Fcc Materials Research Articles

Skin depth, optical density, electron-phonon interaction, steepness parameter, band tail width, carriers transitions and dissipation factors in Cu-Co bilayer films

This work aims to study Cu films, and Cu-Co bilayer deposited using DC Magnetron sputtering system on silicon substrates. It can be seen that as the thicknesses of films were increased, the grain size becomes larger which was in good agreement with the presented SEM and AFM images. The high-intensity peak for FCC materials was generally (111) reflection, which is observed in the films when the Co-doping ratio was increased, the (111) peak became stronger. The PL spectra have shown that the Cu films with 70 nm Cu have a maximum value of the oxygen defect density however Cu-Co bilayer with 70 nm Co have a minimum value of the oxygen defect density. We found that the Cu-Co bilayer with 70 nm Co was more disordered and the disordering energy EU in these bilayers was in a bout of 0.681 eV. The Cu-Co bilayers with 40 nm Co have maximum values of the energy gap about of 3.75 eV. The Cu-Co bilayers with 40 nm Co have maximum values of the electron-phonon interaction Ee-ph in about 0.0180 eV. The maximum value of the steepness parameter has occurred in Cu films with 70 nm Co and was 51.55 eV. It can be seen that the surface and volume dissipation factors in the Cu-Co bilayers with 60 nm Co have the maximum value. The maximum value of skin depth of films has occurred in Cu films with 60 nm Cu and it has a peak maximum about of 280 nm. The Cu-Co bilayers with 70 nm have maximum values of the average crystallite size about of 29.1 nm. Also, it is found that the Cu-Co bilayers with 70 nm have minimum values of micro strain ε. It can be observed that the relation between the optical band gap energy Eg and the Urbach energy Eu by Eg = 0.71 ‒ 0.5Eu were studied.

A new ANN based crystal plasticity model for FCC materials and its application to non-monotonic strain paths

Machine learning (ML) methods are commonly used for pattern recognition in almost any field one could imagine. ML techniques can also offer a substantial improvement in computational time when compared to conventional numerical methods. In this research, a machine learning- and crystal plasticity-based framework is presented to predict stress–strain behaviour and texture evolution for a wide variety of materials within the face-centred cubic family (FCC). Firstly, the process of the framework design is described in detail. The proposed framework was designed to be built of ensemble of artificial neural networks (ANN) and a crystal-plasticity based algorithm. Next, the dataset constituent of crystal plasticity simulations was collected. The dataset consisted of examples of monotonic deformation cases, was prepared for training using mathematical transformations, and finally used to train ANNs used in the framework. Then, the ML framework was demonstrated to predict full stress–strain and texture evolution of different FCC single crystals under uniaxial tension, compression, simple shear, equibiaxial tension, tension–compression–tension, compression–tension–compression, and, finally, for some arbitrary non-monotonic loading cases. The proposed framework predicts the stress–strain response and texture evolution with a high degree of accuracy. The results demonstrated in this research show that the proposed machine learning- and crystal plasticity-based framework exhibits a tremendous computational advantage over conventional crystal plasticity model. Finally, one of the most important contributions of this work is to show the framework’s feasibility. The work demonstrates that machine learning methods can help predict complex strain paths without having to train machine learning models on the infinite set of possible non-monotonic loading scenarios.

Compositional effects on stacking fault energies in Ni-based alloys using first-principles and atomistic simulations

Stacking fault energy (SFE) in fcc materials is a fundamental property that is closely related to Shockley partial dislocations and deformation twinning, the latter of which is potentially responsible for some of the exceptional mechanical properties observed in Ni-based high/medium-entropy alloys. In this study, the SFEs and twinning energies over a wide range of compositions are systematically determined in model Ni-based binary alloys using both first-principles density functional theory (DFT) and atomistic simulations with interatomic potentials. Particularly, different compositional dependences of SFEs are observed in the selected binary alloys (Ni-Cu, Ni-Co, and Ni-Fe) from DFT calculations. We find that SFEs of Ni-Co follow a linear-mixing rule while Ni-Cu and Ni-Fe systems exhibit non-linear compositional dependences, especially in the concentrated region towards the center of the phase diagram. Analyses of the magnetic structures help clarify the origins of the non-linear dependences. The fidelity of existing interatomic potentials for these alloys is evaluated against DFT. Using the interatomic potentials with the overall highest fidelity, the SFE calculations are extended to Cantor-related ternary alloys (Ni-Co-Cr and Ni-Co-Fe) and the spatial features of the fault energies in atomistic simulations are also discussed. These results provide the basis for a systematic understanding of the compositional effects on the SFEs and twinning energies, which could be useful for a systematic tuning of mechanical properties in non-equimolar alloys, thus leading to a broad field in alloy design.

Analytical determination of anisotropic parameters for Poly6 yield function

The present work mainly focuses on the analytical determination of anisotropic parameters for Poly6 yield criterion, which is mathematically equivalent to Yoshida2013 yield function. The parameters of Poly6 yield criterion are expressed with the r-values and yield stresses without any optimization method, which has been successfully applied for highly anisotropic materials. The Poly6 yield criterion is analytically calibrated using two different methods under associated flow rule; i.e, 1) the shear flow stress and rb value and 2) the stresses under σxx = 2σyy and 2σxx =σyy. The first type of analytical calibration from shear stress and rb value is compared with the Yoshida2013 yield criterion whose parameters are obtained from an optimization process, which shows that they are equivalent for describing stress and strain distributions. The yield loci of FCC materials predicted from the second method is similar to Yld2000-2d. Both methods of calibrating the analytical Poly6 yield criterion are shown to describe the r-value and uniaxial yield stress directionalities. The accuracy and effectiveness of the proposed analytical methods are verified through the selected FCC and BCC materials. The ability to describe the evolutions of materials’ anisotropic behaviors with the proposed analytical Poly6 has been verified by applying it to AA5182-O.

Factors affecting Confidence Index in EBSD analysis

Automated EBSD analysis systems have been used for over two decades relying on the fact that the correctness of a particular orientation solution could be assessed through the calculation of a Confidence Index. However, such an index only reports whether a particular solution stands out among any other possible solutions, by receiving a larger number of votes than others, and does not necessarily imply that the corresponding orientation is correct. This issue is addressed in the present paper, where the correctness of solutions and the factors that might affect it have been studied for single crystal Si and polycrystalline Zn. The results were compared to those presented in previous papers where this matter was studied in particular for an FCC material. It was observed that about 90% of the solutions were correct if they were obtained using a confidence index of 0.1 using at least 8 Hough bands regardless of the crystallographic structure or orientation.

Low-temperature tensile properties of Cu-Fe laminated sheets with various number of layers

The Cu-Fe laminated sheets with various number of layers were produced by the accumulative roll bonding process, and the effects of the number of layers on the microstructure and low-temperature tensile properties of the sheets were elucidated. Then the reason for the excellent low-temperature tensile properties in dual-phase materials of Cu and Fe was discussed. The tensile properties of the pure Cu and Fe sheets used for fabricating the Cu-Fe laminated sheets exhibited the typical temperature dependence seen in the case of face-centered cubic (fcc) and body-centered cubic (bcc) metals. The layered structure of the laminated sheets was maintained till the number of layers was 100 but broke in the case of the 1000-layer sheet, which showed a network-like structure. The electrical conductivity of the sheets was independent of the number of layers and significantly higher than that of a Cu-Fe alloy, indicating that Fe had neither dissolved nor precipitated in the Cu layers of the laminated sheets. In the case of the 50-, 100- and 1000-layer sheet, the strength increased with lowering the temperature, while its elongation remained constant. A facet fracture surface was observed until the number of layers was 7, while in the case of the sheets consisting of more than 50-layers, fine dimples were observed on the fracture surfaces of both layers, indicating that ductile fracturing occurred. The strain in the Cu layers around the strain concentration sites of the Fe layers was high in the 50-layer laminated sheet when it was subjected to tensile deformation at 77 K. The strain concentration in the Fe layers might be accommodated by the surrounding soft Cu layers, resulting in the suppression of the nucleation of cleavage cracks in the Fe layers. This should be one of the reasons that the Cu-Fe layered sheets exhibited excellent elongation at low temperatures. Thus, the Cu-Fe layered structure take an important role for the excellent low-temperature tensile properties in the fcc and bcc dual-phase materials.

Analysis of Electron Channeling Contrast of Stacking Faults in fcc Materials.

The characteristics of electron channeling contrast (ECC) of stacking faults in a [101] single-crystal 316L fcc (face-centered cubic) stainless steel have been evaluated. Channeling contrast was analyzed from a series of ECC images taken as a function of sample tilt at ~0.1° increments across the (111) Kikuchi band. The most relevant imaging parameters of the fault contrast, namely the number of fringes, fringe intensity, and fringe spacing, were analyzed under different channeling conditions. The present work shows that the channeling contrast of stacking faults exhibits strong dependence on the sign and magnitude of the deviation parameter, w (w = s ξg, where s is the excitation vector and ξg is the extinction distance). This effect has strong influence on channeling conditions for fault imaging and g.R analysis for determining the nature of a stacking fault. g.R analysis was evaluated by the method developed by Gevers et al. (1963) on g-reversion experiments of ECC images of a stacking fault configuration. The interplay between the stacking fault nature and ε-martensite is analyzed.

A convex fourth order yield function for orthotropic metal plasticity

Gotoh (1977) proposed the first fourth order polynomial yield function under plane stress states that could simultaneously represent both yield stress and r-value directional properties of orthotropic sheet metals. The parameter identification procedure of this model can result in unbalanced overall errors in the anisotropy description, and the convexity of this yield function is not guaranteed. In this regard, a direct method is proposed to uniquely determine the nine model parameters using four yield stress and three r-value measurements. Out of them, only three expressions are new, and the rest remain identical to the published version of Gotoh. However, it is necessary to check the convexity restrictions on the obtained model parameters. Therefore, model parameters of two dissimilar materials that violate these restrictions are accurately identified from the literature. And, an effective procedure to reestablish the yield function convexity using new bounds on equi-biaxial stress is suggested. In the second part, an inverse method for the identification of the yield function parameters is proposed. In this approach, seven nonlinear expressions are solved together to determine the nine parameters of the convex guaranteed Gotoh's yield function. The advantages and disadvantages of two methods are discussed in detail by comparing with the existing ones in recent literature. It is shown that both these methods are successful in representing the anisotropic behavior of both aluminum and steel experimental data's available in the literature. In addition, a new 3D extension of the Gotoh's yield function is suggested by introducing four additional parameters to capture the influence of out of plane shear stresses. When all the parameters are set to a unit value, the yield function exactly coincides with the 3D von-Mises yield function. Further, the direct approach is extended to account for distortional hardening behavior of fcc and bcc materials.

Effect of Alloying Element on Mechanical Properties of High Strength Austenitic Steel

A high strength austenitic steel is expected as a structural material for cryogenic use because fcc material does not cause a cleavage fracture despite high strength. High manganese steel which is a strong candidate material of the cryogenic high strength austenitic steel was originally famous for the Hadfield steel and widely applicable in actual use. In general, an excellent cryogenic toughness of the high manganese steels is achieved by obtaining stable fcc microstructure with an adequate amount manganese which is a typical austenite former alloy. However, as addition of manganese is not effective for increasing strength, other strengthening alloying elements like carbon and chromium need to be added. In this study, an effect of alloying elements on strength and cryogenic toughness of the high manganese austenitic steel is studied.

Effects of stacking fault energy and solute atoms on microstructural evolution of Cu, Ag and Cu–Al alloys processed by equal channel angular pressing

The effect of alloying elements on microstructural evolution during the formation of ultrafine grained (UFG) structures in fcc materials during equal channel angular pressing (ECAP) was investigated. This research focused on the relative influence of stacking fault energy (SFE) and solid solution effect by comparing Cu, Ag and Cu–Al alloys (Cu-4.6 at%Al and Cu-6.8 at%Al). Either the dislocation cell or grain sizes were found to decrease together with a significant accumulation of dislocations during the early stage of successive passes in materials having low SFEs. These changes occurred regardless of the presence of solute atoms. In contrast, the solid solution effect resulted in delayed saturation of the dislocation density and hardening in the later stage of deformation, at which point the formation of a UFG structure was almost complete. The grain size after eight passes decreased with decreasing SFE, following a power law relationship. However, the effect of solute atoms on grain size reduction increased at greater plastic strain values because of steady, ongoing microstructural evolution and hardening resulting from the solid solution effect.

Study of the texture of polycrystalline FCC materials using one incomplete direct pole figure

The article deals with the algorithm for texture analysis of polycrystalline materials using one direct pole figure (DPF). It is shown that the incomplete direct polar figure {111} for fcc materials contains the necessary information about the material texture. The algorithm provides identification of the preferred texture components in a multicomponent texture material and determination of their properties. The proposed algorithm is as follows. The upper hemisphere of the digital representation of the DPF is scanned by a polar complex of vectors that are normal to the reflection planes. Then the reliability parameters for each orientation are calculated and a set of the most reliable orientations is formed. The chosen orientations are recalculated to the Rodrigues space wherein the preferred texture components are formed by clustering. At the same time, an iterative algorithm with symmetry operators is used to avoid the umklapp effect. Each texture component is represented by the following parameters: Rodrigues mean vector, Miller indices, and Euler angles. The share and scattering of the texture component are also calculated. A method for selecting the optimal number of clusters providing presentation of the texture with the desired degree of detail is proposed. This is achieved by comparing two incomplete direct pole figures taken for {111} and {200} to select the maximum cluster scattering value on which the number of formed predominant texture components depend. The developed algorithm seems promising for rapid texture analysis, in analysis of sharp and weak textures and when there are less than three DPFs.

Formation of Structure, Texture and Properties and Control of Continuous Induction Heat Treatment of Brass Strips

The texture, structure and mechanical properties of thick strips from brass L63 produced by different methods and then annealed in a continuous cycle in a transverse magnetic field or statically (in coils) in an in-and-out furnace are studied. The effect of the mode of preliminary treatment on the relation between the texture and the properties of the annealed strips is considered. Reliable enough equations relating the texture parameter used earlier for controlling fcc materials and low-alloy alloys based on them with the properties of the strips are derived. The prospects and the efficiency of online texture control of the properties of thick strips subjected to continuous induction annealing are considered.

Point defects and interstitial climb of 90° partial dislocations in brown type IIa natural diamond

Multiple electron microscopy techniques have been used to study a brown type IIa natural diamond. Electron backscatter diffraction shows evidence of plastic deformation in the form of slip bands, while cathodoluminescence reveals a network of low-angle grain boundaries, also observed in transmission electron microscopy together with long straight dislocations and dislocation dipoles. Aberration-corrected scanning transmission electron microscopy shows interstitial absorption on the 90° partial of both dissociated dislocations and Z-type faulted vacancy dipoles, forming structures similar to that observed in other fcc materials. The observations indicate an interstitial concentration of 1017 to 1019 cm−3 and calculations of point defect concentrations produced by plastic deformation show that this can be produced by strains of the order of 1%. Brown coloration in diamond has been previously attributed to vacancies and vacancy clusters with concentrations around 1018 cm−3, which suggests that roughly equal numbers of interstitials and vacancies are generated in diamond via plastic deformation. Atomic resolution images of Z-type faulted dipoles allowed a stacking fault energy of 472 ± 38 mJ m−2 to be determined.

A compact plane stress yield function formulation

The construction of anisotropic yield function by introducing linear transformations is a common practice with many phenomenological yield criteria. Based on the K1 − K2 representation method of principle stresses (represented by Barlat89 yield function), a compact 13-parameter plane stress function is proposed. The new yield function consists of five different linear transformations, and is designed to have a certain symmetry in both the overall structure and the distribution of linear transformation matrix components. The new 13-parameter yield function is applied to predict the yield surfaces (loci), plastic strain rate directions, flow stresses and r-values for both fcc and bcc materials. Compared with the yield functions of Yld2004-18p and CPB06ex2 (both reduce to 14 parameters in symmetric plane stress condition), the new yield function can achieve similar prediction accuracy with fewer parameters.

Twin related domain networks in René 88DT

Annealing twins in wrought fcc materials have historically been characterized using simple 2D measures including morphology and volume fraction. Accurately describing twin related domain microstructure requires accounting for their complex 3D structure. A large microstructural volume of a nickel-base alloy René 88DT has been collected using TriBeam tomography for full characterization of a large number of complete twin related domains. A network based approach enables visualization of twin domain structures. Metrics adapted from graph theory enable the first quantitative 3D microstructure descriptions. The implications for fatigue properties in twinned nickel-base alloys are discussed.

A phenomenological model based on nanostructured dislocation cluster interactions to predict the work hardening behavior of cryodeformed materials

In the present work, a phenomenological model was developed to understand the different stages of strain hardening in cryodeformed FCC materials with different stacking fault energy (SFE) level, i.e., high, medium, and low. The strain hardening in the materials consists of three stages, i.e. (i) easy-glide (stage-I) (ii) hardening (stage-II) and (iii) dynamic recovery (DRV) (stage-III). The proposed model considers the microstructure of cryodeformed materials which contains a mixture of discrete dislocation clusters including twin boundaries, and randomly arranged dislocations (dislocation debris). The model output predicts the work hardening behavior by using three deciding parameters: (i) normalized nanostructured dislocation cluster velocity (vDC)nor, (ii) normalized nanostructured dislocation cluster acceleration (aDC)nor and (iii) normalized dislocation debris velocity (vDD)nor. The deformation mechanisms during stage-II of strain hardening can be predicted by the nature of (aDC)nor curve. The cyclic trend in the (aDC)nor curve indicates the simultaneous occurrence of dislocation-based strengthening and DRV. The mono-cyclic (aDC)nor curve indicates hardening behavior due to combined effect of twin and dislocations-based deformation. The (aDC)nor curve fails to segregate stage II and stage III of deformation. In order to identify the transition from stage II and stage III mode of deformation, the relationship between (vDC)nor and (vDD)nor was formulated. A new parameter: DRV factor (Sx), was used to estimate the fraction of DRV in the materials. The uniaxial stress vs strain curves at varying strain rates and defect densities calculated from XRD analysis were used as input parameter for the model. A transmission electron microscopy (TEM) was performed to validate the dislocation interactions concluded from the model.

Simulating hydrogen in fcc materials with discrete dislocation plasticity

We have performed discrete dislocation plasticity simulations of hydrogen charged microcantilever bend tests on an fcc material at realistic hydrogen concentrations. This was achieved by accounting for the near-core solute-solute interactions which was found to reduce the dislocation nucleation time and stress. Dislocation pile-ups were observed at the neutral mid plane of the cantilever, and hydrogen was found to increase the number of dislocations in the pile-ups. Meanwhile, hydrogen was observed to decrease the flow stress due to the reduced dislocation core force. This was in contrast to the first-order hydrogen elastic shielding mechanism which was found to be negligible at realistic concentrations. Local stress elevation was observed in the presence of hydrogen in simulations which included an obstacle close to the free surface of the microcantilever, indicating how hydrogen might induce premature stress controlled failure.

Role of deformation twinning and second phase on the texture evolution in a duplex stainless steel during cold rolling: Experimental and modelling study

Deformation twinning is known to be one of the reasons that cause texture transition (copper type to brass type) in single phase fcc materials and is studied extensively. The role of deformation twinning in two phase materials is an area yet to be explored. Similarly in two phase materials, the effect of one phase on the texture evolution of the other phase is not well understood. In this work, a combination of experiments and modelling are used to address their effects on texture evolution in duplex stainless steels. The material is cold rolled to 80% thickness reduction and texture evolution is studied at various strain levels. These are compared with a series of crystal plasticity simulations using the Taylor model and grain interaction based LAMEL model which was extended to a two phase material. Deformation twinning in austenite is incorporated by predominant twin reorientation (PTR) scheme. It is observed that only by accounting for the strong local interactions between the phases, the correct textures are predicted. The texture transition from {001}〈110〉 to {112}〈110〉 orientation observed in ferrite at higher strain levels is attributed to deformation twinning in austenite. A number of simulations with ideal orientations observed in fcc and bcc materials are performed to assess the role of one phase on texture evolution of the other. It is concluded that experimental observations are also required to comment on the dominant phase during texture evolution.

Pattern formation during deformation of metallic nanolaminates

We used nonequilibrium molecular dynamics simulations to study the shear deformation of metallic composites composed of alternating layers of Cu and Au. Our simulations reveal the formation of "vortices" or "swirls" if the bimaterial interfaces are atomically rough and if none of the {111} planes that accommodate slip in fcc materials is exactly parallel to this interface. We trace the formation of these patterns back to grain rotation, induced by hindering dislocations from crossing the bimaterial interface. The instability is accompanied by shear-softening of the material. These calculations shed new light on recent observations of pattern formation in plastic flow, mechanical mixing of materials and the common formation of a tribomutation layer in tribologically loaded systems.

Nitrogen-induced yield point phenomenon in an austenitic steel

Discrepancies remain about nature of dislocation-interstitial interaction governing yielding discontinuity in fcc materials. Here, we show that outward diffusion of nitrogen from stacking fault (SF) into matrix causes strain redistribution, leading to lowering stacking fault energy. Local fluctuation of nitrogen promotes fault growth coupled with nitrogen-enriched zone, contributing to enhanced plasticity.