Nanocrystalline Metals Research Articles

Investigation on the phase transformation and lattice parameters of Sn2+, Cu2+, La3+ and Ce4+ ions doped titania: characterization and solar light activity study

Nanocrystalline titania and metal ion doped titania systems were prepared by combustion method. Titania exhibited bicrystalline phase consisting of both anatase and rutile. The bicrystalline framework in TiO2 powders can be conveniently altered by the incorporation of various bivalent/trivalent/tetravalent metal ions like Sn2+, Cu2+, La3+and Ce4+. The microstructure development and the influence of small amount of dopants on the resulting structural change have been investigated. p-XRD results confirms the stabilization of anatase phase in the case of Cu2+ and Sn2+ doped titania, while the polymorphic interfaces were retained in La3+and Ce4+ doped titania samples. The efficacy of the modified titania was studied on the degradation of Congo Red dye by harvesting solar light. Sn2+/TiO2 shows almost same photodegradation ability as that of pristine TiO2. Subsistence of catalytic hot spots at the interface of anatase-rutile and the synergistic effect of polymorphic phases explains the enhanced activity of La3+/TiO2 and Ce4+/TiO2. But the activity of Cu2+/TiO2 was found to be less due to high electron-hole recombination rate.

Numerical Computation of Material Properties of Nanocrystalline Materials Utilizing Three-Dimensional Voronoi Models

Nanocrystalline metals have been the cause of substantial intrigue over the past two decades due to their high strength, which is highly sensitive to their microstructure. The aim of the present project is to develop a finite element two-phase model that is able to predict the elastic moduli and the yield strength of nanostructured material as functions of their microstructure. The numerical methodology uses representative volume elements (RVEs) in which the material microstructure, i.e., the grains and grain boundaries, is presented utilizing the three-dimensional (3D) Voronoi algorithm. The implementation of the 3D Voronoi particles was performed on the nanostructure investigation of ultrafine materials by SEM and TEM. Proper material properties for the grain interiors (GI) and grain boundaries (GB) were computed using the Hall-Petch equation and a dislocation-based analytical approach, respectively. The numerical outcomes show that the Young’s Modulus of nanostructured copper increased by increasing the crystallite volume fraction, while the yield strength increased by decreasing the grain size. The numerical predictions were strongly confirmed in opposition to finite element outcomes, experimental results from the open literature, and predictions from the rule of mixtures and the Mori-Tanaka analytical models.

In Situ High-Cycle Fatigue Reveals Importance of Grain Boundary Structure in Nanocrystalline Cu-Zr

Nanocrystalline metals typically have high fatigue strengths but low resistance to crack propagation. Amorphous intergranular films are disordered grain boundary complexions that have been shown to delay crack nucleation and slow crack propagation during monotonic loading by diffusing grain boundary strain concentrations, which suggests they may also be beneficial for fatigue properties. To probe this hypothesis, in situ transmission electron microscopy fatigue cycling is performed on Cu-1 at.% Zr thin films thermally treated to have either only ordered grain boundaries or amorphous intergranular films. The sample with only ordered grain boundaries experienced grain coarsening at crack initiation followed by unsteady crack propagation and extensive nanocracking, whereas the sample containing amorphous intergranular films had no grain coarsening at crack initiation followed by steady crack propagation and distributed plastic activity. Microstructural design for control of these behaviors through simple thermal treatments can allow for the improvement of nanocrystalline metal fatigue toughness.

Overcoming the strength-conductivity trade-off dilemma in carbon nanotube/aluminum-copper fibers by diffusion interface and chemical reaction interface

The carbon nanotube fiber has not shown its advantage and been widely used in many applications due to low conductivity. Depositing a metallic layer on the carbon nanotube fiber surface became an effective method. However, the strength of the carbon nanotube fiber decreases with increasing conductivity and coating thickness. Here, we deposited nanocrystalline metal coating on carbon nanotube fibers by magnetron sputtering. The coating consists of an ultrathin Al transitional layer and a Cu layer. After annealing, the Al atoms and Cu atoms diffused into the carbon nanotube fiber to form diffuse interface and chemical reaction interface, leading to the sliding resistance increase and contact resistance decrease between carbon nanotubes. As a result, the new composite fiber with a 2 μm thick Cu layer can exhibit a superhigh effective strength of 996 MPa and electrical conductivity of 2.6 × 107 S/m. This structure achieves both conductivity and strength increasing simultaneously.

Modeling hybrid polymer–nanometal lightweight structures

Increasing the environmental sustainability of aviation is a key design goal for commercial aircraft for the foreseeable future. From the perspective of structural engineering, this is accomplished through reducing the mass of aircraft components and structures. Advanced manufacturing techniques offer new avenues for design, enabling more complex structures which can have highly tailored properties. One advanced manufacturing concept is the use of 3D printed polymer preforms that are coated with nanocrystalline metal through electrodeposition. This enables the use of high-performance materials in virtually any geometry. To exploit this manufacturing approach, it is incumbent to have well-established mechanical models of the behavior of such hybrid structures. In particular, hybrid polymer–nanometal structures tend to fail due to compressive instabilities. This paper describes a model of local shell buckling, a typical compressive instability, as it applies to hybrid polymer–nanometal structures. The analysis depends upon the Southwell stress function model for radially loaded solids of rotation, and couples this with the Timoshenko analysis of local shell buckling. This combination is applicable to a range of practical configurations for truss-like hybrid structures.

Fatigue-driven acceleration of abnormal grain growth in nanocrystalline wires

Molecular dynamics simulations were employed to simulate the mechanical response and grain evolution in a Ni nanowire for both static and cyclic loading conditions at both 300 and 500 K for periods of 40 ns. The loading conditions included thermal annealing with no deformation, constant 1% extension (creep loading) and cyclic loading with strain amplitudes of 0.5% and 1% for 200 cycles. Under cyclic loading, the stress–strain response showed permanent deformation and cyclic hardening behavior. At 300 K, modest grain evolution was observed at all conditions within the 40 ns simulations. At 500 K, substantial grain growth is observed in all cases, but is most pronounced under cyclic loading. This may result mechanistically from a net motion of the boundaries associated with boundary ratcheting. There is a striking qualitative consistency between the present simulation results and the experimental observation of abnormal grain growth in nanocrystalline metals as a precursor to fatigue crack initiation.

Iron and other metal species as phase-composition controllers influencing the photocatalytic activity of TiO2 materials

The goal of the work is to verify which properties of TiO2-based materials, influenced by small amounts of Fe, Co, Ga, Bi, W, Mo, V and Ni species used as modifiers, play the predominant role in changing the photoactivity of TiO2. Two groups of materials were studied: TiO2 synthesized in the presence of nanocrystalline metal oxide and dissolved metal ions (cations or oxyanions). Sols were calcined at 450, 600 or 900°C. Physicochemical properties of the materials were characterized by XRD, DRS, SEM, EPR, porosimetry and photocurrent measurements. The most detailed studies were focused on modifications by iron species. TiO2 doping was achieved for both Fe3+ and Fe2O3 modifiers after calcination at temperatures equal to or higher than 450 and 900°C, respectively. Morphology of the materials (phase composition, specific surface area, etc.) were influenced by iron species even if they were not introduced into the crystal lattice of TiO2. Comparison of activity of photocatalysts and elucidation of the role of various reactive oxygen species were based on photooxidation tests involving Azure B and terephthalic acid. In general, iron species improved photocatalytic activity, nevertheless, doping of the materials appeared detrimental. A particular improvement of the activity was achieved for composites with low iron contents (ca. 0.01%mol Fe:Ti). The studies revealed, that beside the widely discussed mechanisms (photo-Fenton processes, charge separation, photosensitization, etc.) iron species can indirectly influence the photocatalytic activity of TiO2 acting as phase-composition controllers (PCC) during the synthesis of this oxide, which determine morphology of the resulting photocatalyst. To support this hypothesis, several sets of other TiO2 materials were modified with Co, Ga, Bi, W, Mo, V and Ni species. We show, that the use of these modifiers in small amounts can influence indirectly the activity of photocatalysts regardless to the type of modifier. Therefore different photoactivity of the tested materials should be attributed to these modifier-induced structural and electronic changes of the photocatalysts rather than to any other function of dopants, including photosensitization, enhanced charge separation, catalytic activity, at which most of studies are focused.

Rationalizing the Grain Size Dependence of Strength and Strain-Rate Sensitivity of Nanocrystalline fcc Metals

Low strain-rate sensitivity (SRS) of nanocrystalline metals measured by experiments often leads to the claim that grain boundary (GB)-mediated plasticity is insignificant, contrary to molecular dynamics simulation results. Here, we develop an crystal plasticity model to rationalize the important role of GB-mediated plasticity on the rate-controlling deformation of nano-grained (NG) and ultrafine-grained (UFG) face-centered-cubic (fcc) metals. Important phenomena such as the GB strengthening, the stress saturation, and the evolution of SRS are well captured. We show that the main reason for the low SRS measured experimentally in NG metals (several tens of nm) is the dominance of the localized dislocation activities over the GB process on the overall plasticity. Such localization of dislocation process may provide a reason for the formation of shear bands/zones in NG and UFG fcc metals.

Synchrotron X-ray absorption spectroscopy and cation distribution studies of NiAl2O4, CuAl2O4, and ZnAl2O4 nanoparticles synthesized by sol-gel auto combustion method

An array of modern approaches to X-ray including X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) were employed to examine the local atomic structure, cation distribution, and degree of inversion of nanocrystalline metal aluminate MAl2O4 (M = Ni, Cu, Zn) powders produced using the sol-gel auto combustion technique. All samples were revealed to display single-phase cubic spinel structure with Fd-3m space group, as established by the XRD results. The cation distribution within these aluminate spinel structures were completely unique as suggested by the divergent disparity in comparative intensity between (220), (400), (422), and (440) diffraction plans for each XRD pattern. Rietveld refinement examination of the XRD data validates the ZnAl2O4 samples as having normal spinel structure, while CuAl2O4 and NiAl2O4 samples comprise partially-inverse spinel structures with low and high degree of inversion, respectively. The investigative findings gained from experimental Ni, Cu, Zn, and Al K-edge XANES spectra accommodated with the hypothetical computation of absorption spectra using the FEFF9.7 code reinforce this. Four-fold synchronization with oxygen atoms was demonstrated by all Zn ions discovered in the ZnAl2O4 samples. Conversely, the Ni and Cu ions located in the NiAl2O4 and CuAl2O4 samples tend to be synchronized in both tetrahedral and octahedral fashion by four or six oxygen atoms, respectively. Additionally, M2+ altering from Zn2+ ions to be Cu2+ and Ni2+ ions caused a substantial translocation of divalent metal (M2+) ions from the tetrahedral (A) sites to octahedral (B) sites, as clearly demonstrated by the EXAFS spectra. Specifically, the degree of inversion in these metal aluminates is completely dissimilar and identified as 0.83 for NiAl2O4, 0.42 for CuAl2O4 and 0 for ZnAl2O4, as indicated by the curve-fitting analysis of Ni, Cu, and Zn K-edge EXAFS spectra. These values comprise vital data for comprehension of numerous pertinent properties.

Mechanisms of grain boundary migration and growth in nanocrystalline metals under irradiation

Atomistic simulations of radiation damage uncover how grain boundaries (GBs) migrate and coalesce under irradiation in bicrystalline Cu. Planar GB migration biased by defect cluster-mediated attraction first leads to slow and steady motion. Subsequently, adjoining GBs coalesce into curved surfaces, where curvature-driven migration with a velocity three orders of magnitude higher than that of a planar boundary dominates motion, triggering rapid grain growth. This study reveals the atomistic mechanisms of radiation-induced grain growth, and has practical implications towards engineering radiation-tolerant nanostructures.

Amorphous Intergranular Films Mitigate Radiation Damage in Nanocrystalline Cu-Zr

Nanocrystalline metals are promising radiation tolerant materials due to their large interfacial volume fraction, but irradiation-induced grain growth can eventually degrade any improvement in radiation tolerance. Therefore, methods to limit grain growth and simultaneously improve the radiation tolerance of nanocrystalline metals are needed. Amorphous intergranular films are unique grain boundary structures that are predicted to have improved sink efficiencies due to their increased thickness and amorphous structure, while also improving grain size stability. In this study, ball milled nanocrystalline Cu-Zr alloys are heat treated to either have only ordered grain boundaries or to contain amorphous intergranular films distributed within the grain boundary network, and are then subjected to in situ transmission electron microscopy irradiation and ex situ irradiation. Differences in defect density and grain growth due to grain boundary complexion type are then investigated. When amorphous intergranular films are incorporated within the material, fewer and smaller defect clusters are observed while grain growth is also limited, leading to nanocrystalline alloys with improved radiation tolerance.

Encapsulation of yellow phosphors into nanocrystalline metal-organic frameworks for blue-excitable white light emission.

Host-guest based highly emissive and solution processable nanocomposites are achieved by in situ encapsulation of yellow-emitting molecular dyes into ZIF-8 and UiO-66 during their nanocrystal formation, which can be effectively excited by blue light to generate white light.

Mechanisms of Localized Pulsed Electrodeposition (L-PED) for Microscale 3D Printing of Nanotwinned Metals

Nanotwinned metals (nt-metals) show larger ductility and strength compared to nanocrystalline metals facilitated by their special microstructure containing arrays of parallel twin boundaries. The recently-introduced localized pulsed electrodeposition (L-PED) enables 3D printing of nt-metals in complex geometries. Herein, the first computational model incorporating all the involved physics (i.e. electrodeposition, evaporation, fluid flow, and heat transfer) in the L-PED process is presented. The model reveals the critical rules of the pulsed signal and evaporation-driven convection flux in the mass transport mechanism, ion concentration, current density, and printing rate in the L-PED process. Notably, the simulation results predict a very high peak current density (∼130 times of the average current density) during the short (∼ms) ON-time. This high current density in a short period of ON-time results in high deposition rate, of possibly a metal with high internal stress. This prediction may explain the current hypotheses in the literature on formation of nt-metals by stress relaxation during the OFF-time.

Onnanoscale phenomena inthe electroacoustic sputtering process

Introduction. The effect of variable parameters of the electroacoustic sputtering (ELAS) process on the characteristics of the crystalline structure of hardening coatings is studied. The ELAS parameter values providing nanostructured cover coatings for machine parts and cutting tools are determined. Hardening through using such coatings allows achieving a significant (5-10 times) increase in the life of hardenable machine parts and various tools designed for mechanical processing. To obtain coatings with the desired properties of the surface layer, nanocrystalline materials should be selected. In this case, a certain content of the amorphous phase is permissible.Materials and Methods. To carry out the X-ray structural analysis, the X-ray diffraction Russian-made device DRON-3M was used. The Scherrer-Wilson method was applied to determine the granularity of particle blocks from the value of the intrinsic broadening of the diffractogram peaks. The conclusions obtained in this paper are based on the method of separation of the affecting factor contributions into broadening the diffraction reflection peaks (the Warren-Averbach method).Research Results. Depending on the process conditions and the technique for obtaining nanostructured materials, a nonuniquely interpretable change in the indices of the diffraction peaks broadening occurs, which is generally characteristic of nanocrystalline metals. One of the possible explanations for this phenomenon is the presence of a nanosized effect in the hardened layer. The occurrence of the nanocrystalline structure in the sputtered layer verifies the calculated values of the dimensions of the coherent scattering regions (CSR). The occurrence of affecting values of the misorientation angle of the crystal structure is verified by the CSR value for the investigated 110 and 220 reflexes, which is supported by a high percentage of the amorphous phase.Discussion and Conclusions. The electroacoustic scattering method is promising for obtaining nanocrystalline structures in the surface and subsurface layers of the sprayed samples. The ELAS process variables variation leads to the parameter spread of the crystal lattice and coherent scattering areas. In this case, there is no definite trend. In the future it is expected to solve the given problem. First, experiments will be conducted to determine the optimal sputtering regimes that could stimulate the formation of nanocrystalline structures. Secondly, visual observation and evaluation of the sprayed layer structure using electron microscopy is planned.

Interpreting the inverse Hall-Petch relationship and capturing segregation hardening by measuring the grain boundary yield stress through MD indentation

The inverse Hall-Petch relationship, which occurs below a critical grain size, is attributed to the high interface to volume ratio that characterizes nanocrystalline metals. This has led some theoretical models to treat nanocrystals as composite materials in which the grains and grain boundaries behave as two separate phases, which follow their own yield behavior. The present study provides first estimates of a grain boundary yield stress, by employing molecular dynamics indentation. The case system considered was that of nanocrystalline Fe with a Σ5 symmetric boundary and indentations were performed either in the grain interior or on the grain boundary illustrating that grain boundaries yield at less than half the stress that grains do. This can interpret the inverse Hall-Petch relationship, since as the grain size decreases below a critical value, the grain boundary thickness is not negligible and its low yield stress dominates the material behavior. To stabilize the boundary from migration/sliding and capture the effect of segregation the simulations were repeated when 0.008% C impurities were added into the sample. This low C content resulted in a higher grain boundary yield stress, without however increasing the grain interior yield stress, illustrating the sensitivity of grain boundaries to segregant atoms. In addition to shear strain maps, a new visual representation was done by using different colors to indicate the orientation of atoms during indentation, as would be done experimentally with electron backscatter diffraction. It was therefore possible to reveal the occurrence of twinning, in addition to dislocation nucleation.

Anneal hardening and elevated temperature strain rate sensitivity of nanostructured metals: Their relation to intergranular dislocation accommodation

Nanocrystalline materials exhibit various properties or phenomena not common in the conventional grain size regime, including enhanced strain rate sensitivity for FCC metals or strength increase during recovery annealing. These peculiarities are associated with the enhanced confinement of plasticity. Accordingly, the interaction of dislocations with the numerous grain boundaries, the boundary state as well as its local chemistry are of importance for a complete understanding. Due to the various influencing factors, determination of the dominant and rate controlling processes remains challenging. Here, we present a study on selected nanostructured FCC materials where dislocation-grain boundary interactions have been studied earlier for their coarse grained counterparts. High temperature nanoindentation revealed for all materials a pronounced increase of strain rate sensitivity when exceeding a certain temperature, peaking prior to the occurrence of significant grain growth. Static annealing of samples close to these peak temperatures leads, for sufficiently small grain sizes, to a maximum hardness increase. Interestingly, despite the nanocrystalline grain size, these temperatures perfectly agree with those obtained earlier for annihilation of lattice dislocations at grain boundaries in coarse-grained samples. This suggests that at elevated temperatures the dominant mechanism controlling the enhanced rate sensitivities in nanocrystalline metals is the thermally activated annihilation of lattice dislocations at grain boundaries. Measurements of activation energies being close to reported values for grain boundary diffusion further support this concept.

Ultrastrong nanocrystalline steel with exceptional thermal stability and radiation tolerance

Nanocrystalline (NC) metals are stronger and more radiation-tolerant than their coarse-grained (CG) counterparts, but they often suffer from poor thermal stability as nanograins coarsen significantly when heated to 0.3 to 0.5 of their melting temperature (Tm). Here, we report an NC austenitic stainless steel (NC-SS) containing 1 at% lanthanum with an average grain size of 45 nm and an ultrahigh yield strength of ~2.5 GPa that exhibits exceptional thermal stability up to 1000 °C (0.75 Tm). In-situ irradiation to 40 dpa at 450 °C and ex-situ irradiation to 108 dpa at 600 °C produce neither significant grain growth nor void swelling, in contrast to significant void swelling of CG-SS at similar doses. This thermal stability is due to segregation of elemental lanthanum and (La, O, Si)-rich nanoprecipitates at grain boundaries. Microstructure dependent cluster dynamics show grain boundary sinks effectively reduce steady-state vacancy concentrations to suppress void swelling upon irradiation.

Diphenylisobenzofuran Bound to Nanocrystalline Metal Oxides: Excimer Formation, Singlet Fission, Electron Injection, and Low Energy Sensitization

We report the photophysical properties of the dicarboxylated diphenylisobenzofuran dye (1) bound to nanocrystalline metal oxide surfaces. With increased surface loading of 1, emission from the films is significantly quenched but with a small amount of excimer emission at maximum surface loadings. Long-lived triplets were observed by ns TA spectroscopy that are consistent with singlet fission occurring on mesoporous ZrO2. The evolution of these triplets, however, could not be convincingly resolved by our subnanosecond TA spectroscopy. Dye-sensitized devices composed of 1 on a TiO2|Al2O3 core–shell structure exhibited an unusual decrease, increase, and then decrease in Jsc with respect to the thickness of Al2O3. In these films the Al2O3 acts as a tunneling barrier to slow electron injection from the singlet excited state such that singlet fission, and electron injection from the triplet state becomes competitive. Proof-of-principle self-assembled bilayer films that exhibit efficient triplet energy transfer ...

Unprecedented irradiation resistance of nanocrystalline tungsten with equiaxed nanocrystalline grains to dislocation loop accumulation

Nanocrystalline metals are often postulated as irradiation tolerant materials due to higher grain boundary densities. The efficiency of these materials in mitigating irradiation damage is still under investigation. Here, we present an in-situ transmission electron microscopy with ion irradiation study on equiaxed 35 nm grained tungsten (NCW-35 nm) and compare its radiation tolerance, in terms of dislocation loop damage, to several other grades of tungsten with different grain sizes at two temperatures (RT and 1073 K). The NCW-35 nm was shown to possess significant higher radiation tolerance in terms of loop damage. As demonstrated by Kinetic Monte Carlo simulations, at least part of the higher radiation tolerance of the small grains is due to higher interstitial storage (at the grain boundaries) and defect recombination (in the grain interiors) in the small grain material. In addition, experimental observations reveal rapid and efficient dislocation loop absorption by the grain boundaries and this is considered the dominant factor for mass transport to the boundaries during irradiation, enabling the remarkable radiation tolerance of 35 nm grained tungsten. This study demonstrates the possibility of attaining high radiation tolerant materials, in terms of dislocation loop damage, by minimizing grain sizes in the nanocrystalline regime.

Anisotropic grain growth kinetics in nanocrystalline nickel

ABSTRACTIntriguing properties exhibited by nanocrystalline metals, including a high level of mechanical strength, arise from their nanometer-scale grain sizes. It is critical to determine the evolution of grain size of nanocrystalline materials at elevated temperature, as this process can drastically change the mechanical properties. In this work, a nanocrystalline Ni foil with grain size ∼ 25 nm was annealed in situ in an X-ray diffractometer. X-ray diffraction peaks were analysed to determine the grain growth kinetics. The grain growth exponents obtained were ∼ 2–4 depending upon the crystallographic direction, indicating the anisotropic nature of the grain growth kinetics.