Fcc Crystals Research Articles

Origin of fine needle-like M23C6 carbides in a heat resistant steel after extended service

Components in refineries and petrochemical plants exposed to extended service offer a unique opportunity to investigate the effect of a long heat exposure on the microstructure. Twenty years of service at 900 °C of a super austenitic Fe-0.5C-25Cr-35Ni-1Nb-0.1Ti alloy led to the formation of both coarse intergranular equiaxed as well as fine intragranular M23C6 needle-like carbides. Cube-cube and twin-like Σ3 orientation relationships are identified between the matrix and fine needle-like carbides. The former relationship maintains {111}CC//{111}M parallelism while the latter satisfies only one (111)TC//(111)M with 60° twist misorientation similar to that of twins or Σ3 in FCC crystals. Both planar relationships generate low energy {111} carbide-matrix phase boundaries. Cube relationship generates Σ3 phase boundary and a superlattice with 3 times lattice parameter of the matrix while twin-like relationship results in Σ9 phase boundary and a superlattice with 3√3 times lattice parameter of the matrix. The carbide growth direction is determined as 〈011〉C. All six <011>M directions in the matrix accommodate the growth of needle-like carbides resulting in an intricate 3D structure. Twinning is observed in some needle-like carbides resulting in coexisting cube-cube and twin-like orientation relationships with the matrix. Carbide twinning leads to three in-common <011>C directions for preferential growth. Twinning also allows needle-like M23C6 to form serrated cube-cube and straight twin-like {111} phase boundaries with the matrix.

Tensile deformation behaviours of polycrystalline Cu80Ni20 alloy: insights from molecular dynamics simulations

ABSTRACT The polycrystalline Cu80Ni20 sample was simulated by molecular dynamics (MD) approach. This sample consists of the fcc and hcp crystals alternated with disordered (d-) structures. Under the uniaxial tensile test, the sample exhibits elastic and plastic behaviours. The fcc and hcp crystals disintegrate into the d-structures with increasing strain but small clusters of fcc and hcp crystals still exist at the large strain. A very small number of d-atoms recrystallises into the fcc and hcp crystals during the tensile deformation. The distances between atoms are stretched gradually during the tensile loading. The growth and coalescence of big simplexes with R S ≥ 2.0 Å occur only in the d-structures. The shear band (SB) is formed during the tensile deformation. The big simplexes coalesce and propagate in the SB, causing the crack propagation.

A modular spectral solver for crystal plasticity

A fast Fourier transform (FFT) based modular solver for crystal plasticity is presented in this work. In the framework, balance of momentum is solved in a global iterative loop and single crystal plasticity is solved in an inner loop. For the latter, a fully implicit time integration scheme is used in which the correct solution is found using a residual established based on plastic velocity gradient. The elastic–viscoplastic response of single crystals is modeled by coupling slip system based constitutive equations to crystal kinematics. The single crystal response is validated using experimentally observed behavior of fcc and hcp single crystals for uniaxial and cyclic deformation, respectively. The single crystal plasticity module provides solution to the global FFT solver. An FFT scheme based on deformation gradient is derived to address the full-field micromechanical response for finite deformation. Adaptive time increment is used for ensuring convergence. OpenMP based multi-threading is employed for ensuring faster simulations. The current FFT model is validated for an fcc polycrystal by comparing the results against those from an open-source crystal plasticity code, and shows an excellent agreement. In the second study, the FFT solver is used for analyzing the deformation behavior of an α−β Ti polycrystal. In this dual phase polycrystal, plastic deformations of α and β phases are modeled using a dislocation density based and a power law based constitutive framework, respectively. The model is able to capture the expected salient features of deformation of both phases. Simulations for fcc polycrystal and α−β Ti polycrystal also show excellent agreement against an open-source finite element crystal plasticity solver and FFT solver, respectively.

Thermal expansion of FeNi Invar and zinc-blende CdTe from the view point of local structure

Thermal expansion of FeNi Invar and zinc-blende CdTe was investigated from the view point of local structure using the extended x-ray absorption fine structure (EXAFS) spectroscopic data and the path-integral effective classical potential (PIECP) Monte Carlo computational simulations. In this Review article, first the quantum statistical perturbation theory is intuitively described to see different character concerning thermal expansion between the quantum and classical theories. The diatomic Br2 molecule is employed as a simple example. Second, the PIECP theory is briefly described to note advantages and disadvantages of this simulation technique. Historical background is also discussed for the EXAFS investigation of thermal expansion based on the quantum statistical theories. The results of the FeNi Invar alloy are then summarized. The origin of zero thermal expansion in the FeNi alloy is ascribed to the so-called Invar effect that implies the variation of the electronic structure of Fe atoms depending on temperature. Zero thermal expansion at low temperature is however found to originate from the vibrational quantum effect. It is also noted that the interatomic distances of Fe-Fe, Fe-Ni, and Ni-Ni pairs are slightly but meaningfully different from each other, although the alloy exhibit a simple fcc crystal. Such a pair- dependent difference is also true for thermal expansion and we will discuss thermal expansion from the local point of view, which is interestingly different from the lattice thermal expansion significantly. Finally, the results of the zinc blende (or diamond) structure are presented. Although the origin of negative thermal expansion in these tetrahedral crystals is known as a result of classical vibrational anomaly within the Newton dynamics theory, the quantum statistical simulation is found to be essential to reproduce the negative thermal expansion of CdTe. It is emphasized that the vibrational quantum effect and classical anharmonicity are of great importance for the understanding of low-temperature thermal expansion as well as the elastic constants.

Finite-deformation phase-field microelasticity with application to dislocation core and reaction modeling in fcc crystals

The purpose of the current work is the formulation of finite-deformation phase-field microelasticity (FDPFM) and its application to the modeling of (i) the dislocation core and (ii) dislocation interaction/reaction on intersecting glide planes in fcc crystals. The corresponding model formulation is carried out in the context of continuum thermodynamics; for simplicity, isothermal, quasi-static conditions are assumed. The resulting model rests in particular on generalized Ginzburg–Landau relations for dislocation slip-line phase field evolution depending on the Peach-Koehler (PK) force. Restricting attention to (periodic) bulk single crystal behavior, the algorithmic formulation and numerical implementation of FDPFM employs Green function, spectral, and Fourier methods generalized to geometric non-linearity. More specifically, algorithms for control of either the mean deformation gradient, or the mean Cauchy stress (i.e., PK force control) are developed. As in ”standard” linear PFM, the free energy model in FDPFM is based on elastic, crystal and gradient contributions. In order to model dislocation interaction and reactions on multiple intersecting glide planes, the ”standard” 2D stacking-fault-energy-based PFM crystal energy is generalized here to 3D crystallographic form for dissociation, partial fcc dislocations and stacking fault formation. Comparison of FDPFM and atomistic (MS) simulation results for higher-order local continuous and discrete deformation measures (e.g., dislocation tensor) in the core of a dissociated edge dislocation in Au show generally good agreement. In addition, results from FDPFM and MS are compared for dislocation reactions in Al on two intersecting glide planes. In the case of collinear reaction, for example, good qualitative agreement with MS results is obtained only when the dislocation slip on multiple glide planes is energetically coupled in the crystal energy.

Investigation of cutting force, cutting stress, subsurface deformation, and phase change during the macro and nano cutting process of OFHC copper

In this paper, the FEM and MD method are used to establish macro and nano cutting model of OFHC copper, and the cutting force, cutting stress and subsurface deformation and phase change are compared and analyzed under the macro cutting depth (0.2mm, 0.3mm, 0.4mm) and nano cutting depth (0.8nm, 1.2nm, 1.6nm). Meanwhile, experiments are used to verify the simulated cutting force. The conclusions are below: The change trends of macro and nano cutting force are similar, and the ratios of the cutting forces in x and y directions at different cutting depths are similar. In addition, there are lateral chip both in the macro and nano cutting processes, showing a high degree of consistency. However, the nano cutting stress reached 25 times of that in the macro state. Then it was found that the area of unrecoverable plastic deformation on the surface increased with the increase of the cutting depth during the macro cutting process, but the existence depth of pure plastic deformation remained unchanged within 0.05mm. Although in the nano cutting process, the proportion of FCC crystals converted to HCP crystals remains unchanged as the cutting depth increases, but the existence range is increasing. Finally, the amplitude and change trend of experimental and the simulated cutting force are highly consistent, which verifying the rationality of the macro simulated cutting model.

Peierls–Nabarro modeling of twinning dislocations in fcc metals

Metals with fcc structure may exhibit deformation twinning under specific conditions, which is an interesting but somewhat elusive aspect of their deformation behavior. It is well acknowledged that the phenomenon occurs through the activities of twinning partial dislocations. However, the lack of a comprehensive understanding of their fundamental properties obstructs the development of detailed multiscale plasticity models for the fcc metals. Here, we explore the core-structures and lattice friction of twinning partials through atomistically informed numerical modeling. To this end, we choose four fcc crystals with widely differing stacking fault energies. Using the semi-discrete variational Peierls–Nabarro model with non-local and surface corrections, we compute the core-widths and Peierls stresses of edge and screw twinning dislocations. In particular, the alternate-shear mode of twinning has been considered in addition to the regular layer-by-layer mechanism. In the former case, the stable disregistry energy of the middle layer is observed to be significantly less than the unsheared configuration, which is enough to overcome the Peierls barrier spontaneously. This study also highlights the significance of incorporating the correction terms, the absence of which may lead to significant inaccuracy in estimating the intrinsic lattice resistance of the twinning partials.

Study of the stabilization process of gas atomized Al0.5CoCrFeNi2Ti0.5 high-entropy alloy in phase transformation

The phase and microstructure evolution of a precipitation-hardening high entropy alloy (HEA), Al0.5CoCrFeNi2Ti0.5, were studied. In order to promote the applications in surface modification, the proposed HEA was made into powders by using a gas-atomization process. The as-obtained powders presented a spherical shape with uniform element distribution; moreover, their phase constitution transferred from BCC to FCC-dominated structure while particle size varied from several to 120 µm. Further annealing treatments were conducted with the metastable fine powders to investigate the size effect in the microstructure. Below 500 °C, the structure kept a major BCC phase, but the size of grains arranged along (110) plane were getting smaller with temperature. FCC crystal became the dominant structure while the temperature was higher than 550 °C, and the transitional sigma phase existed at 700–800 °C. After annealed at 900 °C, the metastable fine Al0.5CoCrFeNi2Ti0.5 powders fully transferred to a stable structure composed of a dominant FCC plus minor BCC phases. Elemental mappings showed that the Al0.5CoCrFeNi2Ti0.5 HEA transformed from a microstructure with uniform element distribution to a coarsened system comprised of (Fe, Cr)-rich FCC matrix and (Al, Ti)-rich order BCC precipitate. Furthermore, the hardness of the Al0.5CoCrFeNi2Ti0.5 HEA increased by more than 20% on annealing treatment, from 5.50 ± 0.75–6.91 ± 1.49 GPa due to the solid solution and the precipitation strengthening effect brought about by Ti addition.

Anisotropic yield surfaces of additively manufactured metals simulated with crystal plasticity

The mechanical anisotropy created by additive manufacturing (AM) is not yet fully understood and can depend on many factors, such as powder material, manufacturing technology and printing parameters. In this work, the anisotropic mechanical properties of as-built, laser powder bed fusion (LPBF) austenitic stainless steel 316L and titanium alloy Ti-6Al-4V are investigated through crystal plasticity simulations. Periodic representative volume elements (RVEs) are used that are specific to each material. The RVE for austenitic stainless steel consists of FCC crystals with a crystallographic texture measured by X-ray diffraction. The α′ martensite microstructure of Ti-6Al-4V is captured with a multi-scale RVE, including internal lamellar structures, using HCP crystals and a synthetically generated texture. For both materials, the crystal plasticity parameters are calibrated against tensile tests carried out on dog-bone specimens printed in different orientations. The RVEs, calibrated to experiments, are applied in virtual material testing and subjected to multiple load cases to generate the Hill-48 and Yld2004-18p yield surfaces of the materials.

Coherency strengthening of oblate precipitates extended in the {100} plane of fcc crystals: Modeling and experimental validation

Coherency strengthening plays a major role in precipitation strengthening. The governing mechanism is based on the interaction of dislocations with the elastic strain field induced by the lattice misfit of precipitates and matrix. In the case of non-spherical precipitates, the strain field and corresponding stress field is inhomogeneous and depends on the relative orientation of the particle with respect to the Burger's vector of the dislocation. We evaluate the shear stress increment due to inhomogeneous strain fields around an oblate precipitate and suggested a model for coherency strengthening of oblate precipitates. The corresponding results for the weak and strong strengthening mechanisms demonstrate that shape-depending correction factors need to be incorporated in order to estimate the strength precisely. Afterwards, the proposed model was applied for simulation of precipitation strengthening of Inconel 718. Simulation result shows that, the selection of correct aspect ratio can lead to more accurate yield strength predictions that are close to the experimental results.

Numerical recognition of C15 unit in rapid solidification Ni&lt;sub&gt;70&lt;/sub&gt;Ag&lt;sub&gt;30 &lt;/sub&gt;nanoparticles

Simulation has become an important tool in materials science, it is a prerequisite to study the correlation between the structure and properties of materials, in that the structural characteristics of the system from the atomic coordinates output can be obtained by simulations. For simple (FCC, HCP, and BCC) crystals containing only 2-6 atoms, in the numerical analysis method, what needs to be determined is only the local characteristics of each atom. However, it is extremely computationally intensive to determine the cells containing tens or hundreds of atoms. The combination of numerical analysis and visualization is one of the methods to solve this kind of problem. In this work, Ni&lt;sub&gt;70&lt;/sub&gt;Ag&lt;sub&gt;30&lt;/sub&gt; nanoparticles are simulated by molecular dynamics. It is found that the nanoparticles contain FCC crystals and a large number of complex topologically close-packed (TCP) structures. Using the analysis software based on the largest standard cluster analysis (LaSCA), the C15 phase of TCP atoms in nanoparticles is determined by topology configuration analysis and crystallography knowledge. The analytical ideas provide the algorithm logic fordeveloping the numerical recognition software for complex crystal structures in the future.

Controlling the two components modified on nanoparticles to construct nanomaterials.

Nanoparticle self-assembly technology has made great progress in the past 30 years. Many kinds of self-assembly strategies of modifiable nanoparticles have been developed and used to construct nano-aggregates by designing the shape, size and type of nanoparticles and controlling the components modified on nanoparticles. These strategies are widely used in many fields, such as medical diagnosis, biological detection, drug delivery, materials synthesis and sensors. The modified components can be DNA chains, polymer chains, proteins, and even organic molecules based on different molecular conformations and chemical properties. In recent years, the self-assembly of two-component modified nanoparticles has gradually attracted more attention. Nanoparticles modified with two components of different DNA strands can self-assemble to produce a variety of nano arrangement structures, such as BCC, FCC and other cubic crystals, which can be used in crystal materials. Two-component modification of hydrophilic and hydrophobic polymers can produce vesicular aggregates, which can be used for drug delivery. In this review, we summarize the latest experimental progress and theoretical simulation of self-assembly of two-component modified nanoparticles including different DNA chains, different polymer chains, DNA and polymer chains, proteins and polymer chains, and different organic molecules. Their self-assembly characteristics and application prospects were discussed. Compared with single-component modified nanoparticles, two-component nanoparticles have different tethered molecules or molecular chains, which can be multifunctional by regulating different modified components and types of nanoparticles and ultimately expand the scope of applications.

Theoretical Evidence for the Single-Ended-Source Controlled Yield Strengths of Micropillar FCC and BCC Metal Single Crystals

Theoretical Evidence for the Single-Ended-Source Controlled Yield Strengths of Micropillar FCC and BCC Metal Single Crystals

Analysis of Nano Order Structure of Shock Wave Front Generated by HVI and Propagating in &lt;100&gt; Direction of fcc Metal

It is difficult to determine experimentally the nanometer level structure of shockwave propagating in solid materials. In present study, molecular dynamics (MD) simulation is performed to clarify quantitatively the atom level structure of shockwave propagating in fcc crystalline Al or Ni in the <100> direction. Simulated metal to metal collision model is similar to the experimental equipment of the split Hopkinson pressure bar which is used for the high strain rate deformation test. The non-closed pack (100) plane of the fcc crystal is the collision plane.

The effect of ferromagnetism on the CO activation over FCC crystal phase transition metal catalysts: Insights from DFT calculations

In order to understand the relationship between the ferromagnetism and catalytic performance, three pathways of CO activation over transition metal catalysts (Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au) have been studied by using density functional theory (DFT) calculations. It is shown that Co and Ni metal catalysts have better catalytic performance for CO activation than others. The density of states (DOS) and spin density plots reveal that Co and Ni metal possess ferromagnetism. So it is concluded that the catalytic activity connects with the metal ferromagnetism. To further confirm this deduction, the magnetic moments and Bader charge have been calculated when small molecules (CO, CHO, and COH) adsorbed on the Co and Ni metal surface, and the results shows that the electron transfer between the small molecules and metal catalysts happens on the unpaired d-orbital electrons (nd) of the metal, which changes the magnetic moments of Co and Ni metal. Meanwhile, electron transfer also changes the catalytic performance of Co and Ni metal catalysts. Compared with the CO, the electron transfer of CHO and COH is more, the magnetic moments of metal changes greatly, and the carbon-oxygen bonds of CHO and COH are easier to break. So the magnetic moments of metal catalysts can be used as an indicator to select different metal catalysts.

A novel mechanism to generate metallic single crystals

Generally, the evolution of metallic single crystals is based on crystal growth. The single crystal is either produced by growing a seed single crystal or by sophisticated grain selection processes followed by crystal growth. Here, we describe for the first time a fully new mechanism to generate single crystals based on thermo-mechanically induced texture formation during additive manufacturing. The single crystal develops due to two different mechanisms. The first step is a standard grain selection process due to directional solidification, leading to a pronounced fiber texture. The second and new mechanism bases on successive thermo-mechanically induced plastic deformations and texture formation in FCC crystals under compression. During this second step, the columnar grain structure transforms into a single crystal by rotation of individual grains. Thus, the single crystal forms step by step by merging the originally columnar grain structure. This novel, stress induced mechanism opens up completely new perspectives to fabricate single crystalline components and to accurately adjust the orientation according to the load.

Sustainable fabrication of silver-titania nanocomposites using goji berry (Lycium barbarum L.) fruit extract and their photocatalytic and antibacterial applications

• Green synthesis of metal nanocomposites is sustainable and environmentally friendly. • Silver-titania nanocomposites (Ag-TiO 2 NCs) were synthesized using the aqueous extract of goji berries. • FE-TEM morphology shows the embedment of Ag-NPs throughout the surface of TiO 2 -NPs. • Ag-TiO 2 NC (3:1) exhibited the highest photocatalytic degradation of methylene blue. • NCs exhibited potential antibacterial properties against S. aureus and E. coli bacteria. Silver-titania nanocomposites (Ag-TiO 2 NCs) have unique functional attributes due to their photocatalytic and antibacterial properties. In this study, titania nanoparticles (TiO 2 -NPs) were successfully in-situ decorated with silver nanoparticles (Ag-NPs) using the aqueous extract of goji berries (Lycium barbarum L . ) as a bioreducing and stabilizing agent. Different Ag-TiO 2 NCs were synthesized by treating different concentrations of silver nitrate with a specific concentration of TiO 2 -NPs in the presence of fruit extract. The green-synthesized NCs were characterized using several techniques viz., ultraviolet–visible spectrophotometry, X-ray diffractometry (XRD), scanning electron microscopy, field-emission transmission electron microscopy (FE-TEM), Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. XRD analysis revealed the formation of face-centered cubic (fcc) crystals, and FE-TEM analysis revealed the embedment of Ag-NPs throughout the surface of TiO 2 -NPs. The average size of Ag-NPs on TiO 2 -NPs increased from 11.2 ± 3.05 nm to 16.4 ± 4.5 nm with an increase in the concentration of silver ions, and the morphology of Ag-NPs was predominantly quasi-spherical and hexagonal. These NCs exhibited an excellent photocatalytic degradation of an azo dye, methylene blue (MB). The synthesized Ag-TiO 2 NCs (3:1) showed higher photocatalytic degradation efficiency of ∼ 93.4% for MB in 130 min under visible light irradiation. Ag-TiO 2 NC S also exhibited good antibacterial activities towards Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative). Therefore, the formation of Ag-NPs on the surface of TiO 2 -NPs to form Ag-TiO 2 NCs exhibits eco-friendly photocatalytic degradation of azo dye contaminants as well as antibacterial activity.

Localization and delocationzation of surface disordering in surface mediated melting

Melting of solids usually starts with a thin liquid layer forming on the surface at temperatures below their bulk melting point. While the liquid phase formation in the so-called premelting is understood to be governed by the interface energies among the coexisting solid, liquid, and vapor phases, the influence of the underlying crystal structures on its kinetics, and atomistic mechanisms are unknown, and consequently, ignored in the theory of melting. Here we report the first observation of a strong influence of the underlying crystal structure on melting kinetics with the resulting localization and delocalization behavior of the surface disordering and the liquid layers. With increasing temperature, the surface disordering remains in the localized state with a constant thickness in fcc crystals, whereas it becomes delocalized by growing steadily into the bulk in bcc crystals. In both cases, the surface melting is found to occur with highly correlated atomic motion in the form of extended atomic chains and loops that emit from the disordered surface and transmit to the bulk. This newly discovered mechanism in surface melting is behind the surface melting kinetics: The close packed fcc crystal can effectively impede the correlated atomic motion and limit the surface disordering to only the localized region on surface, while the more openly packed bcc crystal allows for its proliferation by tunneling from the surface into the bulk of the crystals. These findings provide valuable insights for future development of new theories of crystal melting.

The roles of Rh crystal phase and facet in syngas conversion to ethanol

Design of Rh catalyst by controlling crystal phase and facet is significant for providing higher mass-specific activity and selectivity for ethanol synthesis from syngas. We demonstrate here the roles of crystal phase and facet in syngas conversion to ethanol on different crystal facets of fcc and hcp Rh by DFT calculations and microkinetic modeling. Compared to hcp Rh, fcc Rh not only exhibits higher intrinsic activity for CH and ethanol formation, but also exposes much denser active facets and inhibits methanol, especially, the mainly exposed (111). Further, for the Fe-modified Rh catalysts, fcc Rh exhibits better activity and selectivity toward ethanol, which is still superior to hcp Rh. Thus, Rh-based catalysts should be focused on the fcc crystal phase instead of hcp crystal phase. The findings in this work are significantly beneficial to the rational design of Rh-based catalysts for CO hydrogenation to desired ethanol.

The effect of HCP on the formation of twin boundaries and dislocations in Ni–Co alloys

In this study, molecular dynamics (MD) was used to simulate the rapid solidification process of Ni47Co53 and Ni48Co52 alloys at a cooling rate of 1012 K/s. The effects of HCP on the formation of twin boundaries and dislocations in two Ni–Co alloys are studied. It is found that the difference of HCP clusters is the main effect that producing discrepancies on microstructure of two alloys. The number of HCP clusters accounted for 9.23% in Ni47Co53 alloy. They are regularly arranged to form the number of single-layer twin boundaries, and each twin boundary ends in a dislocation. The FCC and HCP structures coexist in the same atomic layers, which is easy to create dislocations. The relatively standard FCC crystal and only 0.32% HCP clusters are formed in Ni48Co52 alloy at 300 K. That small amount of HCP clusters are dispersed on the surface, and cause the formation of dislocation in the border with FCC clusters.