For Face-centered Cubic Research Articles

Two-way coupled modeling of dislocation substructure sensitive crystal plasticity and hydrogen diffusion at the crack tip of FCC single crystals

Dislocation substructure-sensitive crystal plasticity (DSS-CP) modeling accounts for the evolution of mesoscale structures using dislocation-based parameters informed by experiments and computation at various lower length scales. To a first-order approximation, DSS-CP model parameters are affected by hydrogen (H) concentration, accounting for both H-dependent yield strength and strain hardening rate. This H-affected DSS-CP model is two-way coupled with H-diffusion to explore both effects of plastic deformation on H-diffusion and effects of H on yield strength and strain hardening in the DSS-CP model. Crack tip simulations are performed for face-centered cubic (FCC) metals under monotonic loading conditions with and without H. Enhanced maximum plastic deformation in the vicinity of the crack tip (i.e., localization or intensification of plastic strain) and crack tip opening displacement (CTOD) are predicted in the presence of H, consistent with experimental observations. In spite of increased initial strength due to H, subsequent reduction of the rate of strain hardening in the presence of H is shown to enhance localization of crack tip plasticity. Furthermore, this modeling framework predicts that higher H-diffusivity (leading to a larger H-affected zone) will enhance the crack tip plasticity, making use of the two-way coupling algorithm implemented in this work. On the other hand, we find that the H-sensitivity of crack tip strain localization response, based only on modification of model parameters, is too weak to explain typical experimental observations. This points to the need to develop more advanced DSS-CP constitutive relations that consider highly complex dislocation interactions with point defects.

PySSpredict: A python-based solid-solution strength prediction toolkit for complex concentrated alloys

The emergence of solid solution high entropy alloys (HEAs) and complex concentrated alloys (CCAs) offers opportunities to design novel alloys with tailored strength and ductility. The growing community of integrated-computational materials engineering (ICME) can benefit from implementing state-of-the-art solid-solution strengthening models to alloy design practices. This paper introduces pySSpredict, an open-source python-based toolkit that automates high-throughput calculations of solid-solution strengths of CCAs and thermodynamic properties. We present the functions of the pySSpredict code: (1) automating high-throughput calculations of strength for CCAs, (2) managing the data of thermodynamic calculations from databases or other software, and (3) visualizing and filtering the data to identify candidate alloys. The toolkit implements the latest theoretical edge dislocation model for face-centered cubic (FCC), and edge/screw dislocation models for body-centered cubic (BCC) alloys. The pySSpredict code is hosted on GitHub and deployed on nanoHUB for demonstrations.

Machine Learning-Based Characterization of the Nanostructure in a Combinatorial Co-Cr-Fe-Ni Compositionally Complex Alloy Film.

A novel artificial intelligence-assisted evaluation of the X-ray diffraction (XRD) peak profiles was elaborated for the characterization of the nanocrystallite microstructure in a combinatorial Co-Cr-Fe-Ni compositionally complex alloy (CCA) film. The layer was produced by a multiple beam sputtering physical vapor deposition (PVD) technique on a Si single crystal substrate with the diameter of about 10 cm. This new processing technique is able to produce combinatorial CCA films where the elemental concentrations vary in a wide range on the disk surface. The most important benefit of the combinatorial sample is that it can be used for the study of the correlation between the chemical composition and the microstructure on a single specimen. The microstructure can be characterized quickly in many points on the disk surface using synchrotron XRD. However, the evaluation of the diffraction patterns for the crystallite size and the density of lattice defects (e.g., dislocations and twin faults) using X-ray line profile analysis (XLPA) is not possible in a reasonable amount of time due to the large number (hundreds) of XRD patterns. In the present study, a machine learning-based X-ray line profile analysis (ML-XLPA) was developed and tested on the combinatorial Co-Cr-Fe-Ni film. The new method is able to produce maps of the characteristic parameters of the nanostructure (crystallite size, defect densities) on the disk surface very quickly. Since the novel technique was developed and tested only for face-centered cubic (FCC) structures, additional work is required for the extension of its applicability to other materials. Nevertheless, to the knowledge of the authors, this is the first ML-XLPA evaluation method in the literature, which can pave the way for further development of this methodology.

A precipitation-strengthened high-entropy alloy prepared by selective laser melting in-situ alloying and post-heat treatment

Achieving a superior strength-ductility combination for face-centered-cubic (FCC) single-phase high-entropy alloys (HEAs) with outstanding ductility is challenging. However, precipitation strengthening provides an effective method to achieve this. In this study, (CoCrFeNi)94Al3Ti3 HEA was prepared by selective laser melting (SLM) and in-situ alloying of a blend of CoCrFeNi pre-alloyed powders and Al, Ti elemental powders. The blended powders show excellent printability and the as-printed sample exhibits good comprehensive tensile properties, of which the yield strength is 744 MPa, ultimate tensile strength is 901 MPa and the uniform elongation is 25.1%. After aging treatment, the tensile strength of the as-printed sample increased significantly. More importantly, the analysis of the strengthening mechanism reveals that the high-density nano-scale L12-(Ni, Co)3(Al, Ti)-type particles with two different morphologies contribute most of the improved strength for the aged sample. In addition to the strengthening effect from the coherent L12 nanoparticles, both grain refinement due to the occurrence of partial recrystallization and remaining dislocations after aging treatment also improved the strength of the aged sample, with the yield strength and ultimate tensile strength determined as 1164 MPa and 1503 MPa, respectively. Moreover, L21 and σ phases were also introduced into the aged sample, but these incoherent brittle phases without causing serious embrittlement due to their low volume fractions; therefore, the aged sample still achieved a tensile ductility of 10.1%.

Micro mechanical property investigations of Ni-based high-temperature alloy GH4169 based on nanoindentation and CPFE simulation

In this research, the microscopic deformation behavior of GH4169 superalloy is investigated by a series of nanoindentation experiments and simulations based on crystal plasticity finite element (CPFE) model, including hardness, creep displacement and stress distribution around indentation. In this regard, the crystal plasticity constitutive model based on dislocation slip for face-centered cubic (FCC) crystal of concern is summarized and numerically implemented at first. Moreover, nanoindentation experiments are performed on specimens with various peak loads and load times to investigate the mechanical response and deformation mechanism at the micro-scale. Nanoindentation test results show that load rate, peak load, and nanoindentation size effect (ISE) all affect the microscopic deformation behavior of GH4169. The value of creep stress exponent, m, calculated from the creep experimental data is greater than 3, indicating that dislocation slip is dominant creep mechanism at room temperature.The nanoindentation simulation data based on the crystal elastic–plastic parameters determined by the trial-and-error method are in good agreement with the experimental data, demonstrating that the CPFE simulation results are credible. Therefore, this study can be used as a reference to investigate other alloy materials.

High-strength AlCoCrFeNi2.1 eutectic high entropy alloy with ultrafine lamella structure via additive manufacturing

AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA), with its unique in-situ composite structure, not only overcomes the shortcoming of insufficient strength for face-centered-cubic (FCC) single-phase high entropy alloy (HEA), but also overcomes the shortcoming of insufficient ductility for body-centered-cubic (BCC) single-phase HEA, thus attracting widespread attention from the academic community. In this study, AlCoCrFeNi2.1 EHEA with a fully nano-lamella structure was prepared by selective laser melting (SLM). Furthermore, massive L12 and BCC nano-precipitates were precipitated out from the FCC and B2 phases, respectively. Compared to AlCoCrFeNi2.1 EHEA prepared by traditional methods, the SLM-ed EHEA sample shows excellent strength and ductility synergy, with the yield strength, ultimate tensile strength and uniform elongation determined as 1329 ± 12 MPa, 1621 ± 16 MPa and 11.7 ± 0.5%, respectively. The strengthening contributions to the high yield strength of the sample come from nano-lamella structure, grain boundaries, dislocations and nano-precipitates. In addition, wear behavior at room temperature and elevated temperatures of the SLM-ed EHEA sample have also been studied. The tribological property is substantially enhanced with increasing temperature from room temperature to 700 °C due to the transformation in wear mechanism from adhesive wear to oxidative wear.

Partially Auxetic Properties of Face‐Centered Cubic Hard‐Sphere Crystals with Nanochannels of Different Sizes, Parallel to [001]‐Direction and Filled by Other Hard Spheres

Results of constant pressure and temperature Monte Carlo (MC) simulations with varying shapes of the periodic box are presented for face‐centered cubic (FCC) hard‐sphere crystals with periodic inclusions––arrays of channels parallel to the crystalline axis and forming a square lattice with and axes in the plane. The channels, referred to as nanochannels because their diameters do not exceed a few lattice constants of the matrix crystal, are filled by hard spheres of slightly different diameters from the spheres forming the matrix crystal. Fourteen types of nanochannels with different sizes, formed by the spheres lying on the nanochannel axis and up to those being their thirteenth nearest neighbor, are simulated. The nanochannel axes are distanced by six lattice constants of the monoatomic FCC crystal and arranged in a square lattice with main axes parallel to one of the main axes of the FCC lattice, formed by identical spheres. It is shown that small differences in: 1) channel sizes, 2) their geometry; and 3) diameter ratios of inclusion to matrix spheres can lead to large differences in the elastic properties of the studied nanocomposite structures. This is well seen in the minimum and maximum values of Poisson's ratio.

The Effects of Stresses and Interfaces on Texture Transformation in Silver Thin Films.

Thin metal films are critical elements in nano- and micro-fabricated technologies. The texture orientation of thin films has a significant effect on applied devices. For Face-Centered Cubic (FCC) metal thin films, when the critical thickness is reached, the texture orientation can transform from (111) to (100) based on the model related to the balance between interfacial energy and strain energy. This research focused on the texture transformation of thin films under two conditions: (1) with or without an adhesion layer in the thin film and (2) with or without initial stress applied through a four-point bending load. In the experiment, two samples (silicon/silver and silicon/titanium/silver) were used to apply different initial stress/strain values and different annealing times. After annealing, an X-ray Diffractometer (XRD) was used to ascertain the preferred orientation of the thin films and the percentage of (111) and (100). Finally, Electron Back-Scattered Diffraction (EBSD) was used to observe the grain size of the thin films. The results showed that, regardless of the existence of an adhesion layer, texture transformation occurred, and this was relatively significant with Ti adhesion layers. Further, the initial stress was found to be small compared to the internal stress; thus, the initial stress imposed in the tests in this research was not significantly influenced by the texture transformation.

EQUILIBRIUM VACANCY CONCENTRATION AND THERMODYNAMIC QUANTITIES OF FCC DEFECTIVE ALLOYS AuCuSi AND PtCuSi UNDER PRESSURE AND TEMPERATURE

We present the analytic expressions of the cohesive energy, the alloy parameters, the equation of state, the mean nearest neighbor distance, the Helmholtz free energy, equilibrium vacancy concentration, and thermodynamic quantities such as the isothermal compressibility, the thermal expansion coefficient, the heat capacities at constant volume and constant pressure for facecentered cubic (FCC) defective ternary substitutional and interstitial alloy ABC derived by the statistical moment method (SMM). The obtained thermodynamic quantities depend on temperature, pressure, the concentration of substitutional atoms, the concentration of interstitial atoms, and equilibrium vacancy concentration. Thermodynamic quantities of FCC defective metal A, FCC defective substitutional alloy AB, and FCC defective interstitial alloy AC are specific cases for thermodynamic quantities of FCC defective ternary substitutional and interstitial alloy ABC. The theoretical results are calculated numerically to alloys AuCuSi and PtCuSi. Our calculated results of thermal expansion coefficient and heat capacities at constant pressure for main metals Au, Pt are in good agreement with experimental data. Our other calculated results for thermodynamic quantities of alloys AuCuSi and PtCuSi at different temperatures, pressure, the concentration of substitutional atoms, and concentrations of interstitial atoms orient and predict new experimental data in the future.

Molecule-like chemical units in metallic alloys

Each conventional alloy has its own specific compositions but the compositional origin is largely unknown due to our insufficient understanding about chemical short-range ordering in the alloy, in particular, in the solid-solution state. In the present paper, the compositions of metallic alloys are discussed and formulated, by unveiling the basic moleculelike structural units in solid solutions. Friedel oscillation theory, which describes the partial charge screening behavior in solid solutions, and henceforth the origin of short-range ordering, is applied to pin down the ideal chemical compositions of conventional metallic alloys. We propose that, at a specific composition, atoms self-assemble into an ideally ordered structure consisting of atoms residing in the nearest-neighbor shell (denoted as cluster) plus those in the next outer shell (denoted as glue atoms), which can be formulated as [cluster](glue atoms). This simplified version of short-range-order structure represents the smallest charge-neutral and mean-density zone (termed as “chemical units”) and can be regarded as the ‘molecules’ of solid solutions. Accordingly, the chemical units and the corresponding molecule-like formulas for face-centered-cubic (FCC), hexagonal close-packed (HCP), and body-centered cubic (BCC) structures are analyzed and equations are obtained to identify the chemical formulas for FCC solid solutions. For instance, well-known α-brass Cu-30Zn alloy is formulated as [Zn-Cu12]Zn4. Examples of aluminum alloys, superalloys and stainless steels are also illustrated, demonstrating the versatility of the present model to interpret chemically complex alloys.

Atomistic simulations of the graded residual elastic fields in metallic nanowires

The local inhomogeneity of bonding environments in nanomaterials should lead to non-uniform distributions of the surface-induced residual stress and strain (residual elastic fields), which is inconsistent with the conventional assumption of uniform residual quantities. Such graded feature of the residual stress and strain is investigated in this paper based on molecular dynamics (MD) simulations for face-centered-cubic (FCC) metallic nanowires. It is found that the residual stress and strain do not only exhibit sharp variations near boundary surfaces but also have noticeable gradients in the interior of nanowires, and the gradients of residual elastic fields inside nanowires gradually vanish with an increasing characteristic size. Meanwhile, the non-uniformities of residual stress and strain inside [100] and [110] axially-oriented nanowires are more significant than those in [111]-oriented nanowires. Furthermore, by fitting the molecular dynamics (MD) data, sinusoidal and polynomial functions can be obtained to describe the size-dependent graded distributions of residual quantities in square and circular-shaped nanowires. The present work enables ones to better understand the stress and strain states in a relaxed nano-solid, which should be helpful for the mechanical design of nanomaterials.

Correlating Strength and Hardness of High‐Entropy Alloys

Strength and hardness of metallic materials are reported to correlate in a specified form. Among various equations, yield strength is generally converted from Vickers hardness (HV) via a three‐time relation due to the simple and nondestructive nature of hardness testing. Herein, a through literature review is made and data of strength and HV for face‐centered cubic (FCC) and body‐centered cubic (BCC) high‐entropy alloys (HEA) are collected. The yield strength and HV visibly deviate from the three‐time relation, but the ultimate tensile strength and HV roughly follow the three‐time relation. New linear relationships, which are universally applied to convert HV to yield strength and ultimate tensile strength, are proposed.

Inverse grain-size-dependent strain rate sensitivity of face-centered cubic high-entropy alloy

It is well documented that the strain rate sensitivity (m) increases at refined grain size for face-centered cubic (FCC) metals and alloys. Through a series of nanoindentation testing, however, we experimentally demonstrated a striking departure from conventional FCC metals that CoCrFeMnNi high entropy alloy (HEA) with FCC lattice structure exhibits monotonously decreased m as grain size reduced from ∼30.3 μm to 7.2 nm. Moreover, the apparent activation volume v*, which generally shows an opposite trend of m, exhibited the identical decreasing trend with reduced grain size as that of m. Such an unusual trend of m and its correlation with v* in the FCC HEA alloys can be understood by a distinct deformation-mechanism-transitions and unique dislocation morphology evolution that differs from conventional FCC metals.

Crystallographic Orientation and Spatially Resolved Damage for Polycrystalline Deformation of a High Manganese Steel

In-situ neutron diffraction investigation on a high manganese steel, which was stretched before or after the fatigue loading, renders a meso-scale damage criterion: the {111} and {422} lattice strains along the transverse direction violating the Poisson effect signifies severe damage. They either showed no more lattice contraction corresponding to increasing of the tensile stress or transversal expansion during the plastic stage of tension. Distribution of the damaged grains was further investigated by the full-width at half-maximum pole figures. The crystal plasticity simulations justify the rationality of the damage criterion, and it could relate to the orientation distribution of the damaged slip/twinning planes. The cracks mainly distributed at the transverse surface. It is shown that the strong interaction between twin boundaries and slip dislocations could result in heavy damage at the surface, with a morphology of curved twin boundaries. Also, grain boundaries and the narrow deformation twins often correspond to different amplitudes of transversal contraction and expansion than other surface areas during the tension-compression fatigue loading, which may trigger the surface cracks. It is due to large crystallographic orientation gradients. The present paper provides a sound routine to identify criterions of the plastic damage for face-centered-cubic (FCC) polycrystals.

Cross stacking faults in Zr(Fe,Cr)2 face-centered cubic Laves phase nanoparticle

Stacking faults (SFs) are mostly related to the glide of partial dislocations. Although the SFs have been extensively studied, the cross stacking faults (CSFs) have hardly been explored. In fact, for face-centered cubic (FCC) structure, the {1 1 1} family planes include four crossed close-packed planes. If the dislocations originate on the crossed planes, cross slips or CSFs can be induced. Here we report CSFs formed in the Zr(Fe,Cr)2 FCC Laves phase nanoparticle (NP) in a zirconium alloy after shear deformation by means of transmission electron microscopy (TEM). Our results reveal that SFs mostly originate from the phase boundary by 1/6 〈1 1 2〉 Shockley partial dislocations on crossed close-packed planes. More importantly, the driving force of CSFs was further discussed. Finally, we propose a model for the formation of CSFs in the FCC NP. That is: when the loading is parallel or close to 〈0 0 1〉 directions, the CSFs can be induced in the FCC crystals. Knowledge about our investigations here will provide a fundamental understanding of the CSFs.

A possibility of Pd based high entropy alloy for hydrogen gas sensing applications

Diffusion of H atoms in Pd-based alloy usually takes place through octahedral voids for face centered cubic (FCC) metals and alloys. Transition metals are preferred as the alloying element for Pd-based alloy because electron removal and addition from Pd, and the number of vacant d-states of Pd can be regulated by lacking the deformation of electronic structure of Pd. In addition, the alloying alters a lattice constant and hydrogen permeability, thereby improving the mechanical, chemical stabilities of Pd-based devices and reduce the expensive cost of Pd raw material. High entropy alloys (HEA) have been investigated as an important contributor in field of microelectronics as a catalyst, diffusion barrier, and anode materials for a solid oxide fuel cell (SOFC). HEA’s of Pd, Ag, Ni, Cu, Au, Rh, Pt and Ir are designed empirically and their lattice constants, corresponding an octahedral void size are analyzed with pure Pd, thus suggesting them as a potential candidate for hydrogen storage applications in the future. The core reason for designing HEA is to overcome the problem of cost and availability of Pd and Pd-rich binary alloys. In this paper, we calculated the lattice parameters using the Vergards law, octahedral void size, lattice distortion and cost for PdAg-based HEA’s of 20 combinations.

Machine Learning\u2013Based Reduce Order Crystal Plasticity Modeling for ICME Applications

Crystal plasticity simulation is a widely used technique for studying the deformation processing of polycrystalline materials. However, inclusion of crystal plasticity simulation into design paradigms such as integrated computational materials engineering (ICME) is hindered by the computational cost of large-scale simulations. In this work, we present a machine learning (ML) framework using the material information platform, Open Citrination, to develop and calibrate a reduced order crystal plasticity model for face-centered cubic (FCC) polycrystalline materials, which can be both rapidly exercised and easily inverted. The reduced order model takes crystallographic texture, constitutive model parameters, and loading condition as inputs and returns the stress-strain curve and final texture. The model can also be inverted and take a stress-strain curve, loading condition, and final texture as inputs and return the initial texture and constitutive model parameters as outputs. Principal component analysis (PCA) is used to develop an efficient description of the crystallographic texture. A viscoplastic self-consistent (VPSC) crystal plasticity solver is used to create the training data by modeling the stress-strain behavior and evolution of texture during deformation processing.

Atomistic Simulation of the Rate-Dependent Ductile-to-Brittle Failure Transition in Bicrystalline Metal Nanowires

The mechanical properties and plastic deformation mechanisms of metal nanowires have been studied intensely for many years. One of the important yet unresolved challenges in this field is to bridge the gap in properties and deformation mechanisms reported for slow strain rate experiments (∼10-2 s-1), and high strain rate molecular dynamics (MD) simulations (∼108 s-1) such that a complete understanding of strain rate effects on mechanical deformation and plasticity can be obtained. In this work, we use long time scale atomistic modeling based on potential energy surface exploration to elucidate the atomistic mechanisms governing a strain-rate-dependent incipient plasticity and yielding transition for face centered cubic (FCC) copper and silver nanowires. The transition occurs for both metals with both pristine and rough surfaces for all computationally accessible diameters (<10 nm). We find that the yield transition is induced by a transition in the incipient plastic event from Shockley partials nucleated on primary slip systems at MD strain rates to the nucleation of planar defects on non-Schmid slip planes at experimental strain rates, where multiple twin boundaries and planar stacking faults appear in copper and silver, respectively. Finally, we demonstrate that, at experimental strain rates, a ductile-to-brittle transition in failure mode similar to previous experimental studies on bicrystalline silver nanowires is observed, which is driven by differences in dislocation activity and grain boundary mobility as compared to the high strain rate case.

Sensitivity of polycrystal plasticity to slip system kinematic hardening laws for Al 7075-T6

The prediction of formation and early growth of microstructurally small fatigue cracks requires use of constitutive models that accurately estimate local states of stress, strain, and cyclic plastic strain. However, few research efforts have attempted to systematically consider the sensitivity of overall cyclic stress-strain hysteresis and higher order mean stress relaxation and plastic strain ratcheting responses introduced by the slip system back-stress formulation in crystal plasticity, even for face centered cubic (FCC) crystal systems. This paper explores the performance of two slip system level kinematic hardening models using a finite element crystal plasticity implementation as a User Material Subroutine (UMAT) within ABAQUS (Abaqus unified FEA, 2016) [1], with fully implicit numerical integration. The two kinematic hardening formulations aim to reproduce the cyclic deformation of polycrystalline Al 7075-T6 in terms of both macroscopic cyclic stress-strain hysteresis loop shape, as well as ratcheting and mean stress relaxation under strain- or stress-controlled loading with mean strain or stress, respectively. The first formulation is an Armstrong-Frederick type hardening-dynamic recovery law for evolution of the back stress [2]. This approach is capable of reproducing observed deformation under completely reversed uniaxial loading conditions, but overpredicts the rate of cyclic ratcheting and associated mean stress relaxation. The second formulation corresponds to a multiple back stress Ohno-Wang type hardening law [3] with nonlinear dynamic recovery. The adoption of this back stress evolution law greatly improves the capability to model experimental results for polycrystalline specimens subjected to cycling with mean stress or strain. The relation of such nonlinear dynamic recovery effects are related to slip system interactions with dislocation substructures.

Fe–Ni Nanoparticles: A Multiscale First-Principles Study to Predict Geometry, Structure, and Catalytic Activity

Nanoparticles of iron and nickel are promising candidates as nanosized soft magnetic materials and as catalysts for carbon nanotube synthesis and CO methanation, among others. To understand geometry- and size-dependent properties of these nanoparticles, phase diagram of Fe/Ni alloy nanoparticles was calculated by density functional theory and cluster expansion method. Ground state convex is presented for face-centered cubic (FCC), body-centered cubic (BCC), and icosahedral (ICO) particles. Previous experimental observations were explained by using multiscale model for particles with realistic size (diameter ≥2 nm). At size 1.5 nm, geometry changes from BCC at low X(Ni) to icosahedral at high X(Ni). FCC is stabilized over icosahedral geometry by increasing number of atoms from 561 to 923. In large FCC particles, there is enrichment of Fe atoms from core to shell beneath surface, while surface and core are enriched by Ni atoms. Catalytic enhancement effect in CO methanation was found to be due to Ni incorpo...