Mechanical Properties Of Nanocrystalline Materials Research Articles

Theoretical Prediction of Strengthening in Nanocrystalline Cu with Multi-Element Grain Boundary Segregation Decoration.

The composition of grain boundaries (GBs) determines their mechanical behavior, which in turn affects the mechanical properties of nanocrystalline materials. Inspired by GB segregation and the concept of high-entropy alloys (HEAs), we investigated, respectively, the mechanical responses of nanocrystalline Cu samples with and without multi-element GBs, as well as the grain size effects, aiming to explore the effects of GB composition decoration on mechanical properties. Our results show that introducing multi-element segregation GBs can significantly improve the mechanical properties of nanocrystalline Cu by effectively inhibiting GB migration and sliding. Additionally, we proposed an improved a theoretical model that can reasonably describe the strengths of the materials with multi-element or single-element segregation GBs. Notably, the introduction of multi-element segregation GBs inhibits both migration and sliding behavior, with migration being more effectively suppressed than sliding. These results present a novel approach for designing high-performance nanometallic materials and offer valuable insights into the role of GB composition decoration in enhancing mechanical properties.

Plastic deformation-driven grain coarsening in nanocrystalline Gd2Zr2O7 ceramics by nanoindentation

Unique deformation mechanisms are responsible for the superior mechanical properties of nanocrystalline materials. However, unlike in nanocrystalline metals, questions remain regarding the underlying microstructural mechanisms governing plastic deformation in nanocrystalline ceramics. In the present study, nanoindentation and postmortem electron microscopy analysis were carried out on the transparent nanocrystalline Gd2Zr2O7 ceramic. Apart from shear bands led from grain boundary (GB) sliding, experimental observations confirm that deformation-driven grain coarsening is controlled by grain rotation accompanying the GB migration mechanism.

Tensile strength and fracture mechanics of two-dimensional nanocrystalline silicon carbide

Two-dimensional Silicon Carbide (SiC) has opened the route to a cornucopia of advanced functionalities in the realm of quantum condensed matter. It holds great promise for highly efficient nanoelectronic, optoelectronic, renewable energy, and spintronic applications thanks to the confluence of a wide spectrum of mesmerizing physical properties like a wide direct bandgap with high exciton binding energy, robust spin–orbit-coupling, excellent photoluminescence, suitable mechanical strength, and thermodynamic stability. Nonetheless, it is still a daunting challenge to incorporate SiC in functional systems since extensive analyses of the mechanical properties, and fracture mechanism of nanocrystalline (NC)-SiC is still obscure. In this light, this work is an attempt to report detailed information concerning the room-temperature tensile mechanical properties and fracture phenomena of NC-SiC executing Molecular Dynamics (MD) simulations. In particular, effects of grain size on the stress–strain profile, fracture strength, fracture strain, and Young’s modulus of the NC-SiC have been thoroughly investigated. It has been found that the strength as a function of grain size can be characterized by the inverse pseudo Hall-Petch relation. Increasing grain size brings about more elasticity in the structure, albeit at the price of fracture strain. The NC-SiC encounters a substantial degradation in mechanical properties relative to its single-crystal counterpart. Afterward, we performed an exhaustive fracture analysis on two NC-SiC samples of different grain sizes. The single-crystal SiC can endure more tensile strain before rupture compared to that of the NC-SiC. At last, the nanosheet exhibits more immunity to fracture with decreasing grain size. This study would lay the groundwork for NC-SiC to be successfully realized in functional systems as well as serving as a solid roadmap for engineering the mechanical properties of nanocrystalline materials.

Molecular dynamics simulation study of interaction mechanism between grain boundaries and subgrain boundaries in nano-cutting

In nano-cutting, grain boundaries with complex network structures hinder the movement of defects like subgrains in nanocrystalline materials, affecting the surface generation process and mechanical properties of the machined materials. In this work, the interaction mechanism between the grain boundaries and the subgrain boundaries in nano-cutting is investigated by molecular dynamics (MD) simulations. The results show that the extrusion action between the grain boundaries and the cutting tool causes the periodic formation and annihilation of the subgrains in nano-grains. The interactions between the grain boundaries and the subgrains lead to continuous pile-up formation, obvious stresses accumulation, elastic recovery of the machined surface, and periodic oscillation of processing force. This study will be helpful for designing the nano-cutting process to get better surface quality and mechanical properties of nanocrystalline materials.

Electrodeposition of bulk nanocrystalline Ni–Fe–P alloys and their mechanical and soft magnetic properties

Electrodeposited Ni–Fe alloys are expected to have an improved mechanical strength because of their nanocrystalline structure. However, unsatisfactory grain refinement yields a higher coercivity compared with conventional permalloys, which is undesirable in soft magnetic materials. We report electrodeposited Ni–Fe–P alloys that combine the mechanical properties of nanocrystalline materials with soft magnetism that is achievable in coarse-grained permalloys. In Ni–Fe–P alloy electrodeposition, the connections between the bath composition and Fe and P contents of the samples were demonstrated. For Ni–Fe alloys, the introduction of a few percent of P led to a preferable microstructure that was composed of ~10-nm-diameter grains. Up to 2.0 at% P, the resultant Ni–Fe–P alloys achieved a tensile strength of 2.1 GPa, along with a plastic deformability. The electrodeposited alloys exhibited a saturation magnetic flux density of 1.1 T and a coercivity of 8.4 A/m. Our approach indicates that grain refinement by the P alloying can result in a lower coercivity, according to the law of grain size and coercivity.

Electrodeposition of Bulk Nanocrystalline Ni-Fe-P Alloys and Their Mechanical and Soft Magnetic Properties

Electrodeposited Ni-Fe alloys are expected to have an improved mechanical strength because of their nanocrystalline structure. However, unsatisfactory grain refinement yields a higher coercivity compared with conventional permalloys, which is undesirable in soft magnetic materials. We report electrodeposited Ni-Fe-P alloys that combine the mechanical properties of nanocrystalline materials with soft magnetism that is achievable in coarse-grained permalloys. Up to 2.0 at% P, the resultant Ni-Fe-P alloys achieved a tensile strength of 2.1 GPa, along with a plastic deformability. The electrodeposited alloys exhibited a saturation magnetic flux density of 1.1 T and a coercivity of 8.4 A/m. Our approach indicates that graine refinement by the P alloying can result in a lower coercivity, according to the law of the grain size and coercivity.

Shear strength of nanocrystalline δ-phase Pu-Ga alloys: Atomistic simulations

The mechanical properties of nanocrystalline materials differ dramatically from those of conventional ones. In particular, the yield strength noticeably grows as grain size decreases. On the other hand, the grain size is not the only factor affecting the yield strength. Extended ingrain defects such as dislocations and stacking faults also contribute to the strength of materials. Modification of the mechanical properties of plutonium and its alloys via grain refinement is of interest because of their use in civil and military nuclear technologies, e.g., as a component of metallic nuclear fuel. But the experimental study of plutonium is hampered by its high chemical and radiological activity harmful to human health. That is why theoretical and computational techniques proved to be useful for investigation into plutonium properties must be applied first, prior to experimental ones. In the paper the atomistic simulation approach is applied to directly calculate the mechanical properties of nanocrystalline face centered cubic δ-phase Pu-Ga alloys, in particular, the ambient conditions quasi-static yield stress dependence on grain size and extended defect concentration. The range of grain sizes studied is 40−200nm. A deviation from the Hall-Petch relation is demonstrated. The effect of the alloying addition redistribution inside grains as well as between the grains is also evaluated.

Effect of blunt nanocracks on the splitting transformation of grain boundary dislocation piled up at triple junctions

Considering that grain boundary (GB) deformation is a structural element that hinders crack growth and promotes superplastic deformation in nanocrystalline materials (NCMs), a theoretical model is suggested to describe the effect of the blunt pre-nanocracks with surface stress on the splitting transformation of the first head grain boundary dislocation (GBD) within the pile-up at the triple junctions (TJs) of GBs in mechanically loaded NCMs. The analytic solution of the total energy change that characterizes this process of splitting transformation of GBD is derived quantitatively by the complex variable method, and then, the very beginning plastic deformation occurrence near the nanocrack tip is predicted. We theoretically evaluated the influence of the various parameters of blunt pre-nanocracks, such as nanocrack blunting and length, the characteristics of grain and GBs, such as grain size, the number of GBDs, and GB angles, and the surface stress at critical conditions for such a splitting transformation. Further analyses revealed that the positive (negative) surface stress significantly decreases (increases) the energy and obviously influences the critical conditions for splitting transformation. Our study identified the beneficial role of GB defect structure transformation, namely the splitting transformation, in enhancing the ductility and fracture toughness of NCMs. It also lays the foundation for investigating how the microstructures caused by GB deformation affect the novel mechanical properties of NCMs.

Prediction of mechanical properties of nanocrystalline materials using Voronoi FE models of representative volume elements

In the present work, a numerical model is developed to predict the mechanical properties of nanocrystalline materials using a Finite Element Analysis. The model is based on Representative Volume Elements (RVE) in which the microstructure of the material is described using the Voronoi tessellation algorithm. The use of the Voronoi particles was based on the observation of the morphology of nanocrystalline materials by Scanning Electron and Transmission Electron Microscopy. In each RVE, three-dimensional modelling of the grain and grain boundaries as randomlyshaped sub-volumes is performed. The developed model has been applied to pure nanocrystallline copper taking into account the parameters of grain size and grain boundary thickness. The mechanical properties of nanocrystalline copper have been computed by loading the RVE in tension. The numerical results gave a clear evidence of grain size effect and the Hall-Petch relationship, which is a consequence of macroscopic strain being preferentially accumulated at grain boundaries. On the other hand, for a given grain volume fraction, the results for elastic moduli showed no effect of the grain size. The model predictions have been validated successfully against numerical results from the literature and predictions of the Rule of Mixtures and the Mori-Tanaka analytical model.

Role of Nanostructure in Electrochemical Corrosion and High Temperature Oxidation: A Review

The extremely fine grain size of nanocrystalline (nc) metallic alloys results in significantly different mechanical, electrochemical and oxidation properties as compared to the coarse-grained alloys of the same composition. Although the synthesis and attractive mechanical properties of nanocrystalline materials have been investigated and reviewed in great depth, the high temperature oxidation and electrochemical corrosion of these materials has received limited attention. The difference in the active dissolution and passivation behavior of nc alloys as compared to their microcrystalline counterparts varies for each alloy system. However, a unified theory explaining these phenomena still eludes us. In this context, this article reviews the progress in the electrochemical corrosion behavior of different nanocrystalline alloys, and hence, develops a better understanding of the effect of grain size, composition, interfacial phenomena and selective dissolution on corrosion of nanocrystalline alloys. This review also presents the role of nanometric grain size and the associated increase in grain boundary diffusion on the high temperature oxidation of nc alloys. The attractive possibility of enhanced oxidation resistance at lower alloying additions as compared to the coarse-grained alloys has been discussed. Although the primary focus of the article is on ferrous alloy systems, however, the lead studies on the role of ultrafine grain size in oxidation/corrosion behavior of other alloys systems have also been reviewed.

“Bulk” Nanocrystalline Metals: Review of the Current State of the Art and Future Opportunities for Copper and Copper Alloys

It is a new beginning for innovative fundamental and applied science in nanocrystalline materials. Many of the processing and consolidation challenges that have haunted nanocrystalline materials are now more fully understood, opening the doors for bulk nanocrystalline materials and parts to be produced. While challenges remain, recent advances in experimental, computational, and theoretical capability have allowed for bulk specimens that have heretofore been pursued only on a limited basis. This article discusses the methodology for synthesis and consolidation of bulk nanocrystalline materials using mechanical alloying, the alloy development and synthesis process for stabilizing these materials at elevated temperatures, and the physical and mechanical properties of nanocrystalline materials with a focus throughout on nanocrystalline copper and a nanocrystalline Cu-Ta system, consolidated via equal channel angular extrusion, with properties rivaling that of nanocrystalline pure Ta. Moreover, modeling and simulation approaches as well as experimental results for grain growth, grain boundary processes, and deformation mechanisms in nanocrystalline copper are briefly reviewed and discussed. Integrating experiments and computational materials science for synthesizing bulk nanocrystalline materials can bring about the next generation of ultrahigh strength materials for defense and energy applications.

A micromechanical approach to the stress–strain relations, strain-rate sensitivity and activation volume of nanocrystalline materials

This review highlights a secant viscosity approach that has wide applicability for the determination of mechanical properties of nanocrystalline materials. Along the way we also add some new elements and provide fresh perspectives. This approach was originally proposed for the nonlinear, time-dependent, work-hardening creep of dual-phase composites (Li and Weng, J Mech Phys Solids 45:1069–1083, 1997a), but by conceiving a nanocrystalline material as a composite of the stronger grain interior and the softer grain-boundary (GB, or grain-boundary affected zone GBAZ), it becomes possible to extend it to calculate the grain-size dependence of their flow stress, strain-rate sensitivity, and activation volume. We also use it to explain how the flow stress first increases and then decreases as the grain size decreases from the coarse grain to the nanometer range, leading to the Hall–Petch and the inverse Hall–Petch relations. The critical state at which the slope of the strength variation with respect to the grain size becomes zero also yields the strongest material state. In this way the two most important parameters for material design—the maximum strength and the critical grain at which it occurs—can be obtained. The strain-rate sensitivity parameters are also shown to follow a similar pattern as the flow stress, but the activation volume varies in an exactly reverse way.

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.

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.

Deformation mechanisms and mechanical properties of nanocrystalline materials

A survey is made of modern views on the structure and mechanical properties of nanosized-grain polycrystalline metals and alloys, grain-refining techniques, special features of the deformation mechanisms observed in the nanometer grain-size range with emphasis on the role of grain-boundary sliding and problems associated with the Hall-Petch parameters in the case of the nanocrystalline grain size.

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.

Deformation Mechanism Map of Nanocrystalline Metallic Materials

In this study, a deformation mechanism map of metallic nanocrystalline materials (NCMs) using the phase mixture model is proposed. It is based on recent modelling that appears to provide a conclusive description of the phenomenology and the mechanisms underlying the mechanical properties of NCMs. The proposed models adopted the concept of a ‘phase mixture’ in which the grain interior and the grain boundaries are treated as separate phases. The volume fraction of this grain boundary ‘phase’ may be quite appreciable in a NCM. Based on the theoretical model that provides an adequate description of the grain size dependence of plasticity covering all grain size range from coarse down to the nanoscale, the tensile deformation response of NCMs, especially focusing on the deformation mechanisms was investigated. The deformation mechanism map is newly proposed with axes of strain rate, grain size and temperature.

Structural nanocrystalline materials: an overview

This paper presents a brief overview of the field of structural nanocrystalline materials. These are materials in either bulk, coating, or thin film form whose function is for structural applications. The major processing methods for production of bulk nanocrystalline materials are reviewed. These methods include inert gas condensation, chemical reaction methods, electrodeposition, mechanical attrition, and severe plastic deformation. The stability of the nanocrystalline microstructure is discussed in terms of strategies for retardation of grain growth. Selected mechanical properties of nanocrystalline materials are described; specifically strength and ductility. Corrosion resistance is briefly addressed. Examples of present or potential applications for structural nanocrystalline materials are given.

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.