Pure Fcc Research Articles

Nucleation of deformation twins in nanocrystalline fcc alloys

An earlier dislocation model for predicting the grain size effect on deformation twinning in nanocrystalline (nc) face-centred-cubic (fcc) metals has been found valid for pure metals but problematic for alloys. The problem arises from the assumption that the stacking-fault energy (γSF) is twice the coherent twin-boundary energy (γfcc), which is approximately correct for pure fcc metals, but not for alloys. Here we developed a modified dislocation model to explain the deformation twinning nucleation in fcc alloy systems, where γSF ≠ 2γtwin. This model can explain the differences in the formations of deformation twins in pure metals and alloys, which is significant in low stacking-fault energy alloys. We also describe the procedure to calculate the optimum grain size for twinning in alloy systems and present a method to estimate γtwin.

New synthetic methodology and magnetic properties of fcc Co–Ni nanostructured alloys embedded in KIT-6 matrix

Abstract

Structural and mechanical study of thermally annealed tungsten nitride thin films

Summary Tungsten nitride films were deposited by RF reactive magnetron sputtering using Tungsten target. The films were subsequently annealed under high vacuum at different temperatures (700–850 °C) for 2 h each. The phase of the films were obtained using X-ray diffraction. EDAX and Nano-indentation tests were carried out to obtain the elemental composition and hardness respectively. XRD results of the as prepared film show the formation of pure fcc W2N phase and it is stable up to 700 °C. Beyond 700 °C mixed phase of W and W2N were observed. Mechanical study shows almost stable hardness value up to 700 °C temperature and beyond that temperature, hardness value reduce drastically, due to transformation of W2N phase to pure W phase. Our preliminary study shows that these films were stable up to

Atomistic Activation Energy Criteria for Multi-Scale Modeling of Dislocation Nucleation in FCC Metals

This study contributes to the development of a ‘fundamental, atomistic basis’ to inform macro-scale models that can provide significant insights about the effect of dislocation microstructure evolution during plastic deformation. Within a mesoscale model, multi-dislocation interactions can be studied which are capable of driving high-stress effects such as dislocation nucleation under low applied stresses, due to stress-concentration in dislocation pile-ups at interfaces. This study establishes a methodology to evaluate a phenomenological model for atomic-scale crystal defect interactions from molecular dynamics simulations, which is a critical step for mesoscale studies of plastic deformation in metals. Dislocations are affected by thermally activated processes that become energetically favorable as the stress approaches a threshold value. The nudged elastic band technique is ideal for evaluating the energetic activation parameters from atomic simulations. With this method, the activation energy and volume were obtained for the process of homogeneous nucleation of a full dislocation loop in pure FCC aluminum. Using the (atomistic) activation parameters, a constitutive mathematical model is developed for simulations at the mesoscale, to evaluate the critical (local) shear stress threshold. The constitutive model is effective for extrapolating from an atomistic timeframe of femtoseconds to experimentally accessible timespans of seconds.

The potential energy landscape for crystallisation of a Lennard-Jones fluid

Crystallisation pathways are explored by direct analysis of the potential energy landscape for a system of Lennard-Jones particles with periodic boundary conditions. A database of minima and transition states linking liquid and crystalline states is constructed using discrete path sampling and the entire potential energy landscape from liquid to crystal is visualised. We demonstrate that there is a strong negative correlation between the number of atoms in the largest crystalline cluster and the potential energy. In common with previous results we find a strong bias towards the growth of FCC rather than HCP clusters, despite a very small potential energy difference. We characterise three types of perfect crystals with very similar energies: pure FCC, pure HCP, and combinations of FCC and HCP layers. There are also many slightly defective crystalline structures. The effect of the simulation box is analysed for a supercell containing 864 atoms. There are low barriers between some of the different crystalline structures via pathways involving sliding layers, and many different defective structures with FCC layers stacked at an angle to the periodic box. Finally, we compare a binary Lennard-Jones system and visualise the potential energy landscape from supercooled liquid to crystal.

Impact of local magnetism on stacking fault energies: A first-principles investigation for fcc iron

A systematic ab initio study of the influence of local magnetism on the generalized stacking fault energy (GSFE) surface in pure fcc iron at 0 K has been performed. In the calculations we considered ferro- and antiferro- (single- and double-layer) magnetic order of local moments as well as their complete disorder, corresponding to paramagnetic (PM) state. We have shown that local magnetism is one of the most important factors stabilizing austenitic structure in iron (with respect to more stable at 0 K hcp) and that the perturbation of magnetic structure by the formation of stacking fault is a short-range effect. Local magnetism also strongly influences the GSFE surface topology and, therefore, the material's plasticity by reducing the energetic barriers that need to be overcome to form the intrinsic stacking fault (ISF) or return from the ISF structure to fcc. The influence of atomic relaxations on such barriers is moderate and does not exceed 15%. In addition, a methodology to evaluate the PM ISF energy using a superposition of the ISF energies obtained for ordered magnetic structures is proposed to overcome computational impediments arising when dealing with disorder in the PM state. The complications of the proposed methodology together with the ways to overcome them are also discussed.

Facile bio-inspired synthesis of zinc sulfide nanoparticles using Chlamydomonas reinhardtii cell free extract: optimization, characterization and optical properties

Abstract This study describes an eco-friendly, economical method to synthesize semiconductor zinc sulfide (ZnS) nanoparticles using the cell free extract of Chlamydomonas reinhardtii. Physicochemical parameters like pH, temperature and cell free extract concentration were optimized. Spherical particles measuring 8–12 nm were observed under high-resolution transmission electron microscopy (HRTEM). Elemental analysis proved that the nanoparticles were composed of zinc and sulfur, while powder X-ray diffraction (XRD) demonstrated the pure FCC crystal structure. Examination of the functional groups by Fourier transform infrared (FTIR) spectroscopy showed that algal proteins were involved in the synthesis of the nanoparticles. These nanoparticles demonstrated unique optical properties that were probed with UV-visible and photoluminescence (PL) spectroscopy. A peak at 310 nm was detected that was significantly blue-shifted from the bulk counterpart. Broad emission peaks at 410 nm and 430 nm were seen. The former was due to radiative recombination while the latter was attributed to defect states. In an effort to understand the molecular mechanism, the proteins bound to the nanoparticle surface were studied using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE) and numerous proteins that are part of the cells’ oxidoreductive machinery were identified. These cellular proteins probably play a pivotal role in the synthesis and stabilization of ZnS nanoparticles.

Thermo-kinetic mechanisms for grain boundary structure multiplicity, thermal instability and defect interactions

Grain boundaries (GBs) provide a source and/or a sink for crystal defects and store elastic energy due to the non-uniform atomic bonding structure of the GB core. GB structures are thermodynamically driven to transition to the lowest energy configuration possible; however to date there has been little evidence to explain why specific GB structures have a low energy state. Furthermore, there is little quantitative demonstration of the significance of physical and GB structure characteristics on the GB energy, thermal stability, and the effect of temporary local GB structure transformations on defect interactions. This paper evaluates the defect interactions and structure stability of multiple Σ5(310) GB structures in bi-crystals of pure aluminium, and systematically investigates the features at 0 K to characterise multiple metastable structures. Structure stability is evaluated by utilising unstable vacancy defects to initiate GB transformations, and using nudged elastic band simulations to quantify this with the activation energy. The emission of stable vacancy defects from the ‘stable’ and metastable grain boundaries is also evaluated in the same manner. A detailed analysis of dislocation nucleation at the atomistic scale demonstrates that local transformations of GB structure between stable and metastable intermediates can provide a mechanism to accommodate the generation of crystal defects. Kinetic (time-dependent) effects that compete with energetic driving forces for structural transformations of GBs are shown to cause a significant effect on the activation properties that may exceed the influence of GB potential energy. The results demonstrate that GB structural multiplicity can be associated with the generation and absorption of dislocations and vacancies. This paper demonstrates the suitability of atomistic simulations coupled with nudged elastic band simulations to evaluate fundamental thermodynamic properties of pure FCC metals. Overall, this paper demonstrates an inherent link between the atomic structure and the thermal, energy and structure stability properties of grain boundaries; and reveals the mechanisms for GB-defect interactions involving localised GB structure transformation.

A unified relation for the solid-liquid interface free energy of pure FCC, BCC, and HCP metals.

We study correlations between the solid-liquid interface (SLI) free energy and bulk material properties (melting temperature, latent heat, and liquid structure) through the determination of SLI free energies for bcc and hcp metals from molecular dynamics (MD) simulation. Values obtained for the bcc metals in this study were compared to values predicted by the Turnbull, Laird, and Ewing relations on the basis of previously published MD simulation data. We found that of these three empirical relations, the Ewing relation better describes the MD simulation data. Moreover, whereas the original Ewing relation contains two constants for a particular crystal structure, we found that the first coefficient in the Ewing relation does not depend on crystal structure, taking a common value for all three phases, at least for the class of the systems described by embedded-atom method potentials (which are considered to provide a reasonable approximation for metals).

Damage accumulation in ion-irradiated Ni-based concentrated solid-solution alloys

Irradiation-induced damage accumulation in Ni0.8Fe0.2 and Ni0.8Cr0.2 alloys are investigated using molecular dynamics (MD) simulations to assess possible enhanced radiation-resistance in these face-centered cubic (fcc), single-phase, concentrated solid-solution alloys, as compared with pure fcc Ni. The Ni0.8Cr0.2 and Ni0.8Fe0.2 alloys demonstrate higher radiation resistance compared to Ni. The total number of point defects produced in Ni0.8Cr0.2 and Ni0.8Fe0.2 is approximately 2.5 and 1.4 times lower than in Ni, respectively, due to efficient defect recombination in the chemically disordered alloys. Both interstitial and vacancy clusters are formed in all three materials. In Ni, large interstitial clusters are produced; whereas in Ni0.8Cr0.2, smaller interstitial clusters are produced but with a higher number. This indicates a higher mobility of interstitials in Ni compared to Ni0.8Cr0.2. Moreover, Ni0.8Cr0.2 shows better radiation resistance than Ni0.8Fe0.2. Larger interstitial clusters and 1.7 times higher numbers of accumulated point defects are observed in Ni0.8Fe0.2, in comparison with Ni0.8Cr0.2. Due to the low mobility of vacancies on the MD time scales, they are found primarily as single point defects and small clusters in all materials. While performance improvement is observed in the alloys, the difference in irradiation response between Ni0.8Cr0.2 and Ni0.8Fe0.2 indicates the importance of element choice to achieve the desired property.

Transitional grain boundary structures and the influence on thermal, mechanical and energy properties from molecular dynamics simulations

The thermo-kinetic characteristics that dictate the activation of atomistic crystal defects significantly influence the mechanical properties of crystalline materials. Grain boundaries (GBs) primarily influence the plastic deformation of FCC metals through their interaction with mobile dislocation defects. The activation thresholds and atomic mechanisms that dictate the thermo-kinetic properties of grain boundaries have been difficult to study due to complex and highly variable GB structure. This paper presents a new approach for modelling GBs which is based on a systematic structural analysis of metastable and stable GBs. GB structural transformation accommodates defect interactions at the interface. The activation energy for such structural transformations was evaluated with nudged elastic band analysis of bi-crystals with several metastable 0 K grain boundary structures in pure FCC Aluminium (Al). The resultant activation energy was used to evaluate the thermal stability of the metastable grain boundary structures, with predictions of transition time based on transition state theory. The predictions are in very good agreement with the minimum time for irreversible structure transformation at 300 K obtained with molecular dynamics simulations. Analytical methods were used to evaluate the activation volume, which in turn was used to predict and explain the influence of stress and strain rate on the thermal and mechanical properties. Results of molecular dynamics simulations show that the GB structure is more closely related to the elastic strength at 0 K than the GB energy. Furthermore, the thermal instability of the GB structure directly influences the relationship between bi-crystal strength, temperature and strain rate. Hence, theoretically consistent models are established on the basis of activation criteria, and used to make predictions of temperature-dependent yield stress at a low strain rate, in agreement with experimental results.

Effect of Co2+ concentration on the crystal structure of electrodeposited Co nanowires

The structure of Co nanowires deposited at the same potential depends on Co2+ concentration in solution. When depositing at −1.6V, the formed Co nanowire are hcp phase in 0.356M solution, a mixture of hcp and fcc phases in 0.53M solution, almost fcc phase in 0.71M solution and pure fcc phase in 1.06M solution. The transient curves show two interesting observations. First, the imax increases with increasing concentration of Co2+ ions while the tm decreases with increasing concentration. Second, the imax and tm observed in depositing Co nanowires at −1.6V in the 0.71M solution are close to those in depositing Co nanowires at −3.0V in the 0.356M solution. A higher imax and shorter tm can represent a larger Ns (saturation nucleus density). Therefore we believe that the deposition at −1.6V in higher concentrations such as 0.71 and 1.067M can lead to a larger Ns, indicating the formation of smaller critical nuclei. The structure of Co can be determined by the critical nucleus size and smaller critical nuclei favor the formation of fcc Co. Therefore the fcc Co nanowires were observed when depositing in the high concentration solution such as 0.71 and 1.067M.

How Deposition Parameters Affect Phase Formation in Metals

To understand the mechanism for formation of fcc-cobalt nanowires in electrodeposition, we have systematically studied the effect of deposition potential, pH, and deposition temperature on the formation of fcc Co nanowires by X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM). The Co nanowires deposited at the potential of -1.6V are pure hcp phase. When increasing the value of potential to -2.0V, there are hcp Co and fcc Co crystals in the deposited nanowires. The fraction of fcc Co crystals in the nanowires increases with increasing the potential value. At -3.0V, the nanowires are pure fcc Co. The pH of the solution has little effect on formation of fcc Co nanowires. We have also seen that high concentration and low temperature favors fcc phase whereas low concentration and high temperature favors hcp phase. However, at 35°C the co-occurrence of hcp and fcc phases were also observed. These experimental results can be explained by the classical electrochemical nucleation theory. The formation of fcc Co crystals can be attributed to smaller critical clusters formed at a higher potential value since the smaller critical clusters favor formation of fcc nuclei.

Biosynthesis of Ag/MWf-CNT Nanocomposites Using Aspergillus fumigatus as the CO Oxidation Catalyst

In this research, the biosynthesis process of an Ag nanoparticles/multiwalled carbon nanotubes (Ag/MWf-CNT) catalyst system has been investigated by Aspergillus fumigatus fungus. Characteristic peaks of pure silver and FCC crystallization systems were characterized by X-ray diffraction pattern. TEM images show a well-dispersed spherical silver nanoparticles (≤10 nm) on the carbon nanotubes. Based on the results, reduction of silver ions is mainly dependent on the biomass weight applied in the biosynthesis process. Catalytic activity in CO/CO2 conversion process revealed approximately 100% conversion of CO at 220°C.

Influence of bath temperature and pH on the structure of electrodeposited cobalt nanowires

Abstract To fully understand the mechanism of forming fcc Co in electrodeposition, the effect of bath temperature and pH on the structure of electrodeposited Co nanowires is studied by means of X-ray diffraction and scanning electron microscopy. At −3.0 V and pH 2.5, the fraction of fcc Co decreases with increasing temperature, ranging from 1 (25 °C, pure fcc Co) to 0 (45 °C, pure hcp Co). The formation of hcp Co can be attributed to larger critical clusters formed at higher temperatures. The pH value has no appreciable effect on the formation of fcc Co nanowires. This is because the H adatoms produced at the cathodic surface can penetrate quickly through the thin Au film and desorb into air.

Generation of Interstitial Atoms in FCC Single Crystals

A mathematical model of generation and accumulation of interstitial atoms in plastically deformable pure FCC metals is suggested based on the concept of hardening and recovery that links the phenomena proceeding in the deformable crystal material with the behavior of crystal structure defects. The model comprises kinetics equations for point defects – mono- and bivacancies and interstitial atoms – written with allowance for mechanisms of their generation and precipitation on sinks. Special attention is given to investigation of the influence of the velocity and character of motion of helical segments of expanding dislocation loops on generation of interstitial atoms. Concentrations of interstitial atoms generated in the process of plastic deformation are calculated.

Robust Phase Control through Hetero-Seeded Epitaxial Growth for Face-Centered Cubic Pt@Ru Nanotetrahedrons with Superior Hydrogen Electro-Oxidation Activity

Controllable synthesis of metallic nanocrystals (NCs) with tunable phase, uniform shape, and size is of multidisciplinary interests but has still remained challenging. Herein, a robust phase control strategy is developed, in which seeds with a given phase are added to guide the epitaxial growth of the target metal to inherit the seeds’ phase. Through this strategy, M@Ru (M = Pt, Pd) NCs in the face-centered cubic (fcc) phase, a metastable phase for Ru under ambient conditions, were synthesized with the hydrothermal method. The Pt@Ru NCs showed not only the pure fcc phase but also high morphology selectivity to tetrahedrons surrounded by {111} facets. As revealed by density function theory (DFT) calculations, the preferentially epitaxial growth of Ru atom layers on the nonclosest-packed facets of hetero fcc metal seeds led to the formation of fcc Ru shells. Furthermore, the fcc Pt@Ru tetrahedrons/C showed electrocatalytic activity enhancement with more than an order of magnitude toward hydrogen oxidation r...

Surface composition of PdCuAu ternary alloys: a combined LEIS and XPS study

PdCuAu ternary alloy samples with different composition were synthesized on top of ZrO2‐modified porous stainless steel disks by the sequential electroless deposition technique. The structure, morphology and bulk composition of the samples were characterized by X‐ray diffraction (XRD), scanning electron microscopy and energy dispersive X‐ray spectroscopy (EDX). Complete alloy formation with a pure fcc phase for the Pd71Cu26Au3, Pd70Cu25Au5 and Pd67Cu24Au9 samples and a bcc structure for the Pd62Cu36Au2 and Pd60Cu37Au3 samples were obtained upon annealing at 500 °C for 120 h as revealed by XRD. A combination of low‐energy ion scattering (LEIS) and X‐ray photoelectron spectroscopy (XPS) was used to investigate the surface properties of the PdCuAu alloys. XPS results confirmed alloy formation under the annealing conditions. XPS analysis also revealed that the near‐surface regions of the alloys became enriched in Pd with respect to the bulk composition determined by EDX. In contrast, LEIS and angle‐resolved XPS analyses showed that the top‐most surface layers in all samples were copper‐rich compared with the bulk composition. This high Cu surface concentration could impart resistance to bulk sulfide formation to the PdCuAu alloy membranes. Copyright © 2015 John Wiley & Sons, Ltd.

Features of Contact Interaction of BCC and FCC Metals

We carried out simulations of contact interaction between BCC iron crystallite and various pure FCC metals under shear loading by means of molecular dynamics. It was shown that the result of this interaction is the transformation of FCC atomic lattice of contacted material into BCC one within a thin layer in the contact zone. The results of simulations can be used to control strength of the interfacial layers of coated materials, as well as to understand the processes which are taking place in surface layers of materials under the contact.

The hydrogen permeability of Pd–Cu based thin film membranes in relation to their structure: A combinatorial approach

Pd–Cu is a well-known alloy for H2 separation membranes. Using a new optical combinatorial method we determined the H2 permeability of Pd–Cu alloys at room temperature in relation to their crystal structure and microstructure.Compositional gradient samples allow us to determine the intrinsic permeability as a function of the alloy composition. From a detailed XRD, TEM and AFM analysis we find that the pure fcc and bcc phases have a very low permeability, whereas the mixed bcc/fcc phases show the highest permeability for thin film alloys prepared at higher temperatures. Therefore interstitial-bulk hydrogen diffusion is most probably not the main transport mechanism. Instead we argue that the hydrogen transport occurs mainly along the heterogeneous grain boundaries. As compared to the bulk phase diagram we find that the mixed bcc/fcc region in our Pd–Cu thin films is larger than reported in literature (ranging from 43 at% – 65 at% Pd at 483 K).