grain boundaries (GBs) play an important role in the mechanical behavior of polycrystalline materials. Despite decades of investigation, the atomic-scale dynamic processes of GB deformation remain ...elusive, particularly for the GBs in polycrystals, which are commonly of the asymmetric and general type. We conducted an in situ atomic-resolution study to reveal how sliding-dominant deformation is accomplished at general tilt GBs in platinum bicrystals. We observed either direct atomic-scale sliding along the GB or sliding with atom transfer across the boundary plane. The latter sliding process was mediated by movements of disconnections that enabled the transport of GB atoms, leading to a previously unrecognized mode of coupled GB sliding and atomic plane transfer. These results enable an atomic-scale understanding of how general GBs slide in polycrystalline materials.
Twin nucleation in a face-centered cubic crystal is believed to be accomplished through the formation of twinning partial dislocations on consecutive atomic planes. Twinning should thus be highly ...unfavorable in face-centered cubic metals with high twin-fault energy barriers, such as Al, Ni, and Pt, but instead is often observed. Here, we report an in situ atomic-scale observation of twin nucleation in nanocrystalline Pt. Unlike the classical twinning route, deformation twinning initiated through the formation of two stacking faults separated by a single atomic layer, and proceeded with the emission of a partial dislocation in between these two stacking faults. Through this route, a three-layer twin was nucleated without a mandatory layer-by-layer twinning process. This route is facilitated by grain boundaries, abundant in nanocrystalline metals, that promote the nucleation of separated but closely spaced partial dislocations, thus enabling an effective bypassing of the high twin-fault energy barrier.
With continuous minimization of nanodevices, the dimensions of metallic materials used in nanodevices decrease to a few nanometers. Understanding the structural stability and deformation behavior of ...these small-sized metallic materials is important for their practical applications. Here we report our atomic-resolution observation of the deformation processes of Ag nanowires with widths of ∼3 nm. The nanowires under tension experienced plastic deformation via partial dislocation activities, which led to deformation twinning in and homogeneous elongation of the nanowires, and surface atom diffusion that reduced the nanowires’ width but did not contribute to the nanowire elongation. The diffusion of surface atoms was initiated at surface steps introduced by the partial dislocation activities, leading to fracture of the nanowires with relatively low homogeneous elongation.
In this study, the plastic behaviors of nanocrystalline Au with an average grain size of 18nm were investigated in situ using a home-made tensile device in a transmission electron microscope. We ...provide the direct experimental results revealed the process of grain boundary migration. The results show that dislocation behaviors are prevalent for large grains. However, for grain sizes below ~15nm, grain boundary migration occurs frequently. The results of our statistical analyses show that grain boundary migration occurs more frequently than grain boundary sliding and rotation in nanocrystalline Au.
Display omitted
Magnetoelasticity is the bond between magnetism and mechanics, but the intricate mechanisms via which magnetic states change due to mechanical strain remain poorly understood. Here, we provide direct ...nanoscale observations of how tensile strain modifies magnetic domains in a ferromagnetic Ni thin plate using in situ Fresnel defocus imaging, off-axis electron holography and a bimetallic deformation device. We present quantitative measurements of magnetic domain wall structure and its transformations as a function of strain. We observe the formation and dissociation of strain-induced periodic 180° magnetic domain walls perpendicular to the strain axis. The magnetization transformation exhibits stress-determined directional sensitivity and is reversible and tunable through the size of the nanostructure. In this work, we provide direct evidence for expressive and deterministic magnetic hardening in ferromagnetic nanostructures, while our experimental approach allows quantifiable local measurements of strain-induced changes in the magnetic states of nanomaterials.
The atomic-scale in situ tensile tests of twin-structured Pt (d = ~25 nm) nanocrystals were conducted using a home-made device in a high-resolution transmission electron microscope. We showed that ...the twin-structured Pt nanocrystals exhibit as large as 21.9% homogeneous elongation before crack emergence. The atomic-scale and time-resolved in situ investigation indicated that, full and partial dislocations that intersect with coherent twin boundaries (TBs), as well as TB migration resulting from partial dislocations glide adjacent to the TBs, and the rapid migration of incoherent TBs all contribute the large homogeneous elongation capability of the twin-structured nanocrystals.
Display omitted
Abstract
Understanding the competing modes of brittle versus ductile fracture is critical for preventing the failure of body-centered cubic (BCC) refractory metals. Despite decades of intensive ...investigations, the nanoscale fracture processes and associated atomistic mechanisms in BCC metals remain elusive due to insufficient atomic-scale experimental evidence. Here, we perform in situ atomic-resolution observations of nanoscale fracture in single crystals of BCC Mo. The crack growth process involves the nucleation, motion, and interaction of dislocations on multiple 1/2 < 111 > {110} slip systems at the crack tip. These dislocation activities give rise to an alternating sequence of crack-tip plastic shearing, resulting in crack blunting, and local separation normal to the crack plane, leading to crack extension and sharpening. Atomistic simulations reveal the effects of temperature and strain rate on these alternating processes of crack growth, providing insights into the dislocation-mediated mechanisms of the ductile to brittle transition in BCC refractory metals.
Metallic nanomaterials are widely used in micro/nanodevices. However, the mechanically driven microstructure evolution in these nanomaterials is not clearly understood, particularly when large stress ...and strain gradients are present. Here, we report the in situ bending experiment of Ni nanowires containing nanoscale twin lamellae using high-resolution transmission electron microscopy. We found that the large, localized bending deformation of Ni nanowires initially resulted in the formation of a low-angle tilt grain boundary (GB), consisting of randomly distributed dislocations in a diffuse GB layer. Further bending intensified the local plastic deformation and thus led to the severe distortion and collapse of local lattice domains in the GB region, thereby transforming a low-angle GB to a high-angle GB. Atomistic simulations, coupled with in situ atomic-scale imaging, unravelled the roles of bending-induced strain gradients and associated geometrically necessary dislocations in GB formation. These results offer a valuable understanding of the mechanically driven microstructure changes in metallic nanomaterials through GB formation. The work also has implications for refining the grains in bulk nanocrystalline materials.
Although their mechanical behavior has been extensively studied, the atomic-scale deformation mechanisms of metallic nanowires (NWs) with growth twins are not completely understood. Using our own ...atomic-scale and dynamic mechanical testing techniques, bending experiments were conducted on single-crystalline and twin-structural Ni NWs (D=∼40nm) using a high-resolution transmission electron microscope (HRTEM). Atomic-scale and time-resolved dislocation nucleation and propagation activities were captured in situ. A large number of in situ HRTEM observations indicated strong effects from the twin thickness (TT) on dislocation type and glide system. In thick twin lamella (TT>∼12nm) and single-crystalline NWs, the plasticity was controlled by full dislocation nucleation. For NWs with twin thicknesses of ∼9nm<TT<∼12nm, full and partial dislocation nucleation occurred from the free surface, and the dislocations glided on multiple systems and interacted with each other during plastic deformation. For NWs with twin thicknesses of ∼6nm<TT<∼9nm, partial dislocation nucleation from the free surface and the gliding of those dislocations on the plane that intersected the twin boundaries (TBs) were the dominant plasticity events. For the NWs with twin thicknesses of ∼1nm<TT<∼6nm, the plasticity was accommodated by a partial dislocation nucleation process and glide parallel to the TBs. When TT<∼1nm, TB migration and detwinning processes resulting from partial dislocation nucleation and glide adjacent to the TBs were frequently observed.
It is known that Ni-based superalloys experience microstructural rafting during high temperature straining. This paper presents a comparative study of the different rafting models by means of finite ...element modeling and experimentation investigation. It was found that uniaxial elastic loading does not alter the misfit or misfit isotropy of the structure, but causes repartitioning of the misfit into elastic strains in the γ and γ' phases. This causes an elastic strain energy anisotropy and hydrostatic stress anisotropy among the vertical and horizontal γ phase channels. Both anisotropies predict a raft structure that is contrary to the experimental observation. Through a carefully designed pre-treatment at 750 °C and under a uniaxial stress of 750 MPa, an anisotropic dislocation structure was created on the {001} γ'/γ interfaces without altering the cuboidal morphology of the γ' phase. This allows the evaluation of the influence of dislocations on the rafting behavior of the alloy. It was found that rafting occurred in the pre-treated samples containing anisotropic dislocation structures during thermal exposure without applied stress, instead of isotropic corsening, and that the raft structures conform to the expectations base don the anistropic dislocation strucutres. This demonstrates that an anisotropic dislocation structure on the {001} γ'/γ interfaces is a direct cause of rafting. This is attributted to the fact that dislocations on γ'/γ interfaces help to relax the local misfit strains, causing some to expand at the expense of others. At the same time, the formation of the anisotropic dislocation structure is a direct result of the anisotropy of elastic strains induced by the applied bia stess in during the pre-treatment. These explain the complex interplay of the elastic and plastic models reported the literature.
Display omitted
•The elastic stretching alters the partitioning of elastic strains in two phases, whereas the misfit remains unchanged.•The elastic strain energy and hydrostatic stress anisotropies are ruled out to be responsible for rafting.•The anisotropic dislocation on γ'/γ interfaces wtith γ' phase cuboid morphology unchanged were created by pre-straining.•The dislocation anisotropy is effective in causing rafting in Ni-based superalloys by causing misfit anisotropy.
PDF
