Bicrystal Interface Research Articles

Visualizing Shock Induced Thermo-Mechanical Change at Bi-Crystal Interface Using Laser Array Based Nano-Second Raman Spectral Imaging

Visualizing Shock Induced Thermo-Mechanical Change at Bi-Crystal Interface Using Laser Array Based Nano-Second Raman Spectral Imaging

Laser shock peening-induced misfit dislocation at interfaces between HCP and BCC crystals of Ti6Al4V alloy and the corresponding dislocation patterns in two phases

Understanding of laser shock peening (LSP)-generated microstructural characteristics at the interfaces between hexagonal close-packed (HCP) and body-centered cubic (BCC) crystals and dislocation patterns could provide useful insights into designing the interface between both phases of dual- or multi- phase metallic materials, which is a key information for higher properties. Herein, with a severe plastic deformation process of LSP, a systematic work of the mechanism behind misfit dislocations at different grain size has been carried out. It demonstrates that, as the grain gradually is refined, extrusion and torsion of the lattice are important contributions to the modified d-spacing and quantity of misfit dislocation. Hence, the growth orientation and the style of bicrystal interface between HCP and BCC crystals are affected. Furthermore, the size effect of the LSP-generated dislocation pattern has a significant attribution to the stress of misfit dislocation originating from neighboring interfaces.

General model for the kinetics of solute diffusion at solid-solid interfaces

Solute diffusion through solid-solid interfaces is paramount to many physical processes. From a modeling point of view, the discontinuities in the energy landscape at a sharp interface represent difficulties in predicting solute diffusion that, to date, have not been solved in a consistent manner across length scales. Using an explicit finite volume method, this work is the first to derive numerical solutions to the diffusion equations at a continuum level while including discrete variations in the energy landscape at a bicrystal interface. An atomic jump equation consistent with atomistic descriptions is derived and scaled up into a compendium of model interfaces: monolayer energy barriers, monolayer interfacial traps, multilayered traps, and heterogeneous interfaces. These can track solute segregation behavior and long-range diffusion effects. We perform simulations with data for hydrogen diffusion in structural metals, of relevance to the assessment of the hydrogen embrittlement phenomenon, and point defects in electronic devices. The approach developed represents an advancement in the mathematical treatment of solute diffusion through solid-solid interfaces and an important bridge between the atomistic and macroscopic modeling of diffusion, with potential applications in a variety of fields in materials science and physics.

Conceptual Framework for Dislocation-Modified Conductivity in Oxide Ceramics Deconvoluting Mesoscopic Structure, Core, and Space Charge Exemplified for SrTiO3.

The introduction of dislocations is a recently proposed strategy to tailor the functional and especially the electrical properties of ceramics. While several works confirm a clear impact of dislocations on electrical conductivity, some studies raise concern in particular when expanding to dislocation arrangements beyond a geometrically tractable bicrystal interface. Moreover, the lack of a complete classification on pertinent dislocation characteristics complicates a systematic discussion and hampers the design of dislocation-modified electrical conductivity. We proceed by mechanically introducing dislocations with three different mesoscopic structures into the model material single-crystal SrTiO3 and extensively characterizing them from both a mechanical as well as an electrical perspective. As a final result, a deconvolution of mesoscopic structure, core structure, and space charge enables us to obtain the complete picture of the effect of dislocations on functional properties, focusing here on electric properties.

Effects of Crystal Orientation and Grain Boundary Inclination on Stress Distribution in Bicrystal Interface of Austenite Stainless Steel 316L

Nuclear structural material austenitic stainless steel 316L is a polycrystalline composed of single crystals with a face-centered cubic (FCC) structure, and the intergranular stress corrosion cracking (IGSCC) is closely related to the crystal orientation. A constitutive model is presented to assess the elastic response of anisotropic behavior of single crystals in 316L in this study. With a bicrystal model built by the finite element method, the effects of crystal orientation and grain boundary (GB) inclination on the stress state nearby a symmetric tilt GB were discussed under the constant-displacement condition. The results indicate that when tensile axes are perpendicular to the GB, the stress and strain are equal at the GB and inside the grain, and the crystal misorientation has little effects on the stress and strain distribution. If the GB is not perpendicular to the load direction, the GB inclination angle will change the equivalent elastic modulus along the load direction and result in a larger stress in the grain with larger equivalent elastic modulus, but the stress tends to be equal inside the two grains. The grain size effects verification shows that the conclusions are independent of grain size.

Atomic and electronic structure of Lomer dislocations at CdTe bicrystal interface.

Extended defects are of considerable importance in determining the electronic properties of semiconductors, especially in photovoltaics (PVs), due to their effects on electron-hole recombination. We employ model systems to study the effects of dislocations in CdTe by constructing grain boundaries using wafer bonding. Atomic-resolution scanning transmission electron microscopy (STEM) of a [1–10]/(110) 4.8° tilt grain boundary reveals that the interface is composed of three distinct types of Lomer dislocations. Geometrical phase analysis is used to map strain fields, while STEM and density functional theory (DFT) modeling determine the atomic structure at the interface. The electronic structure of the dislocation cores calculated using DFT shows significant mid-gap states and different charge-channeling tendencies. Cl-doping is shown to reduce the midgap states, while maintaining the charge separation effects. This report offers novel avenues for exploring grain boundary effects in CdTe-based solar cells by fabricating controlled bicrystal interfaces and systematic atomic-scale analysis.

A phase field dislocation dynamics model for a bicrystal interface system: An investigation into dislocation slip transmission across cube-on-cube interfaces

In this work, we present a phase field dislocation dynamics formulation designed to treat a system comprised of two materials differing in moduli and lattice parameters that meet at a common interface. We apply the model to calculate the critical stress τcrit required to transmit a perfect dislocation across the bimaterial interface with a cube-on-cube orientation relationship. The calculation of τcrit accounts for the effects of: 1) the lattice mismatch (misfit or coherency stresses), 2) the elastic moduli mismatch (Koehler forces or image stresses), and 3) the formation of the residual dislocation in the interface. Our results show that the value of τcrit associated with the transmission of a dislocation from material 1 to material 2 is not the same as that from material 2 to material 1. Dislocation transmission from the material with the lower shear modulus and larger lattice parameter tends to be easier than the reverse and this apparent asymmetry in τcrit generally increases with increases in either lattice or moduli mismatch or both. In efforts to clarify the roles of lattice and moduli mismatch, we construct an analytical model for τcrit based on the formation energy of the residual dislocation. We show that path dependence in this energetic barrier can explain the asymmetry seen in the calculated τcrit values. Significantly, the analysis reveals that τcrit scales with a(2)G(2)a(1)+a(2)(a(1)a(2)−G(1)G(2))2, where G is the shear modulus, a is the lattice parameter, and the superscripts (1) and (2) indicate quantities for material 1 and material 2, respectively.

Electron-Beam-Induced Current Study of Electronic Property Change at SrTiO<sub>3</sub> Bicrystal Interface Induced by Forming Process

We investigated the spatial variation of energy band structure in a SrTiO3 (001) bicrystal with the (100) 10° tilt boundary before and after the annealing process at 973 K under a vacuum of ~10-7 Torr and after the subsequent forming process with electrical dielectric breakdown, using electron beam induced current (EBIC) method in the temperature range between 200 and 300K. Although the EBIC contrast at the tilt boundary was weak before and after the annealing, it became visible after the forming process and stronger as the observation temperature decreased to less than 260 K. It was found that the EBIC direction at the tilt boundary is opposite to that in the matrix, i.e. the single crystalline regions of both sides of the tilt boundary. In the matrix, the EBIC increased after the annealing and decreased again after the forming process. We propose energy band structures of the bicrystalline SrTiO3 after the annealing and after the forming process.

Measurement of the Kapitza resistance across a bicrystal interface

The Kapitza resistance across a Si bicrystal interface was measured using a pump probe optical technique. This approach, termed time resolved thermal wave microscopy (TRTWM), uses ultrafast laser pulses to image lateral thermal transport in bare semiconductors. The sample geometry is that of a Si bicrystal with the vertically oriented boundary intersecting the sample surface. High resolution transmission electron microscopy of the boundary region revealed a thin SiO2 layer at the interface. By comparing experimental results with a continuum thermal transport model the Kapitza resistance between the Si and SiO2 was estimated to be 2.3 × 10−9 m2K/W.

The Scratch Behaviors of Copper Bi-Layers by a Diamond Tip: A Molecular Statics Study

The scratch deformation behaviors of two bicrystal coppers (Cu(100)/Cu(110) and Cu(110)/Cu(100)) during the nanoscratching process were explored and compared with their single crystal ingredients by the molecular statics simulations. The effects of lattice configuration and scratch depth were investigated in this study. The results showed that the motion of dislocations was blocked in the bicrystal interface until the dislocations accumulated enough energy to move. From the study, it was found that the bicrystal interfaces can provide resistance to the motion of dislocations, and can strengthen the mechanical properties of copper materials.

The impedance caused by interfacial current constriction: a case study on BaF2/YSZ bicrystals

Abstract BaF2/YSZ (yttria stabilized zirconia) bicrystals are investigated by means of impedance spectroscopy. The spectra show two semicircles in the complex impedance plane, and both arcs exhibit very similar temperature dependences. The high frequency semicircle can be attributed to the ideal (quasi one-dimensional) bulk resistance of BaF2, and the low frequency arc is caused by an additional resistance in BaF2 due to the current constriction taking place close to the contact spots at the imperfect bicrystal interface. This interpretation is supported by finite element calculations revealing that bicrystals with imperfect contacts indeed exhibit an additional semicircle in the complex impedance plane even without any ion transfer resistance between the two crystals. The low frequency arc of the BaF2/YSZ bicrystal drastically increases upon bias voltage, but relaxes to its original value after removing the bias. This phenomenon can be associated with a strong decrease of the vacancy concentration in YSZ (close to the interfacial contact spots), which is caused by F− ions being pumped into YSZ and acting as a counter dopant to Y3+.

Influence of defects on mechanical properties of bicrystal copper grain boundary interfaces

Defects play a key role in the determination of material properties, especially at small scales. The influence of several kinds of defects (point vacancies, line vacancies and cracks) on the deformation and fracture characteristics of a planar copper grain boundary interface with a 45° lattice misorientation is explored using molecular dynamics simulations and the embedded-atom method. Both tensile and shear modes of interfacial separation are considered. The results show that the crystalline defects can have a strong influence on the interfacial behaviours. The sensitivity of the mechanical properties of the interface to a defect type may be different under tension than under shear. It is found that some defect topologies can improve certain properties (e.g. strength and fracture strain) of the bicrystal interface system.

Dislocation nucleation from bicrystal interfaces with dissociated structure

Atomistic calculations are used to model the nucleation of partial dislocations during a tensile deformation from bicrystal interfaces with dissociated structure. Interfaces with this type of structure occur primarily in materials with low intrinsic stacking fault energies. In this work, the initial structure of each bicrystal interface is refined using energy minimization techniques. Molecular dynamics simulations are then used to study the deformation of each interface in uniaxial tension perpendicular to the boundary plane at a constant strain rate. Analysis focuses on the evolution of the dissociated interface structure prior to the dislocation nucleation event and the resulting structure of the boundary after the emission of partial dislocations from the interface. Dislocation nucleation occurs predominantly at the dissociated interface structural unit, while the spacing between interface features is identified as an important length scale that affects the failure mode. The evolution of the dissociated interface structure and the nucleation of partial dislocations are found to be similar to results obtained in a previous atomistic study of the stress dependence of a lock formation containing a stair-rod dislocation.

Magnetization reversal processes in magnetic bicrystal junctions

We provide experimental data for the magnetoresistance in epitaxial ferromagnetic manganite $({\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3})$ bicrystals with various angles. Experiments were conducted using samples with in-plane magnetization as well as with out-of-plane magnetization. From the shape of the magnetoresistance hysteresis we draw conclusions on the different magnetization reversal processes. We show that it is possible to set the electrode magnetization at remanence such that the spins at the bicrystal interface are either parallel or inclined to each other at an angle determined by the crystal orientations. Finally, we demonstrate the feasibility of using epitaxial magnetic bicrystals to probe the transport properties in magnetic junctions with well-determined angles of magnetization.

Metal Oxide Bicrystal Josephson Junctions of a New Type with High Critical Parameters

Film bicrystal Josephson junctions of a new type based on a metal oxide high-Tc superconductor of the YBa2Cu3O7−x (YBCO) system are suggested and studied. In junctions of the new type, in contrast to the known ones, the working surfaces situated on the opposite sides of the bicrystal interface have different crystal orientations: one is (001)YBCO, and the other is rotated (misoriented) relative to it by an asymmetric bicrystal angle around a certain axis lying in the substrate plane. The main electrical and dynamic (microwave) characteristics have been determined for junctions of the new type based on NdGaO3 bicrystal substrates with the misorientation angle varying from 13° to 28°. These junctions exhibit high values of the critical parameters at T=77 K, including the critical current density IC= (2–5)×105 A/cm2 and the characteristic voltage VC=ICRN=0.6–0.9 mV. Advantages of the bicrystal Josephson junctions of the new type over the known junctions are considered.

Effect of deformation path sequence on the behavior of nanoscale copper bicrystal interfaces

Molecular dynamics calculations are performed to study the effect of deformation sequence and history on the inelastic behavior of copper interfaces on the nanoscale. An asymmetric 45 deg tilt bicrystal interface is examined, representing an idealized high-angle grain boundary interface. The interface model is subjected to three different deformation paths: tension then shear, shear then tension, and combined proportional tension and shear. Analysis shows that path-history dependent material behavior is confined within a finite layer of deformation around the bicrystal interface. The relationships between length scale and interface properties, such as the thickness of the path-history dependent layer and the interface strength, are discussed in detail.

Crystal-crystal interfaces: Representation and impact of periodic cross-interface interaction

The cross-interface interaction potential V of atoms on either side of a misfitting bicrystal interface varies periodically along the interface. This phenomenon has previously been invoked in dealing with crystal dislocations, including misfit dislocations. Frenkel and Kontorowa have modelled this in terms of a truncated Fourier series. Applications to epitaxy have exposed the importance of higher Fourier coefficients. In this contribution we report on (i) the derivation of appropriate Fourier series by using reciprocal space formalism, (ii) the defining and deriving of optimum coefficients for a truncated Fourier series and (iii) showing how the interfacial energy, an important quantity in epitaxy, involved in device fabrication, varies with values of Fourier coefficients and modes of misfit accommodation.

Stress and strain energy of a periodic array of interfacial wedge disclination dipoles in a transversely isotropic bicrystal

Analytical closed-form solutions are obtained for the elastic stress and strain energy density fields of a periodic array of interfacial wedge disclination dipoles in a bicrystal. The adjoining crystals are transversely isotropic with maximum dissimilar in-plane crystallographic orientations (0 and π/2). The solutions are obtained by the method of image dislocations. The strain energy per unit area of the bicrystal interface is also obtained numerically. The results show that significant discrepancies can exist between the bicrystal and the isotropic homogeneous solutions. The rates of decrease/increase of the strain energy density and stresses from the interface are smaller in the crystal whose larger stiffness direction is perpendicular to the interface. Also, the strain energy of the bicrystal boundary is a function of the dipole arm length (2 a) and period ( L). The maximum strain energy occurs at a/ L=0.25 and is estimated to be ∼8.9 J/m 2 if the dipole period is 10 nm and the disclination strength is π/2.

Continuum simulations of directional dependence of crack growth along a copper/sapphire bicrystal interface. Part I: experiments and crystal plasticity background

Cracks that exhibit relative amounts of ductility along a copper/sapphire bicrystal interface are simulated within the context of continuum mechanics. The specimen in question exhibits a directional dependence of fracture; that is a crack oriented in one direction along the copper/sapphire interface propagates much more during a given load increment than does the crack oriented to propagate in the opposite direction along the interface. This phenomenon had previously been explained on the basis of an energetic competition between dislocation nucleation and cleavage failure at the two crack tips using both the Rice and Thomson (Philos. Mag. 29 (1974) 73) model as well as the more recent type of dislocation nucleation analysis by Rice (J. Mech. Phys. Solids 40 (1992) 239) based on a Peierls-like stress vs. displacement relationship on the slip plane. However, recent experiments by Kysar (Acta Mater. 48 (2000) 3509) have shown that the orientation of the directional dependence of fracture in the copper/sapphire bicrystal is opposite to that predicted on the basis of dislocation nucleation arguments. The goal of the present work is to attempt to explain the directional dependence of fracture solely on the basis of continuum mechanics. In Part I of this pair of papers we review the main results of the experiments and then set the stage for a series of finite element analyses of the bicrystal specimen by reviewing the fundamentals of single crystal plasticity and the general features of crack tip fields in single crystals. We then discuss two different constitutive hardening models for single crystals and predict that, depending on the hardening model, regions of single-slip around the crack tip may degenerate into regions of triple slip. This leads to a discussion of how the near-tip displacement field can change dramatically with constitutive models. Next the constitutive properties used in the simulations are fit to experimental data. Finally we describe the finite element meshes and procedures for simulating the stationary and quasistatically growing cracks. The simulation results are reported in Part II.

Continuum simulations of directional dependence of crack growth along a copper/sapphire bicrystal interface. Part II: crack tip stress/deformation analysis

The goal of this work is to explain the directional dependence of fracture of interfacial cracks in a copper/sapphire bicrystal in terms of continuum stress and deformation fields. In Part I of this pair of papers, we briefly review the main results of experiments by Kysar (Acta Mater. 48 (2000) 3509) and discuss how the orientation of the directional dependence of fracture is contrary to predictions made on the basis of crack tip dislocation nucleation concepts. We then set the stage for a series of finite element analyses of the bicrystal specimen. In Part II the simulation results are presented. We first conclude that the assumptions which enter into the crack tip dislocation nucleation analyses are valid. Therefore, the orientation of the directional dependence is opposite that of the dislocation nucleation analyses, in spite of the fact that crack tip dislocation nucleation may occur as predicted. We then show that the directional dependence of fracture, at least for the copper/sapphire bicrystal specimen, can be explained by the fact that the quasistatically growing brittle crack has the propensity to generate a significantly higher normal opening stress along its prolongation than does the ductile crack. This conclusion is valid for a wide range of crack growth criteria as well as material constitutive models and parameters. We also present results of the simulated crack opening displacement profiles of the two crack and compare them to experimental measurements. The results do not satisfactorily explain the qualitative features of the normal crack opening displacement profile; however we discuss some possible reasons why the finite element method may not be able to accurately model the crack opening displacement profile.