Extrinsic Grain Research Articles

Intrinsic heterogeneity of grain boundary phase transitions in the Cu–Bi system: insights from grain boundary diffusion measurements

Abstract Diffusion of Bi and Ag in a series of polycrystalline Cu–Bi alloys is investigated using a radiotracer technique and applying the 207Bi and 110m Ag isotopes, respectively. Together with the previous measurements (Divinski S., Lohmann M., Herzig C., Straumal B., Baretzky B., Gust W. Grain-boundary Melting Phase Transition in the Cu−Bi System. Phys. Rev. B 2005, 71, 104104), a temperature–concentration interval of strong, by orders of magnitude, enhancements of Bi grain boundary diffusion rates is distinguished and the results are interpreted in terms of a grain boundary pre-wetting/wetting phase transition. Grain boundary diffusivity of Ag exhibits as well a step-wise increase with rising Bi content, mirroring the behaviour observed for the Bi tracer. However, contrary to the Bi tracer atoms for which grain boundary enhancement is observed at about 60 ppm of Bi in Cu–Bi alloys, this transition is revealed by the Ag tracer atoms at a significantly higher concentration, specifically between 90 and 100 ppm of Bi at 1080 K. The Ag diffusion rates in alloys with a moderate Bi content turn out to be not affected by the Bi-induced grain boundary phase transition and the measured grain boundary diffusion coefficients of Ag are nearly the same as those determined for pure polycrystalline Cu. This spectacular result suggests a strong heterogeneity of Bi segregation and Bi-induced phase transition for general high-angle grain boundaries in a given alloy. The behaviour is discussed in terms of the extrinsic grain boundary defects and their impact on mechano-chemical coupling which is accompanying the grain boundary phase transitions.

Investigating Metal Solidification with X-ray Imaging

In the last two decades, X-ray imaging techniques have been used increasingly to study metal solidification in real-time as, thanks to advances in X-ray sources (synchrotron and laboratory-based) and detector technology, images can now be obtained with spatio-temporal resolutions sufficient to record key phenomena and extract quantitative information, primarily relating to crystal growth. This paper presents an overview of the research conducted at the University of Oxford over the last 6 years as a partner in the UK’s Future Liquid Metal Engineering (LiME) Manufacturing Hub. The focus is on in situ X-ray radiography to investigate the solidification of Al alloys, including the formation of primary α-Al crystals, and the formation and growth of secondary intermetallic phases. Technologically, the thrust is to understand how to control as-cast phases, structures and element distributions, particularly elements associated with recycling, as a means to facilitate greater recirculation of aluminium alloys. We first present studies on refinement of primary α-Al, including extrinsic grain refinement using inoculation and intrinsic refinement based on dendrite fragmentation. Second, we describe studies on intermetallic phase formation and growth, because intermetallic fraction, morphology and distribution are frequently a limiting factor of alloy mechanical properties and recyclability. Then we present some of the latest progress in studying liquid flow during solidification and associated hot tear formation. Finally, future research directions are described.

Intercrystalline defects and some properties of electrodeposited nanocrystalline nickel and its alloys

Abstract Experimental results are presented concerning the influence of primary intercrystalline defects, i. e., grain boundaries and triple junctions, on the properties of nanocrystalline nickel and its alloys. The properties considered are largely based on nanostructured materials relatively free of secondary defects introduced during synthesis, such as porosity, residual amorphous regions, lattice dislocations and extrinsic grain boundary dislocations. These property studies include thermal stability and grain growth, mechanical, electrical, corrosion and hydrogen transport.

Molecular dynamics simulation of the effect of cyclic stresses on nanocrystals with nonequilibrium grain boundaries: the role of the grain size

Grain boundaries (GBs) in bulk nanostructured materials processed by severe plastic deformation (SPD) have a nonequilibrium structure caused by extrinsic grain boundary dislocations (EGBDs) absorbed during deformation. Under external influences (annealing or cyclic straining) these GBs relax towards equilibrium resulting in a release of the excess energy of nanomaterials. Using our earlier developed method, atomic models of nanocrystals with nonequilibrium GBs having grain sizes of 10, 15, and 20 nm are constructed and relaxed by molecular dynamics. Then, the effect of oscillating tension-compression stresses with amplitudes of 2, 3 and 4 GPa on the systems thus obtained is simulated. Relaxation of nonequilibrium GBs by dislocation emission is found and the effect is shown to be independent of the grain size.

In Situ Grain Boundary Functionalization for Stable and Efficient Inorganic CsPbI2Br Perovskite Solar Cells

AbstractThe phase instability and large energy loss are two obstacles to achieve stable and efficient inorganic‐CsPbI3−xBrxperovskite solar cells. In this work, stable cubic perovskite (α)‐phase CsPbI2Br is successfully achieved by Pb(Ac)2functioning at the grain boundary under low temperature. Ac−strongly coordinates with CsPbI2Br to stabilize the α‐phase and also make the grain size smaller and film uniform by fast nucleation. PbO is formed in situ at the grain boundary by decomposing Pb(Ac)2at high‐temperature annealing. The semiconducting PbO effectively passivates the surface states, reduces the interface recombination, and promotes the charge transport in CsPbI2Br perovskite solar cells. A 12% efficiency and good stability are obtained for in situ PbO‐passivated CsPbI2Br solar cells, while Pb(Ac)2‐passivated device exhibits 8.7% performance and the highest stability, much better than the control device with 8.5% performance and inferior stability. This article highlights the extrinsic ionic grain boundary functionalization to achieve stable and efficient inorganic CsPbI3−xBrxmaterials and the devices.

Molecular Dynamics Simulation of Nonequilibrium Grain Boundaries in Ultrafine-Grained Nickel and their Relaxation under Cyclic Loading

Atomistic simulations of the structure, energy and relaxation under the action of high frequency cyclic straining are carried out for columnar nickel nanocrystals with [112] column axis, the grain boundaries (GBs) of which are in a nonequilibrium state caused by the presence of extrinsic grain boundary dislocations (EGBDs). A special method of introducing EGBDs is used to create initial structures with nonequilibrium GBs. Energy of GBs as a function of the degree of nonequilibrium is evaluated and qualitatively compared to the results of dislocation and disclination modeling. It is shown that under loading by symmetrically oscillating stresses the nonequilibrium GBs generate lattice dislocations, which travel across the grains and are absorbed by opposite GBs thus resulting in a relaxation of the structure, long-range stress fields and the energy of GBs.

Nonequilibrium grain boundaries and their relaxation under oscillating stresses in columnar nickel nanocrystals studied by molecular dynamics

Atomic structure of columnar nickel nanocrystals with [1 1 2] column axis having nonequilibrium grain boundaries (GBs) containing extrinsic grain boundary dislocations (EGBDs) and its evolution under oscillating stresses are studied by molecular dynamics method. Energy of GBs as a function of the degree of nonequilibrium is evaluated. It is found that under loading by symmetrically oscillating stresses the nonequilibrium GBs generate lattice dislocations, which travel across the grains and are absorbed by opposite GBs thus resulting in a relaxation of the structure, long-range stress fields and the energy of GBs.

A method for the construction of initial structures for molecular dynamics simulations of nanocrystals with nonequilibrium grain boundaries containing extrinsic dislocations

A method for the construction of initial atomic models of nanocrystals with extrinsic grain boundary dislocations (EGBDs) in grain boundaries (GBs) for molecular dynamics (MD) simulations is developed. The method is realized for f.c.c. nanocrystals with columnar grains having common crystallographic axis [112] parallel to the column axis and thus divided only by [112] tilt GBs. This system is convenient for studies of interactions between GBs and lattice dislocations, since each grain can be deformed by edge dislocations of only one slip system, which have lines parallel to the [112] axis. In order to introduce extrinsic dislocations to the boundaries of a selected grain, its contour is assumed to be strained by a given shear strain  so that a contour of a freely sheared grain is formed. This contour is filled in by atoms of a f.c.c. lattice with [112] direction parallel to the column axis and then the grain thus formed is subjected to an elastic shear strain -. This results in a deformed grain having the original shape, on the boundaries of which precursors of EGBDs are formed. In order to prevent these precursors from spontaneous annihilation during MD relaxation, one can temporarily fix GB atoms, or apply a proper external stress, or do both. A case study is carried out using two different protocols of MD relaxation to determine atomic structures and energies of nonequilibrium GBs.

Influence of stacking fault energy on friction of nanotwinned metals

The unique dislocation–twin boundary (TB) interactions that govern the extraordinary mechanical properties of nanotwinned (NT) metals have the strong intrinsic effect of material energy and the extrinsic effect of feature size. In this work, we perform molecular dynamics (MD) simulations to elucidate fundamental deformation mechanisms of two NT face-centered cubic (FCC) metals (Cu and Pd) under probe-based friction, with an emphasis on evaluating the influence of both material’s intrinsic energy barrier and extrinsic grain size on the microscopic deformation behavior and correlated macroscopic frictional results of the materials. Simulation results reveal that individual deformation modes of dislocation mechanisms, dislocation–TB interactions, TB-associated mechanisms, deformation twinning and grain boundary (GB) accommodation work in parallel in the plastic deformation of the materials, and their competition is strongly influenced by both the intrinsic energy barriers for the nucleation of stacking faults and twin faults, and the extrinsic grain size. Consequently, both the frictional response and worn surface morphology present strong anisotropic characteristics. It is also found that the deformation behavior of NT Pd under a localized multi-axis stress state is significantly different from that which occurs under a uniaxial stress state. These findings will advance the rational design and synthesis of nanostructured materials with advanced frictional properties.

Basal-plane thermal conductivity of nanocrystalline and amorphized thin germanane

Germanane (GeH), a hydrogen-terminated layered germanium structure, has recently been synthesized. Here, we employed a four-probe thermal transport measurement method to obtain the basal-plane thermal conductivity of thin exfoliated GeH flakes and correlated the measurement results with the crystal structure. The obtained thermal conductivity increases with increasing temperature, suggesting that extrinsic grain boundary and defect scattering dominate over intrinsic phonon-phonon scattering. Annealing a polycrystalline GeH sample at 195 °C caused it to become amorphous, reducing the room-temperature thermal conductivity from 0.53 ± 0.09 W m−1 K−1, which is close to the value calculated for 16 nm grain size, to 0.29 ± 0.05 W m−1 K−1, which approaches the calculated amorphous limit in the basal plane thermal conductivity.

Strain-Rate Sensitivity Enhanced by Grain-Boundary Sliding in Creep Condition for AZ31 Magnesium Alloy at Room Temperature

This study investigated strain-rate sensitivity (SRS) in an as-extruded AZ31 magnesium (Mg) alloy with grain size of about 10 mm. Although the alloy shows negligible SRS at strain rates of >10-5 s-1 at room temperature, the exponent increased by one order from 0.008 to 0.06 with decrease of the strain rate down to 10-8 s-1. The activation volume (V) was evaluated as approximately 100b3 at high strain rates and as about 15b3 at low strain rates (where b is the Burgers vector). In addition, deformation twin was observed only at high strain rates. Because the twin nucleates at the grain boundary, stress concentration is necessary to be accommodated by dislocation absorption into the grain boundary at low strain rates. Extrinsic grain boundary dislocations move and engender grain boundary sliding (GBS) with low thermal assistance. Therefore, GBS enhances and engenders SRS in AZ31 Mg alloy at room temperature.

Electric transport properties study of bulk BaFe12O19 by complex impedance spectroscopy

Polycrystalline M-type hexagonal Barium ferrite (BaFe12O19) was prepared by the solid state reaction method. The single phase and well grain growth of micrometer range was confirmed by XRD pattern and SEM image respectively. The presence of Fe2+ and Fe3+ states were verified by XPS study. In this report, the electric transport properties of BaFe12O19 (BaM) was systematically investigated and analyzed in the frequency range of 100Hz to 1MHz and temperature range of 30–200°C. The relaxation behaviors were examined by considering both impedance and modulus formalism, to differentiate intrinsic (grain) and extrinsic (grain boundary and electrode polarization) conduction process. The co-contribution effect of grain and grain boundary was verified which appeared as a single relaxation process in the chosen frequency and temperature range. However, the sample experienced the early (even at room temperature) arrival of surface conduction which dominates over grain boundary with increasing of temperature.

Non-stoichiometry in “CaCu3Ti4O12” (CCTO) ceramics

A combined powder X-ray lattice parameter and ceramic impedance spectroscopy study is presented on materials within the CaO–CuO–TiO2 ternary phase diagram. Several compositions containing CaCu3Ti4O12 (CCTO) and small amounts of secondary phases such as TiO2, CaTiO3 and CuO are analysed and two different defect mechanisms are identified as the cause of the non-stoichiometry in CCTO. The first mechanism involves a variation in the Cu content, which explains the large differences in the intrinsic bulk and extrinsic grain boundary (GB) resistance, and the formation of the ceramic internal barrier layer capacitor (IBLC) structure. The second mechanism is associated with Ca–Cu anti-site disorder causing an unusually high intrinsic bulk permittivity above that predicted from Clausius–Mossotti calculations.

1536 焼鈍による余分な粒界転位コア構造の変化と粒界の転位ピン留め効果

1536 焼鈍による余分な粒界転位コア構造の変化と粒界の転位ピン留め効果

Grain Boundary Structure Dependence of Extrinsic Grain Boundary Dislocation Emission Phenomena: A Molecular Dynamics Study

Recently, it has been reported that extrinsic grain boundary dislocations (EGBDs) are often present in the grain boundaries of ultrafine-grained (UFG) metals produced by severe plastic deformation; therefore, the emission of EGBDs from the grain boundaries could afect the mechanical characteristics of UFG metals. In this paper, we use molecular dynamics to simulate the emission of EGBDs from grain boundaries, and we examine the grain boundary structure dependence of the emissions. Then we apply J-integral analysis to evaluate the Peach–Koehler force required for the grain boundaries to emit the EGBD. It can be confirmed that the Peach–Koehler force required to emit the EGBD is highly dependent on the relationship between the Burgers vector components of the EGBD and intrinsic grain boundary dislocations (IGBDs), which form the equilibrium grain boundaries. Comparing analyses of the linear elastic theory with atomic simulations, we confirm that nonlinear structural changes in the dislocation cores of the EGBD and IGBDs, which can only be expressed by atomic scale resolution, are responsible for such strong grain boundary structural dependence. We also verify that normal stress components perpendicular to the slip plane of EGBDs have a significant effect on the emission of EGBDs.

911 IntrinsicとExtrinsic粒界転位の相互作用に関する原子シミュレーション(OS2-3計算固体力学の新展開一ナノからマクロまで一)

911 IntrinsicとExtrinsic粒界転位の相互作用に関する原子シミュレーション(OS2-3計算固体力学の新展開一ナノからマクロまで一)

Coupling of grain boundary sliding and migration within the range of boundary specialness

Stress-induced behavior of high-angle near-coincidence symmetric tilt boundaries has been examined in bicrystal specimens of zinc. Parameters of coupling between boundary sliding and migration were determined. The angular deviation from the coincidence misorientation within the range of boundary specialness has a noticeable effect on the sliding-to-migration ratio, called “coupling factor”. Mechanisms of coupled boundary sliding and migration based on the motion of edge-type extrinsic and intrinsic grain boundary dislocations are discussed. It has been demonstrated that the observed alteration of the coupling factor with the change in boundary misorientation is due to the change of the parameters of extrinsic secondary grain boundary dislocations. The obtained results have also shown the limitation of the coincidence site lattice/displacement shift complete lattice model for the quantitative description of the structure of near-coincidence boundaries.

Endurecimiento por deformación de materiales cristalinos

Para describir el proceso de endurecimiento de materiales cristalinos en deformación uniaxial, un modelo de gradiente variable interna simple (GM1) construido en base del conocido modelo de Kocks-Mecking es propuesto. La dependencia en tiempo y temperatura de la tensión de flujo es considerada vía migración de bordes de grano, y la migración es relacionada al aniquilamiento de dislocaciones de borde de grano extrínsecas (EGBD’s) mediante climb vía difusión de red de vacancias en puntos triples. Para modelar el comportamiento policristalino de níquel comercialmente puro (desde un punto de vista de un compuesto) por medio del método del elemento finito, probetas virtuales con dimensiones en el rango de 3-25 μm deberían ser usadas.

J-Integral Evaluation of Continuous Dislocation Emissions from the Crack Tip near Grain Boundaries in Atomic Simulations

Extrinsic grain boundary dislocations (EGBDs) are often observed in ultra fine-grained metals produced by severe plastic deformation processes. The mechanical properties of such metals are different from those of coarse-grained metals, so EGBDs could be important factor to elucidate the unique mechanical properties. In this paper, in order to clarify the influence of EGBDs on the emission of dislocations from the crack tip, we evaluate the mechanical field around the crack tip using J-integral in atomic simulations. It is found that quantitative evaluations of the crack-tip shielding effect due to EGBDs can be performed by J-integral taking grain boundary structures into account.

T0101-3-2 余分な粒界転位成分が及ぼすき裂先端に対する遮へい・反遮へい効果の分子動力学解析([T0101-3]マイクロ・ナノ材料システムの力学と強度・機能評価(3):先端材料システムの強度・機能解析(マクロ～マイクロ・ナノ))

Extrinsic grain boundary dislocations (EGBDs) are often observed in ultra fine-grained metals produced by severe plastic deformation (e.g. ECAP or ARB etc.). The mechanical properties of such metals are different from those of coarsegrained metals. That is, EGBDs could be important factor to elucidate the unique mechanical properties. In this study, introducing lattice dislocations into an equilibrium grain boundary by pre-deformation, we evaluate the influence of the EGBD component on the crack tip shielding effect using J-integral analyses.