Polycrystalline Metals Research Articles

A model of the spin Hall effect in polycrystalline nonmagnetic metals

In this paper, we theoretically investigated the effect of the polycrystalline structure of metal samples on the magnitude of the spin Hall effect. The equation for the dynamics of the averaged over the crystallite momentum of the collectivized conduction electron in the crystalline field of a homogeneous and isotropic polycrystalline metal is obtained. Averaging the obtained equation over all randomly oriented crystallites resulted in an expression for the electric field of the spin Hall effect. It significantly depends on the form of the single-electron atomic radial wave function and on the Hall coefficient. The coefficients of the spin Hall effect for some metals of 3rd–6th periods are calculated. It is shown that the results for nonmagnetic d- and f-metals within the measurement error are consistent with the experimental data. The best match is observed for Pt, [Formula: see text]-W, [Formula: see text]-W, Pd, Nb, Ag, Mo, Cu and Al. A discrepancy of less than 20% was obtained for Au, Ti, Mn. For the considered rare-earth metals (Lu, Ho, Dy, Gd) only for paramagnetic Lu the correspondence of calculation results and experimental data is obtained. For Ho, Dy, and Gd, which exhibit ferromagnetic and antiferromagnetic states at low temperatures, the discrepancy is more than 35%. Based on the obtained results, the limitations of the developed approach are formulated.

Modelling thermal and irradiation creep with crystal plasticity theory and self-consistent method

In order to help comprehend the underlying deformation mechanisms related to irradiation creep of metallic polycrystalline materials, the crystal plasticity theory and viscoplastic self-consistent method are combined in this work. In the developed constitutive laws for single crystals, the influence of irradiation-induced defects on the creep strain rate has been effectively addressed by considering dislocation climb and glide. For the former, dislocation climb with irradiation effect can be enhanced through the absorption of point defects by dislocations, where the steady-state concentration of point defects is affected by the defect sinks including dislocations and defect clusters. As a comparison, dislocation glide may be retarded through the impediment of mobile dislocations by irradiation-induced defects. More importantly, the effect of dislocation static recovery has been incorporated into the dislocation evolution law, which is noticed to play a deterministic role for the annihilation of dislocations during the long-term creep process. In addition, the viscoplastic self-consistent method is taken as a cross-scale way to predict the irradiation creep properties of polycrystals. To validate the developed creep model, experimental data of both single crystalline and polycrystalline nickel has been considered under both thermal and irradiation creep. A good agreement between the theoretical results and experimental data is achieved, which offers a solid basis to further analyze the macroscopic irradiation creep deformation from the aspect of microstructure evolution.

Correlating Corrosion to Surface Grain Orientations by Scanning Electrochemical Cell Microscopy

The study of grain-dependent corrosion behaviors of practical polycrystalline metals remains challenging due to the difficulty in eliminating the influences of other microstructural features, such as intermetallic particles and grain boundaries. In this work, we performed thousands of microscopic potentiodynamic polarization measurements on a polycrystalline alloys using the spatially resolved oil-immersed scanning electrochemical cell microscopy measurement. This work highlights the capability of oil-immersed scanning electrochemical cell microscopy in resolving small electrochemical differences, which will greatly promote the study of grain-dependent behaviors of practical polycrystalline samples.

Enabling low-drift flexible perovskite photodetectors by electrical modulation for wearable health monitoring and weak light imaging

Metal halide perovskites are promising for next-generation flexible photodetectors owing to their low-temperature solution processability, mechanical flexibility, and excellent photoelectric properties. However, the defects and notorious ion migration in polycrystalline metal halide perovskites often lead to high and unstable dark current, thus deteriorating their detection limit and long-term operations. Here, we propose an electrical field modulation strategy to significantly reduce the dark current of metal halide perovskites-based flexible photodetector more than 1000 times (from ~5 nA to ~5 pA). Meanwhile, ion migration in metal halide perovskites is effectively suppressed, and the metal halide perovskites-based flexible photodetector shows a long-term continuous operational stability (~8000 s) with low signal drift (~4.2 × 10−4 pA per second) and ultralow dark current drift (~1.3 × 10−5 pA per second). Benefitting from the electrical modulation strategy, a high signal-to-noise ratio wearable photoplethysmography sensor and an active-matrix photodetector array for weak light imaging are successfully demonstrated. This work offers a universal strategy to improve the performance of metal halide perovskites for wearable flexible photodetector and image sensor applications.

High performance, single crystal gold bowtie nanoantennas fabricated via epitaxial electroless deposition

Material quality plays a critical role in the performance of nanometer-scale plasmonic structures and represents a significant hurdle to large-scale device integration. Progress has been hindered by the challenges of realizing scalable, high quality, ultrasmooth metal deposition strategies, and by the poor pattern transfer and device fabrication yields characteristic of most metal deposition approaches which yield polycrystalline metal structure. Here we highlight a novel and scalable electrochemical method to deposit ultrasmooth, single-crystal (100) gold and to fabricate a series of bowtie nanoantennas through subtractive nanopatterning. We investigate some of the less well-explored design and performance characteristics of these single-crystal nanoantennas in relation to their polycrystalline counterparts, including pattern transfer and device yield, polarization response, gap-field magnitude, and the ability to model accurately the antenna local field response. Our results underscore the performance advantages of single-crystal nanoscale plasmonic materials and provide insight into their use for large-scale manufacturing of plasmon-based devices. We anticipate that this approach will be broadly useful in applications where local near-fields can enhance light–matter interactions, including for the fabrication of optical sensors, photocatalytic structures, hot carrier-based devices, and nanostructured noble metal architectures targeting nano-attophysics.

Ultrasonic Identification of Polycrystalline Metal Materials based on Linear Prediction Analysis

Ultrasonic Identification of Polycrystalline Metal Materials based on Linear Prediction Analysis

Machine learning based approach to predict ductile damage model parameters for polycrystalline metals

Damage models for ductile materials typically need to be parameterized, often with the appropriate parameters changing for a given material depending on the loading conditions. This can make parameterizing these models computationally expensive, since an inverse problem must be solved for each loading condition. Using standard inverse modeling techniques typically requires hundreds or thousands of high-fidelity computer simulations to estimate the optimal parameters. Additionally, the time of a human expert is required to set up the inverse model. Machine learning has recently emerged as an alternative approach to inverse modeling in these settings, where the machine learning model is trained in an offline manner and new parameters can be quickly generated on the fly, after training is complete. This work utilizes such a workflow to enable the rapid parameterization of a ductile damage model called TEPLA with a machine learning inverse model. The machine learning model can efficiently estimate the model parameters much faster, as compared to previously employed methods, such as Bayesian calibration. The results demonstrate good accuracy on a synthetic test dataset and is validated against experimental data.

Columnar grain morphology and mechanical anisotropy of face-centered cubic metals and alloys

The columnar grain morphology (grain shape and orientation) in polycrystalline metals is generally considered to contribute to mechanical anisotropy due to the interaction of dislocations with grain boundaries. However, past investigations have failed to reach a consensus on the role of grain morphology on property anisotropy. In this study, the influence of columnar morphology as a potential source for anisotropy is investigated for directed energy deposited (DED) stainless steel 316L (SS 316L). Macroscopic stress-strain behaviors and microscopic slip band evolution of DED SS 316L specimens were obtained under uniaxial tensile testing. The stress-strain behaviors show that the influence of grain morphology on anisotropy is negligible. The characteristics of slip band evolution reveal that the influence of grain morphology on anisotropy depends on the outcome of two competing choices for slip selection: the least resistant glide path versus the one that offers the greatest compatibility with the intergranular deformation.

Effect of sample size on the maximum value distribution of fatigue driving forces in metals and alloys

An analytical framework is presented to predict the effects of sample size on the maximum value distribution (MVD) of the driving forces for fatigue crack formation in metals and alloys. The distribution of the maximum driving force for fatigue crack formation over the domain follows the generalized extreme value theory in the limit as the domain size increases to infinity. However, a simulation-based analysis of microstructure influences on fatigue resistance for polycrystalline metals and alloys is very costly, and reaching those limits is intractable. This work models the MVD of Fatigue Indicator Parameters (FIPs), which serve as surrogate measures for the driving force for fatigue crack formation, at finite sample sizes prior to their convergence to a limiting extreme value distribution. Large-scale crystal plasticity finite element (CPFE) simulations of FCC Al 7075-T6 with microstructure realizations of various sizes are incorporated to calibrate and evaluate the developed framework, and a total of ∼6.5 million grains of Al 7075-T6 are examined. The calibrated analytical solution agrees well with the brute force Monte Carlo simulation results extracted from the CPFE simulations. Furthermore, the developed formulation can predict the MVD of FIPs for different sample sizes using a size-dependent parameter, and it is capable of accurately extrapolating the MVD of FIPs for much larger microstructure sample sizes than the size used for its calibration.

Local Stress Measurements in Polycrystalline Metallic Tensile Specimens Using High Resolution EBSD.

Local Stress Measurements in Polycrystalline Metallic Tensile Specimens Using High Resolution EBSD.

A Lie-algebra-based description of disclination densities and the quantification of partial disclinations in deformed polycrystalline metals

Crystalline defects (e.g., dislocations, disclinations and grain boundaries) play a critical role in determining the mechanical and functional properties of metallic materials. From a physical point of view, dislocations and disclinations are the two fundamental types of topological defects in crystals, which originate from the breaking of translational symmetry and rotational symmetry, respectively. In spite of extensive studies on dislocations in the literature, the effects of disclinations on mechanical properties of metals have not been well recognized, partially due to the lack of an appropriate description for the rotational nature of disclinations. Here we suggest a Lie-algebra-based method to quantify the rotational properties of disclinations, which leads to a convenient way to determine disclination density distribution directly from Electron Backscatter Diffraction data. Through quasi-in-situ Electron Backscatter Diffraction characterizations, three major formation mechanisms of disclinations have been identified in deformed polycrystalline Mg alloys, i.e., dislocation-grain boundary interaction, dislocation redistribution and twin-twin reaction, all of which can be treated as topological reactions among various types of defects under a general framework. Our work not only suggests a new mathematical tool to investigate the interactions and reactions among multiple types of crystalline defects, but also provides a new insight to understand the deformation behaviors of metals and alloys based on dislocation/disclination theory.

An explicit dynamic FFT method for homogenizing heterogeneous solids under large deformations

We propose an explicit displacement-based FFT method for computational homogenization and multi-scale modeling of heterogeneous solids. Unlike existing FFT methods that solve the static equilibrium equation using implicit iterative techniques, our proposed explicit method solves dynamic equations with the Central Difference method. The voxel-like discretizations of the periodic domain and the direct calculation of gradient and divergence operations using Discrete Fourier Transform and its inverse result in a straightforward algorithm that avoids convergence issues and does not require material consistent tangents. The stability condition of this explicit method is also addressed. Four numerical examples (2D particle-reinforced composites of hyperelasticity and viscoelasticity, a 3D fiber-reinforced composite, and a 3D polycrystalline metal) illustrate that the proposed method calculates as accurately and efficiently as implicit FFT methods for quasi-static problems. Furthermore, the proposed explicit displacement-based FFT method outperforms implicit FFT methods when dealing with microstructures comprising phases with very high contrast. Moreover, since the method does not require material consistent tangents, it extends the applicability of FFT methods to the homogenization of materials with complex or black-boxed constitutive models, where consistent tangents are challenging to compute.

Thermo-kinetic characteristics on stabilizing hetero-phase interface of metal matrix composites by crystal plasticity finite element method

Using dislocation-based constitutive modeling in three-dimension crystal plasticity finite element (3D CPFE) simulations, co-deformation and instability of hetero-phase interface in different material systems were herein studied for polycrystalline metal matrix composites (MMCs). Local stress and strain fields in two types of 3layer MMCs such as fcc/fcc Cu–Ag and fcc/bcc Cu–Nb have been predicted under simple compressive deformations. Accordingly, more severe strain-induced interface instability can be observed in the fcc/bcc systems than in the fcc/fcc systems upon refining to metallic nanolayered composites (MNCs). By detailed analysis of stress and strain localization, it has been demonstrated that the interface instability is always accompanied by high-stress concentration, i.e., thermodynamic characteristics, or high strain prevention i.e., kinetic characteristics, at the hetero-phase interface. It then follows that the thermodynamic driving force ΔG and the kinetic energy barrier Q during dislocation and shear banding can be adopted to classify the deformation modes, following the so-called thermo-kinetic correlation. Then by inserting a high density of high-energy interfaces into the Cu-Nb composites, such thermo-kinetic integration at the hetero-phase interface allows a successful establishment of MMCs with the high ΔG-high Q deformation mode, which ensures high hardening and uniform strain distribution, thus efficiently suppressing the shear band, stabilizing the hetero-phase interface, and obtaining an exceptional combination in strength and ductility. Such hetero-phase interface chosen by a couple of thermodynamics and kinetics can be defined as breaking the thermo-kinetic correlation and has been proposed for artificially designing MNCs.

A crystal plasticity model of low cycle fatigue damage considering dislocation density, stress triaxiality and Lode parameter

To capture the mechanical behavior and low cycle fatigue damage of metals, a crystal plasticity model combined with isotropic continuum damage mechanics model of low cycle fatigue is developed. In this constitutive model, the Lemaitre model of low cycle fatigue damage is modified accounting for the effect of stress triaxiality and Lode parameter, the slip strength is associated with the evolution of the density of statistically stored forest dislocations and debris dislocations, and a non-linear kinematic hardening rule is employed to capture the cyclic response. Simulation is implemented by applying this developed constitutive model to the Voronoi polycrystalline aggregation of 7075 aluminum alloy. The studied results show that the grains with low Schmid factor generally suffer more severe damage compared to those with high Schmid factor. In addition, due to the orientation mismatch of adjacent grains, localized stress concentrations and the resulting severe damage generate near the grain boundaries, especially near the grain boundaries with high misorientation angles. The proposed constitutive model manifests the capacity for the determination of damage evolution of the polycrystalline metals.

Nucleation and Growth of Cu2O: Role of Potential, Electrolyte pH, and Substrate

The nucleation and growth mechanism of functional oxides has a direct bearing on the structural and electronic properties of the deposit. We study the effect of electrolyte pH and deposition potential on the nucleation and growth of Cu2O on polycrystalline metal oxide (FTO) & metal (Au) substrates. Modelling of the recorded current-time transients indicates that both instantaneous and progressive nucleation occur with growth limited by diffusion or lattice incorporation of electro-active species or both. The preferred orientation of Cu2O shows a strong dependence on electrolyte pH. The films are (100) oriented on both substrates at pH 9 except at high applied potential on FTO where the orientation changes to (111). Interestingly, irrelevant of electrolyte pH, the grain size of Cu2O decreases with potential on FTO whereas it increases on Au substrates. We attribute this to a difference in the number of active nucleation sites between the two substrates. The nucleation and growth at pH 12 is observed to be dependent both on diffusion and lattice incorporation of electro-active species. Additionally, the films are primarily (111) oriented on both substrates, which is correlated to the availability of OH− ions.

Synthesis of α-Amino Acids via Electrochemical Fixation of CO2 to Imines Using Different Metal Cathodes

The electrochemical fixation of CO2 by imines has recently attracted an increased interest as sustainable strategy for the synthesis of α-amino acids and a green alternative to the traditional Strecker synthesis, which relies on highly toxic precursors. Despite the industrial prospects of the electrochemical approach, the catalyst material effects on the selectivity of the process are still purely understood, hindering rational catalyst design. Herein, we study the electrochemical fixation of CO2 by N-benzylideneaniline using a wide variety of cathode materials, including 10 polycrystalline metals (Ti, Zn, Au, Pd, Pt, Sn, Ag, Ni, Fe, Cu), glassy carbon, and Pd nanoparticles of different shapes. We found that among all studied bulk metals, Ti and Zn show the best results with above 93% faradaic efficiency of α-amino acid, while other materials show from good to low selectivity (12% for Sn). We also demonstrate that especially high current densities and nearly quantitative faradaic efficiency and selectivity of α-amino acids can be achieved by employing Pd nanoparticles.

Young's modulus prediction model for metal carbides based on first-principles calculations

The stability of metal carbides is calculated and the mechanical properties of stable metal carbides are presented. Primitive volume (V), electronegativity (δ) and number of valence electrons (N) of the metal element are chosen to predict Young's modulus (E) of polycrystalline metal carbides. A prediction equation is established (E=982.822V−0.263δ−0.831+N2.610), which can accurately predict the E value of metal carbides with an R2 value of 0.90. It is interest of selecting an appropriate compositional design of carbides in terms of overall properties as well as elastic properties by using these three parameters.

Nonmetallic trace elements induced rhenium co-segregation in nickel Σ5 [001](210) symmetrical tilt grain boundary

The thermally/mechanically-driven structural instability of nano- and coarse-grained polycrystalline metals originates from the unstable grain boundaries (GBs). GB segregation of solute elements could not only stabilize but also embrittle GBs. To improve GB stability and cohesion in parallel via solute segregations, in this work the segregation behavior of six non-metallic impurities X at the Σ5 [001](210) GB of nickel-based alloys, their co-segregations with Re element that has no GB segregation tendency, and segregation-induced changes in GB properties were investigated comprehensively from first-principles calculations. We discovered that these nonmetallic elements can segregate to the Ni GB and thus stabilize it, but they are GB embrittlers except B. Annealing temperature and bulk concentration have no effect on the S segregation. With the X pre-segregations, induced GB segregations of Re were observed, which lowers the excess energy and enhances the cohesive strength of all X-segregated GBs. The attractive interactions between C/N/B and Re atoms promote the Re segregation. The stabilizing and strengthening mechanisms of Re segregation were mainly attributed to the changes in Fermi level location and the increased interplanar bonding, respectively. Our findings may gain more insights into the atomic-scale GB co-segregations and stabilization of polycrystalline Ni-based alloys.

Measurement and In-Depth Analysis of Higher Harmonic Generation in Aluminum Alloys with Consideration of Source Nonlinearity.

Harmonic generation measurement is recognized as a promising tool for inspecting material state or micro-damage and is an ongoing research topic. Second harmonic generation is most frequently employed and provides the quadratic nonlinearity parameter (β) that is calculated by the measurement of fundamental and second harmonic amplitudes. The cubic nonlinearity parameter (β2), which dominates the third harmonic amplitude and is obtained by third harmonic generation, is often used as a more sensitive parameter in many applications. This paper presents a detailed procedure for determining the correct β2 of ductile polycrystalline metal samples such as aluminum alloys when there exists source nonlinearity. The procedure includes receiver calibration, diffraction, and attenuation correction and, more importantly, source nonlinearity correction for third harmonic amplitudes. The effect of these corrections on the measurement of β2 is presented for aluminum specimens of various thicknesses at various input power levels. By correcting the source nonlinearity of the third harmonic and further verifying the approximate relationship between the cubic nonlinearity parameter and the square of the quadratic nonlinearity parameter (β∗β), β2≈β∗β, the cubic nonlinearity parameters could be accurately determined even with thinner samples and lower input voltages.

Coupling Computational Homogenization with Crystal Plasticity Modelling for Predicting the Warm Deformation Behaviour of AA2060-T8 Al-Li Alloy.

This study aimed to propose a new approach for predicting the warm deformation behaviour of AA2060-T8 sheets by coupling computational homogenization (CH) with crystal plasticity (CP) modeling. Firstly, to reveal the warm deformation behaviour of the AA2060-T8 sheet, isothermal warm tensile testing was accomplished using a Gleeble-3800 thermomechanical simulator at the temperatures and strain rates that varied from 373 to 573 K and 0.001 to 0.1 s-1. Then, a novel crystal plasticity model was proposed for describing the grains' behaviour and reflecting the crystals' actual deformation mechanism under warm forming conditions. Afterward, to clarify the in-grain deformation and link the mechanical behaviour of AA2060-T8 with its microstructural state, RVE elements were created to represent the microstructure of AA2060-T8, where several finite elements discretized every grain. A remarkable accordance was observed between the predicted results and their experimental counterparts for all testing conditions. This signifies that coupling CH with CP modelling can successfully determine the warm deformation behaviour of AA2060-T8 (polycrystalline metals) under different working conditions.