Properties Of Nanocrystalline Materials Research Articles

3D Nanocrystallography and the Imperfect Molecular Lattice.

Crystallographic analysis relies on the scattering of quanta from arrays of atoms that populate a repeating lattice. While large crystals built of lattices that appear ideal are sought after by crystallographers, imperfections are the norm for molecular crystals. Additionally, advanced X-ray and electron diffraction techniques, used for crystallography, have opened the possibility of interrogating micro- and nanoscale crystals, with edges only millions or even thousands of molecules long. These crystals exist in a size regime that approximates the lower bounds for traditional models of crystal nonuniformity and imperfection. Accordingly, data generated by diffraction from both X-rays and electrons show increased complexity and are more challenging to conventionally model. New approaches in serial crystallography and spatially resolved electron diffraction mapping are changing this paradigm by better accounting for variability within and between crystals. The intersection of these methods presents an opportunity for a more comprehensive understanding of the structure and properties of nanocrystalline materials.

Nanocrystalline materials: recent advances in crystallographic characterization techniques.

Most properties of nanocrystalline materials are shape-dependent, providing their exquisite tunability in optical, mechanical, electronic and catalytic properties. An example of the former is localized surface plasmon resonance (LSPR), the coherent oscillation of conduction electrons in metals that can be excited by the electric field of light; this resonance frequency is highly dependent on both the size and shape of a nanocrystal. An example of the latter is the marked difference in catalytic activity observed for different Pd nanoparticles. Such examples highlight the importance of particle shape in nanocrystalline materials and their practical applications. However, one may ask 'how are nanoshapes created?', 'how does the shape relate to the atomic packing and crystallography of the material?', 'how can we control and characterize the external shape and crystal structure of such small nanocrystals?'. This feature article aims to give the reader an overview of important techniques, concepts and recent advances related to these questions. Nucleation, growth and how seed crystallography influences the final synthesis product are discussed, followed by shape prediction models based on seed crystallography and thermodynamic or kinetic parameters. The crystallographic implications of epitaxy and orientation in multilayered, core-shell nanoparticles are overviewed, and, finally, the development and implications of novel, spatially resolved analysis tools are discussed.

The corrosion behaviour of nanograined metals and alloys

There has been considerable interest in the properties of nanocrystalline materials over the last decade. Such materials include metals and alloys with a crystal size within the order of 1 to 100 nm. The interest arises due to the substantial differences in electrical, optical and magnetic properties and also due to their high adsorption capability and chemical reactivity compared to their larger grained counterparts. In this paper, the corrosion of nanocrystalline metals and alloys is investigated and compared to the corrosion of microcrystalline materials having a similar composition. The focus is on the corrosion of nickel, copper, cobalt and iron alloys. Key aspects of different corrosion behaviour such grain boundaries and size are identified.

Coupled effects of grain size and orientation on properties of nanocrystalline materials

In this paper, an elastic visco-plastic constitutive model for nanocrystalline materials considering the coupled effects of grain size and orientation was proposed for the first time. In the frame work of the model, three cases of different grain size and orientation distribution were analyzed to quantitatively describe the size and orientation effects. Then numerical simulations on the mechanical behaviors of the different cases were performed to figure out the grain size and orientation effects on the overall mechanical behavior of nanocrystalline materials. Especially, the coupled effect of grain size and orientation on the local shear deformation was analyzed and discussed.

Formation mechanism and characterization of nanostructured Ti6Al4V alloy prepared by mechanical alloying

In recent years, researches on properties of nanocrystalline materials in comparison with coarse-grained materials have attracted a great deal of attention. The present investigation has been based on production of nanocrystalline Ti6Al4V powder from elemental powders by means of high energy mechanical milling. In this regard, Ti, Al and V powders were milled for up to 90h and heat treated at different temperatures. The structural and morphological changes of powders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA) and microhardness measurements. The results demonstrated that Ti(Al) and Ti(Al, V) solid solutions with grain size of 95 and 20nm respectively form during mechanical alloying. In addition, an amorphous structure was obtained at longer milling times. The crystallization of amorphous phase upon annealing led to the formation of nanostructured Ti6Al4V phase with a grain size of 20–50nm. The as-milled Ti6Al4V powder with amorphous structure exhibited a high microhardness of ∼720Hv. Upon crystallization the hardness value reduced to ∼630Hv which is higher than those reported for Ti6Al4V alloys processed by conventional routes.

Novel solution routes of synthesis of metal oxide and hybrid metal oxide nanocrystals

Novel solution routes covering a wide range of processes like hydrothermal, solvothermal and supercritical techniques have been described in detail with reference to the processing of a wide range of advanced inorganic nanocrystalline materials and organic-inorganic hybrid nanocrystalline materials. The significance of the thermochemical calculations, in situ surface modification and the experimental parameters has been discussed. One step in situ fabrication of advanced functional nanocrystalline materials by soft solution processing has also been discussed briefly. Synthesis of nanocrystals of metal oxides and hybrid nanocrystals and also processing several nanocomposites like carbon nanotubes: metal oxide, activated carbon: metal oxide, etc. has been reviewed in relation to various process parameters. The effect of doping, size, shape and quality on the properties of nanocrystalline materials has been discussed in relation to the photoluminescence and photocatalytic characteristics.

Identification of the model describing viscoplastic behaviour of high strength metals

Ultrafine grained (UFG) and nanocrystalline metals (nc-metals) are studied. Experimental investigations of the behaviour of such materials under quasistatic as well as dynamic loading conditions related with microscopic observations show that in many cases the dominant mechanism of plastic strain is a multiscale development of shear deformation modes. The comprehensive discussion of these phenomena in UFG and nc-metals is given in M.A. Meyers, A. Mishra and D.J. Benson [Mechanical properties of nanocrystalline materials, Progr. Mater. Sci. 51 (2006), pp. 427–556], where it has been shown that the deformation mode of nanocrystalline materials changes as the grain size decreases into the ultrafine region. For smaller grain sizes (d < 300 nm) shear band development occurs immediately after the onset of plastic flow. Significant strain-rate dependence of the flow stress, particularly at high strain rates, was also emphasized. Our objective is to identify the parameters of Perzyna constitutive model, a new description of viscoplastic deformation, which accounts for the observed shear banding. The viscoplasticity model proposed earlier by Perzyna [Fundamental problems in viscoplasticity, Adv. Mech. 9 (1966), pp. 243–377] was extended in order to describe the shear banding contribution in Z. Nowak, P. Perzyna, R.B. Pȩcherski [Description of viscoplastic fow accounting for shear banding, Arch. Metall. Mater. 52 (2007), pp. 217–222]. The shear banding contribution function, which was introduced formerly by Pȩcherski [Modelling of large plastic deformation produced by micro-shear banding, Arch. Mech. 44 (1992), pp. 563–584] and applied in continuum plasticity accounting for shear banding in R.B. Pȩcherski [Macroscopic measure of the rate of deformation produced by micro-shear banding, Arch. Mech. 49 (1997), pp. 385–401] plays pivotal role in the viscoplasticity model. The derived constitutive equations were identified and verified with the application of experimental data provided in the article by D. Jia, K.T. Ramesh and E. Ma [Effects of nanocrystalline and ultrafne grain sizes on constitutive behavior and shear bands in iron, Acta Mat. 51 (2003), pp. 3495–3509], where quasistatic and dynamic compression tests with UFG and nanocrystalline iron specimens of a wide range of mean grain size were reported. Numerical simulation of the compression of the prismatic specimen was made by the ABAQUS FEM program with UMAT subroutine. Comparison with experimental results proved the validity of the identified parameters and the possibilities of the application of the proposed description for other high strength metals.

Investigation of microstructure thermal evolution in nanocrystalline Cu

The microstructure of nanocrystalline Cu prepared by compacting nanoparticles (50–60 nm in diameter) under high pressures has been studied by means of positron lifetime spectroscopy and X-ray diffraction. These nanoparticles were produced by two different methods. We found that there are order regions interior to the grains and disorder regions at the grain boundaries with a wide distribution of interatomic distances. The mean grain sizes of the nanocrystalline Cu samples decrease after being annealed at 900 °C and increase during aging at 180 °C, which are observed by X-ray diffraction, revealing that the atoms exchange between the two regions. The positron lifetime results clearly indicate that the vacancy clusters formed in the annealing process are unstable and decomposed at the aging time below 6 hours. In addition, the partially oxidized surfaces of the nanoparticles hinder grain growth when the samples age at 180 °C, and the vacancy clusters inside the disorder regions, which are related to Cu 2O, need longer aging time to decompose. The disorder regions remain after the heat treatment in this work, in spite of the grain growth, which will be good for the samples keeping the properties of nanocrystalline material.

Thermomagnetic and Mössbauer studies of structural transformations caused in the amorphous Nd9Fe85B5 alloy by severe plastic deformation and annealing

Thermomagnetic analysis and Mossbauer spectroscopy were used to study the effect of severe plastic deformation (SPD) by high-pressure torsion (HPT) and subsequent annealing on structural transformations and formation of magnetic properties of rapidly quenched Nd9Fe85B6 alloy. The HPT of the Nd9Fe85B6 amorphous alloy was found to result in the precipitation of α-Fe nanocrystals and in changes in the structural state of the residual amorphous phase A′. In the annealed samples, there was revealed a great amount of nonequilibrium phases with different magnetizations. The total content of nonequilibrium phases depends on the annealing temperature and affects the exchange interaction between magnetically soft α-Fe nanocrystals and Nd2Fe14B nanocrystalline grains. The results obtained in this study can explain the differences between the high level of hysteresis properties of nanocrystalline materials, which was predicted theoretically, and low magnitudes realized in practice.

Fabrication and characterization of nanostructured Ti6Al4V powder from machining scraps

In recent years researches on properties of nanocrystalline materials in comparison with coarse-grained materials has attracted a great deal of attention. The present investigation has been based on production of nanocrystalline Ti6Al4V powder by means of high energy mechanical milling. In this regard, Ti6Al4V powder was produced by ball milling of machining scraps of Ti6Al4V. The structural and morphological changes of powders were investigated by X-ray diffraction, scanning electron microscopy, and microhardness measurements. The results revealed that ball milling process reduced the size of the coherent-scattering region of Ti6Al4V to approximately 20 nm. Also a remarkable change in morphology and particle size was occurred during ball milling. Moreover, phase evolution during milling was taken into consideration. The as-milled Ti6Al4V powder exhibited higher microhardness comparing to the original samples.

Synchrotron X-ray line-profile analysis experiments for the in-situ microstructural characterisation of SPD nanometals during tensile deformation

Abstract There is a great interest in the understanding of mechanical properties of nanocrystalline materials, especially of those processed by severe plastic deformation, since those exhibit both high strength and considerable ductility. A special setup for X-ray line profile analysis was developed for monitoring the microstructural evolution of in-situ tensile tests of high pressure torsion deformed samples. A comprehensive evaluation procedure is presented which allows the determination of several physical microstructural parameters. This includes a careful estimation of the error of the evaluation procedure. As an example, nickel shows a slight decrease of the size of the coherently scattering domains while the dislocation density tends to increase. The dislocation arrangement exhibits strain-field-screening of neighboring dislocations and an equal amount of screw and edge dislocations.

Low-frequency vibrational properties of nanocrystalline materials: Molecular dynamics simulations of two-dimensional systems

Low-frequency vibrational properties of nanocrystalline materials: Molecular dynamics simulations of two-dimensional systems

Molecular Dynamics Analysis on Crack Growth Behavior in Single and Nano-Crystalline Fe by the Use of FS-2NNMEAM Hybrid Potential

In recent years, nano-crystalline materials have attracted many researchers’ attention, but the fracture mechanism has not been fully clarified. In a molecular dynamics (MD) simulation, grain size and crystal orientation can be chosen, and their effects on the mechanical properties of nano-crystalline materials can be evaluated clearly. This research first compares the results of crack growth behavior in single crystalline Fe for three typical interatomic potentials (Embedded Atom Method (EAM), Finnis Sinclair (FS), and Second Nearest Neighbor Modified EAM (2NNMEAM) potentials) and a Hybrid potential method, which uses FS potential for bcc structure atoms and 2NNMEAM potential for non-bcc structure atoms. The 2NNMEAM potential is accurate, but the computation time is dozens of times that of FS potential, which is the simplest of the three interatomic potentials. Therefore, the 2NNMEAM potential requires too much calculation for the purpose of this research that analyzes the crack growth behavior in nano-crystalline metals. However, Hybrid potential is able to give results similar to those of the 2NNMEAM potential, and the calculation time is close to that of the FS potential. From these results, the crack extension behavior in relatively large nano-crystalline models is analyzed using the Hybrid potential, and we demonstrate the grain-size dependency of the fracture behavior.

Scale effect and geometric shapes of grains

The rule-of-mixture approach has become one of the widely spread ways to investigate the mechanical properties of nano-materials and nano-structures, and it is very important for the simulation results to exactly compute phase volume fractions. The nanocrystalline (NC) materials are treated as three-phase composites consisting of grain core phase, grain boundary (GB) phase and triple junction phase, and a two-dimensional three-phase mixture regular polygon model is established to investigate the scale effect of mechanical properties of NC materials due to the geometrical polyhedron characteristics of crystal grain. For different multi-sided geometrical shapes of grains, the corresponding regular polygon model is adopted to obtain more precise phase volume fractions and exactly predict the mechanical properties of NC materials.

Tensile properties of electrodeposited nanocrystalline nickel

For the usage and shaping of engineering parts of nanocrystalline (NC) materials, deformation behavior at both the service temperature and the forming temperature is necessary. For characterization of the mechanical properties of metallic NC materials, NC nickel has been synthesized using electrodeposition processing. In this study, experimental uniaxial tensile testing was performed to elucidate the deformation behavior of NC nickel of 12 nm grain size under various strain rate conditions. In addition to the experimental approach, we provide an account of the mechanical properties of NC nickel materials from our current viewpoint. This is based on recent modeling that appears to provide a conclusive description of the phenomenology and the mechanisms underlying the mechanical properties of NC materials. The experimental results indicate that the deformation in an elastic range is large (∼2%) and strain rate insensitive whereas in a plastic deforming range the deformation is small and strain rate sensitive. The fracture strain and stress increases with increasing strain rate. Overall, limited fracture strain results from the absence of strain hardening soon after the elastic limit. Phase mixture modeling could simulate the absence of strain hardening associated with the grain boundary mediated plastic deformation mechanism.

Elastic stiffness of interfaces studied by Rayleigh waves

The properties of nanocrystalline materials are often dominated by the behavior of their interfaces. We use Rayleigh waves to measure in situ the elastic stiffness of elemental and alloy thin films while they are being deposited. Of special interest are changes in stiffness of the multilayer upon switching the deposition from one metal to another. For most film/substrate combinations, the stiffness changes abruptly, as found for Ir/Pd, Pd/Ag, Pd/Co, Au/Co, Ag/Pd, Ag/Co, and Pt/Pd interfaces. For other metal–metal combinations, the stiffness change shows transient effects. These changes reflect either an elastic softening, as found at Co/Pd, Co/Ag, Co/Au, Pd/Pt, and Ir/Pt interfaces, or an elastic stiffening, as found at Pd/Ir and Pt/Ir interfaces. The stiffening at Pd/Ir interfaces is studied in greater detail. X-ray photoelectron spectroscopy (XPS) data are used in the interpretation of the results.

Physical-mechanical properties of nanocrystalline materials based on ultrafine-dispersed diamonds

The physical-mechanical properties of polycrystals produced on the basis of ultrafine-dispersed diamonds (UDDs) at high static pressures with the use of thermal treatment in vacuum are investigated. The properties of the polycrystals are shown to depend on the sintering conditions and sintering technique, as well as on the modification of the starting powders. With the addition of metals (Co, Ti) to UDD powders, the mechanical strength of compacts increases, their structure improves, and their parameters can be optimized at lower thermobaric treatment temperatures. By studying the products of thermal treatment of UDDs in vacuum, it is established that the compacts and individual particles that form during thermal treatment differ significantly in microhardness.

Nanocrystalline fcc metals: bridging experiments with simulations

AbstractAtomistic simulations have provided unprecedented insight into the structural and mechanical properties of nanocrystalline materials, highlighting the role of the non-equilibrium grain boundary structure in both inter- and intra-grains deformation processes. One of the most important results is the capability of the nanosized grain boundary to act as a source and sink for dislocations. However the extrapolation of this knowledge to the experimental regime requires a clear understanding of the temporal and spatial scales of the modelling technique and a detailed structural characterisation of the simulated samples. In this contribution some of the synergies that can be develo ped between atomistic simulations and experiments for this research field are briefly discussed by means of some typical examples.

Nanocrystalline fcc metals: bridging experiments with simulations

AbstractAtomistic simulations have provided unprecedented insight into the structural and mechanical properties of nanocrystalline materials, highlighting the role of the non-equilibrium grain boundary structure in both inter- and intra-grains deformation processes. One of the most important results is the capability of the nanosized grain boundary to act as a source and sink for dislocations. However the extrapolation of this knowledge to the experimental regime requires a clear understanding of the temporal and spatial scales of the modelling technique and a detailed structural characterisation of the simulated samples. In this contribution some of the synergies that can be developed between atomistic simulations and experiments for this research field are briefly discussed by means of some typical examples.

The structure and properties of nanocrystalline materials: Issues and concerns

In this article, issues and concerns arising from the present research into nanocrystalline materials are high-lighted.