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Register a new userStructural phase transformations in metallic grain boundaries
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Structural transformations at interfaces are of profound fundamental interest as complex examples of phase transitions in low-dimensional systems. Despite decades of extensive research, no compelling evidence exists for structural transformations in high-angle grain boundaries in elemental systems. Here we show that the critical impediment to observations of such phase transformations in atomistic modeling has been rooted in inadequate simulation methodology. The proposed new methodology allows variations in atomic density inside the grain boundary and reveals multiple grain boundary phases with different atomic structures. Reversible first-order transformations between such phases are observed by varying temperature or injecting point defects into the boundary region. Due to the presence of multiple metastable phases, grain boundaries can absorb significant amounts of point defects created inside the material by processes such as irradiation. We propose a novel mechanism of radiation damage healing in metals which may guide further improvements in radiation resistance of metallic materials through grain boundary engineering.
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Nano-crystallize materials have been known for decades to potentially owe the novel self-healing ability for radiation damage, which has been demonstrated to be especially linked to preferential occupation of interstitials at grain boundary (GB) and promoted vacancy-interstitial annihilation. A major obstacle to better understanding the healing property is the lack of an atomistic picture of the interstitial states near GBs, due to severely separation of the timescale of interstitial segregation from other events and abundance of interstitials at the GB. Here, we report a generic self-blocking effect of the interstitial cluster (SIAn) near the metallic GB in W, Mo and Fe. Upon creating a SIAn near the GB, it is immediately trapped by the GB during the GB structural relaxation and blocks there, impeding GBs further spontaneous trapping of the SIAn in the vicinity and making these SIAns stuck nearby the GB. The SIAn in the stuck state surprisingly owes an exceptionally larger number of annihilation sites with vacancies near the GB than the SIAn trapped at the GB due to maintaining its bulk configuration basically. Besides, it also has an unexpectedly long-ranged repelling interaction with the SIA in the bulk region, which may further affect the GBs trap of the SIAn. The self-blocking effect might shed light on more critical and extended role of the GB in healing radiation-damage in NCs than previously recognized the GBs limited role based on GBs trap for the SIA and resulted vacancy-SIA recombination.
Previous simulation and experimental studies have shown that some grain boundaries (GBs) can couple to applied shear stresses and be moved by them, producing shear deformation of the lattice traversed by their motion. While this coupling effect has been well confirmed for symmetrical tilt GBs, little is known about the coupling ability of asymmetrical boundaries. In this work we apply a combination of molecular dynamics and phase field crystal simulations to investigate stress-driven motion of asymmetrical GBs between cubic crystals over the entire range of inclination angles. Our main findings are that the coupling effect exists for most of the asymmetrical GBs and that the coupling factor exhibits a non-trivial dependence on both the misorientation and inclination angles. This dependence is characterized by a discontinuous change of sign of the coupling factor, which reflects a transition between two different coupling modes over a narrow range of angles. Importantly, the magnitude of the coupling factor becomes large or divergent within this transition region, thereby giving rise to a sliding-like behavior. Our results are interpreted in terms of a diagram presenting the domains of existence of the two coupling modes and the transition region between them in the plane of misorientation and inclination angles. The simulations reveal some of the dislocation mechanisms responsible for the motion of asymmetrical tilt GBs. The results of this study compare favorably with existing experimental measurements and provide a theoretical ground for the design of future experiments.
Nanocrystals can exist in multiply twinned structures like the icosahedron, or single crystalline structures like the cuboctahedron or Wulff-polyhedron. Structural transformation between these polymorphic structures can proceed through diffusion or displacive motion. Experimental studies on nanocrystal structural transformations have focused on high temperature diffusion mediated processes. Thus, there is limited experimental evidence of displacive motion mediated structural transformations. Here, we report the high-pressure structural transformation of 6 nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell that is driven by displacive motion. In-situ X-ray diffraction and transmission electron microscopy were used to detect the transformation of multiply twinned nanocrystals into single crystalline nanocrystals. High-pressure single crystalline nanocrystals were recovered after unloading, however, the nanocrystals quickly reverted back to multiply twinned state after redispersion in toluene solvent. The dynamics of recovery was captured using transmission electron microscopy which showed that the recovery was governed by surface recrystallization and rapid twin boundary motion. We show that this transformation is energetically favorable by calculating the pressure-induced change in strain energy. Molecular dynamics simulations showed that defects nucleated from a region of high stress region in the interior of the nanocrystal, which make twin boundaries unstable. Deviatoric stress driven Mackay transformation and dislocation/disclination mediated detwinning are hypothesized as possible mechanisms of high-pressure structural transformation.
Flexoelectricity is a type of ubiquitous and prominent electromechanical coupling, pertaining to the response of electrical polarization to mechanical strain gradients while not restricted to the symmetry of materials. However, large elastic deformation in most solids is usually difficult to achieve and the strain gradient at minuscule is challenging to control. Here we exploit the exotic structural inhomogeneity of grain boundary to achieve a huge strain gradient (~ 1.2 nm-1) within 3 ~ 4 unit-cells, and thus obtain atomic-scale flexoelectric polarization up to ~ 38 {mu}C/cm2 at a 24 LaAlO3 grain boundary. The nanoscale flexoelectricity also modifies the electrical activity of grain boundaries. Moreover, we prove that it is a general and feasible way to form large strain gradients at atomic scale by altering the misorientation angles of grain boundaries in different dielectric materials. Thus, engineering of grain boundaries provides an effective pathway to achieve tunable flexoelectricity and broadens the electromechanical functionalities of non-piezoelectric materials.
Grain boundaries (GBs) are structural imperfections that typically degrade the performance of materials. Here we show that dislocations and GBs in two-dimensional (2D) metal dichalcogenides MX2 (M = Mo, W; X = S, Se) can actually improve the material by giving it a qualitatively new physical property: magnetism. The dislocations studied all have a substantial magnetic moment of ~1 Bohr magneton. In contrast, dislocations in other well-studied 2D materials are typically non-magnetic. GBs composed of pentagon-heptagon pairs interact ferromagnetically and transition from semiconductor to half-metal or metal as a function of tilt angle and/or doping level. When the tilt angle exceeds 47{deg} the structural energetics favor square-octagon pairs and the GB becomes an antiferromagnetic semiconductor. These exceptional magnetic properties arise from an interplay of dislocation-induced localized states, doping, and locally unbalanced stoichiometry. Purposeful engineering of topological GBs may be able to convert MX2 into a promising 2D magnetic semiconductor.
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