Nanocrystalline Metals Research Articles

Shear-induced directional grain growth in Ag nanocrystalline films under nanoindentation

Nanocrystalline metallic materials exhibit different mechanical properties compared with coarse-grained counterparts. In essence, the high density of grain boundaries contributes to significant portion of plasticity, which is more pronounced in thin films. Due to intrinsic geometric limitation, the responses of nanocrystalline films under shear-dominant loading are seldom understood. Here, we take Ag film as an example, with a larger indentation depth than film thickness, the deformation beneath the pyramidal surfaces of the indenter is considered to be dominated by in-plane shear stress with gradient distribution. The shear-dominant loading leads to directional grain elongation within indentation region, accompanied with frequent formation of nanotwins across the grain boundaries. These findings enrich our understanding of stress induced grain growth under nanoindentation, and provide a novel way to study shear-dominated plastic deformation in metallic nanocrystalline films.

Hydrostatic pressure-induced transition in grain boundary segregation tendency in nanocrystalline metals

Impurity segregation at grain boundaries (GBs) can have a profound impact on the mechanical behavior of nanocrystalline metals. In this study, we employ atomistic simulations to investigate the effects of hydrostatic pressure on the GB segregation energy spectra in a series of binary alloy systems. Our results demonstrate that, depending on the alloy system, hydrostatic pressure can either enhance or hinder GB segregation tendencies. However, in specific alloys, such as AlMg and NiNb, hydrostatic pressure can also result in transitions in the preferred segregation sites, characterized by an initial increase followed by a decrease at certain pressures. The underlying mechanisms for these transitions can be attributed to the changes in the elastic component brought about by hydrostatic pressure, which dominate the other contributions to the total segregation energy.

Role of geometric dynamic recrystallization in nanocrystalline alloys

Nanocrystalline and near-nano-grained metals are plagued with an overall lack of ductility and formability. This is in part due to their inability to support the dislocation mechanisms or undergo the same types of transformations that conventional coarse-grained metals access inorder to increase their workability. Therefore, despite their advanced properties such as high strength, the manufacturing of such fine-grained metals, which exposes them to multi-axial states of stress in operations such as rolling, drawing, extrusion, forging, and bending, has not been possible. In this work, we address this limitation and highlight an important aspect of geometric dynamic recrystallization (GDRX) in nanocrystalline materials as a way to improve manufacturability. In particular, using two different alloy compositions and two different processing techniques, this study points to the fact that if nanocrystalline or near nano metals can be truly stabilized, they can repeatedly undergo the same types of processes that conventional coarse-grained metals endure, increasing both their formability and hence manufacturability. The results reveal that due to a large number of initial high-angle grain boundaries and large applied compressive strains, GDRX is activated in the absence of any discontinuous dynamic recrystallization (DDRX) or continuous dynamic recrystallization (CDRX). This is a unique aspect of stabilized nanocrystalline materials, as traditionally fine-grained metals are not microstructurally stable enough to accommodate such an event.

Exceptional thermal stability of nanostructured FeCoNiCrCu high entropy alloy facilitated by unusual grain boundary segregation

Nanocrystalline metals often suffer from grain coarsening at elevated temperatures, which deteriorates their mechanical properties. One important strategy to retard grain growth is solute segregation at grain boundaries, which can reduce excess Gibbs free energy of grain boundaries and thus impede their motion. The strategy has been widely utilized in binary and ternary alloy systems to stabilize their nanostructures. In present work, we report superior thermal stability of nanostructured FeCoNiCrCu high entropy alloy film prepared by ion beam deposition. In contrast to Cu segregation at grain boundaries when the alloy is fabricated by solidification, the nanostructured FeCoNiCrCu film fabricated via ion beam deposition exhibits segregation of Cr at the boundaries. The segregation of Cr persists at grain boundaries even at high annealing temperature and effectively prevents grain growth. This study indicates that multi-elemental alloys can provide plenty of opportunities to obtain nanostructured alloys with high thermal stability.

Effect of constrained inter-granular regions on the inverse Hall-petch phenomena

The paper makes an effort to lay out a critical assessment of grain size and its role in limiting strength gains in stable nanocrystalline (NC) metals. In particular, this study uses copper-tantalum binary alloys, in which the predominate mechanisms of grain boundary sliding and rotation are shut down, to decipher the breakdown of classical Hall-Petch behavior and its underlining mechanisms. Through varying grain sizes and tantalum concentrations, the results show Cu–Ta's strength exceeds the traditional strength limits anticipated when strictly applying smaller grain size translates to greater strength than that from the Hall-Petch relationship for NC Cu. Within this work, we also highlight the consistent linear behavior holding up to the point of losing full crystallinity given this alloy's ability to constrain failures occurring within other non-stabilized NC systems.

Thermal stability of Ti-Nb-Fe ternary nanocrystalline alloys related to grain boundary segregation

Reducing the grain size of metal materials to nanometer scale is an important method to improve their strength. However, nanocrystalline materials in most cases corresponds to a non-equilibrium state and have a relatively strong tendency to reduce the total grain boundary (GB) area to diminish the energy of the system, which results in significant grain coarsening even at low temperatures. This work is based on the thermodynamic model of the ternary nanocrystalline system, selected TiNbFe alloys as an interesting ternary system and explored its thermodynamic stability. The conspicuous GB segregation tendency of Fe atoms in annealed films is the decisive factor that achieves sufficient thermodynamic stability of nanocrystalline. Ti83.20Nb15.03Fe1.76, Ti79.71Nb15.43Fe4.86, Ti77.28Nb15.88Fe6.85 and Ti75.09Nb15.25Fe9.66 nanocrystalline samples showed excellent thermodynamic stability, which against grain growth up to 400 °C. The calculation of thermodynamic model indicates that Nb solute isn't a GBs segregator in the TiNb binary system, while the Nb element greatly promotes the GBs segregation of Fe atoms in the TiNbFe ternary system. These results enrich the experimental support for solute GB segregation in ternary systems and provide a new strategy for the design of Ti-based nanocrystalline metals with excellent thermal stability.

Complex strengthening mechanisms in nanocrystalline Ni-Mo alloys revealed by a machine-learning interatomic potential

A nanocrystalline metal’s strength increases significantly as its grain size decreases, a phenomenon known as the Hall-Petch relation. Such relation, however, breaks down when the grains become too small. Experimental studies have circumvented this problem in a set of Ni-Mo alloys by stabilizing the grain boundaries (GB). Here, using atomistic simulations with a machine learning-based interatomic potential (ML-IAP), we demonstrate that the inverse Hall-Petch relation can be correctly reproduced due to a change in the dominant deformation mechanism as the grain becomes small in the Ni-Mo polycrystals. It is found that the atomic von Mises strain can be significantly reduced by either solute doping and/or annealing for small-grain-size polycrystals, leading to the increased strength of the polycrystals. On the other hand, for large-grain-size polycrystals, annealing weakens the material due to the large atomic movements in GB. Over a broad range of grain size, the superposition of the solute and annealing effects on polycrystals enhances the strength of those with small grain size more than those with large ones, giving rise to the continuous strengthening at extremely small grain sizes. Overall, this study not only demonstrates the reliability of the ML-IAP, but also provides a comprehensive atomistic view of complex strengthening mechanisms in nanocrystals, opening up a new avenue to tailor their mechanical properties.

Deformation behavior of nanostructured aluminum: Experiment and computational study

Nanocrystalline metals have been processed from powder predecessors in recent times in significant ways, and nowadays, materials are starting to be manufactured which are not only strong but also ductile. Nanocrystalline aluminum (Average grain size 51 nm) was synthesized through high-energy ball milling at the room temperature of microcrystalline powder. The particle size and crystallite sizes were obtained by Williamson Hall and found to be in good correlation with transmission spectroscopy (TEM) data. There was a significant increase in the mechanical properties of nanostructured aluminum in comparison to coarse-grained aluminum. Moreover, a phenomenological model of large-deformation, isotropic, rate-dependent plasticity is developed, which takes into account pressure dependency, plastic dilatation, and non-normal flow. The model has been incorporated into a finite element program. Compression and tension experiments were performed on nanocrystalline aluminum, and the constitutive parameters within the model were estimated from these experiments. The present study shows that the constitutive model successfully simulates the mechanical response of nanocrystalline aluminum with reasonable accuracy using our numerical finite-element capability.

Correlation between stabilizing and strengthening effects due to grain boundary segregation in iron-based alloys: Theoretical models and first-principles calculations

Grain boundary (GB) segregation can drastically affect both the thermal stability and fracture strength of nanocrystalline metals. Underlying physics of the correlation between the above two aspects, however, remains unclear, since a comprehensive study addressing a variety of properties and involving a wide range of solute types seems not available. In this work, we theoretically modeled the GB segregation and strengthening energies considering both the chemical and elastic contributions of segregated solutes. Then using first-principles method, the GB segregation behaviors of twenty-six 3d, 4d and 5d transition metal atoms at three typically symmetrical tilt GBs (∑3(112), ∑5(210) and ∑5(310)), with iron as representative, were studied, where the GB segregation and strengthening energies of solutes were calculated, thus revealing their dependence on GB structures, group number and solute parameters (such as atomic radius and electronegativity). Arising from the competition between the chemical (related to the electronegativity) and the elastic (related to the atomic radius) contributions from solutes, a concave-down (concave-up) parabolic-like dependency of the GB segregation (strengthening) energy on the group number has been discovered. Furthermore, the stabilizing and the strengthening mechanisms of the segregants were revealed by analyses of bond length and charge density. The strengthening energy generally decreases with increasing the GB segregation energy, as reflected by a so-called trade-off relationship between the strengthening effect and the stabilizing effect. This work could be beneficial for understanding the GB segregation and the strengthening behaviors of the segregants in iron-based alloys and designing the iron-based alloys with comprehensively desired performances.

Thermal and Radiation Stability in Nanocrystalline Cu

Nanocrystalline metals have presented intriguing possibilities for use in radiation environments due to their high grain boundary volume, serving as enhanced irradiation-induced defect sinks. Their promise has been lessened due to the propensity for nanocrystalline metals to suffer deleterious grain growth from combinations of irradiation and/or elevated homologous temperature. While approaches for stabilizing such materials against grain growth are the subject of current research, there is still a lack of central knowledge on the irradiation-grain boundary interactions in pure metals despite many studies on the same. Due to the breadth of available reports, we have critically reviewed studies on irradiation and thermal stability in pure, nanocrystalline copper (Cu) as a model FCC material, and on a few dilute Cu-based alloys. Our study has shown that, viewed collectively, there are large differences in interpretation of irradiation-grain boundary interactions, primarily due to a wide range of irradiation environments and variability in materials processing. We discuss the sources of these differences and analyses herein. Then, with the goal of gaining a more overarching mechanistic understanding of grain size stability in pure materials under irradiation, we provide several key recommendations for making meaningful evaluations across materials with different processing and under variable irradiation conditions.

Metal forming and working of stabilized nanocrystalline Cu-Ta for electrical contacts

The commercialization of nanocrystalline metals and alloys is currently entering a renaissance period. Many of the processing and consolidation challenges that have haunted them are now more fully understood, opening the doors for stabilized nanocrystalline metals to be produced on a bulk scale. While challenges remain, the increased volume at which these materials are being supplied is for the first time allowing for investigations into more traditional methods of metal working, such as extruding, rolling, forming, and forging. Recently, the manufacturing science has been developed to allow nanocrystalline Cu-Ta alloys to progress to this point. This article therefore builds upon the last decade of evolutionary progression within the family of stabilized nanocrystalline Cu-Ta alloys by presenting some of the first findings related to scaled powder synthesis, production of billets, thin sheets, and foils. The mechanical performance and physical properties relevant to forming electrical contacts and pins including, tensile, J-integral fracture toughness, Charpy Impact, bi-axial tension, and conductivity are reported. This introductory investigation into forming such a novel material, provides evidence to these alloys potential at bridging the gap between being a scientific curiosity to that of a real engineering material.

Deformation-induced grain rotation and grain boundary formation achieved through dislocation-disclination reactions in polycrystalline hexagonal close-packed metals

Grain boundary mediated plasticity (e.g., grain rotation and grain boundary sliding) plays a critical role in determining the deformation behavior of polycrystals, which is especially important for nanocrystalline metals or plastic deformation at elevated temperatures. However, due to the lack of a theoretical framework, the mechanism of deformation-induced grain rotation has not been well understood. By introducing a disclination-based description, we show that crystal rotation can be captured conveniently through the characteristic rotational vector of a disclination (i.e., the Frank vector). Under the framework of topological defect theory, grain boundary mediated plasticity can be treated as topological reactions between dislocations and disclinations, leading to rigorous predictions of grain/subgrain rotation through a pseudo-inverse solution for the reformulated Frank-Bilby equation. Systematic classifications of grain/subgrain rotation and grain boundary formation are raised for hexagonal close-packed (HCP) metals, which has been validated by Electron-Backscattered Diffraction characterizations in Mg alloys. Our work not only suggests a theoretical approach to investigate grain/subgrain rotation based on the topological correlation between dislocations and disclinations, but also provides a new insight into the plastic behaviors of polycrystalline HCP metals.

Revealing the structure-property relationships of amorphous carbon tribofilms on platinum-gold surfaces

Nanocrystalline metal alloys have shown great promise as electrical contact materials, given their mechanical and tribological properties. In particular, platinum-gold (Pt–Au) nanocrystalline alloys have demonstrated coefficients of friction as low as 0.01 and specific wear rates on the order of 10−9 mm3 N−1 m−1, largely due to the formation of carbon-based tribofilms at the sliding interfaces. In this study, we advance our understanding of the Pt–Au tribofilm structure-property relations and growth mechanisms via high-throughput and high-resolution measurements as a function of Pt–Au composition. As the solute content increased from 0 at. % to 10 at. % Au, cross-sectional and plan-view transmission electron microscopy demonstrated a decrease in average grain size d and an accompanied increase in grain boundary (GB) segregation. The decrease in d and increase in GB solute segregation translated to a decrease in modulus Er and an increase in hardness H as determined via nanoindentation; the Er trend was mainly described using a rule-of-mixtures approximation, whereas the H trend was ascribed to solid solution strengthening and GB stabilization. The steady state-friction μ and wear rate decreased with the addition of Au; low Au-content films showed substrate wear, while high Au-content films showed stable tribofilm growth in both macroscale and nanoscale friction tests. The carbon bonding configuration of the tribofilms was investigated by near-edge X-ray absorption fine structure spectroscopic analyses and found to be similar to that of hydrogenated amorphous carbon films. Altogether, the study provided insight into the mechanistic origins of the tribofilms, thus opening the door to tunable properties ranging from mitigation for electrical contacts to the creation of self-healing films for solid lubricants.

A challenge to the sintering of nanocrystalline metals: Organic burnout and oxide reduction compete with powder consolidation

In nanocrystalline materials atomic transport is enhanced by diffusion along grain boundaries and triple junctions, aggressively accelerating sintering kinetics to the point that the onset of consolidation can precede organic debinding reactions at low temperature. What is more, nanocrystalline metals typically have (or at one time had) more surface area, and thus a greater degree of native oxide that can react with organics upon sintering. In this paper, we explore the sintering behavior of high-energy ball-milled nanocrystalline Ni-10Fe powder, quantitatively evaluating the interference of densification with the process of organic removal. Early sintering leads to trapping of evolved gas and substantial swelling of a nominally sintering compact. Volatilizing process additives remain trapped until very high temperatures, causing microstructural degradation through creep foaming. Finally, in an alloy system with more than one native oxide phase, there are shown to be multiple oxide reduction events that can renew gas evolution to very high temperatures. After quantitatively evaluating these mechanisms in the Ni-10Fe powder specifically, we also generalize their consideration to a range of nanocrystalline metals, to provide guidance for development of powder processing routes for those materials.

Gradient nanostructuring via compositional means

Nanocrystalline metals are inherently unstable against thermal and mechanical stimuli, commonly resulting in significant grain growth. Also, while these metals exhibit substantial Hall-Petch strengthening, they tend to suffer from low ductility and fracture toughness. With regard to the grain growth problem, alloying elements have been employed to stabilize the microstructure through kinetic and/or thermodynamic mechanisms. And to address the ductility challenge, spatially-graded grain size distributions have been developed to facilitate heterogeneous deformation modes: high-strength at the surface and plastic deformation in the bulk. In the present work, we combine these two strategies and present a new methodology for the fabrication of gradient nanostructured metals via compositional means. We have demonstrated that annealing a compositionally stepwise Pt-Au film with a homogenous microstructure results in a film with a spatial microstructural gradient, exhibiting grains which can be twice as wide in the bulk compared to the outer surfaces. Additionally, phase-field modeling was employed for the comparison with experimental results and for further investigation of the competing mechanisms of Au diffusion and thermally induced grain growth. This fabrication method offers an alternative approach for developing the next generation of microstructurally stable gradient nanostructured films.

Discontinuous segregation patterning across disconnections

Twinning is a frequent deformation mechanism in nanocrystalline metals, and segregation of solute atoms at twin boundaries is a thermodynamic process that plays an important role in the stability and strengthening of these materials. In pristine, defect-free twin boundaries, solute segregation generally follows a single- or multilayer patterned coverage of solutes that is uniformly and symmetrically distributed at segregation sites across the boundary. However, when a disconnection, a type of interfacial line defect, is present at the twin boundary, we report a possible discontinuity of the segregation patterns across this defect for a broad range of binary alloys. The change of segregation pattern is explained by a break of the local symmetry across the disconnection terraces. The characteristics of this change are dictated by the orientation of the dislocation content sitting at the step region of the disconnection and its synergistic/antagonistic interactions with the step character. These findings not only advance our understanding of the origin of the interface segregation phenomena and the key contribution from interfacial defects, but they also shed light on applications for tailoring atomically precise interfacial structures to design alloys with emerging properties.

Overcoming strength–ductility trade-off of nanocrystalline metals by engineering grain boundary, texture, and gradient microstructure

The strength–ductility trade-off has been a long-standing dilemma for polycrystalline metals. Though many strategies such as introduction of gradient and twinned microstructure have been proposed to overcome the strength–ductility trade-off of nanocrystalline (NC) metals, a higher strength–ductility synergy is always called for. Here schemes taking advantage of texture engineering with incorporation of grain boundary (GB) strengthening and gradient microstructure design are proposed to achieve a more desirable strength–ductility synergy. Taking into account the size-dependent storage of dislocations in grains, GB sliding, and evolution of the void volume fraction, crystal plasticity finite element simulations incorporating the Gurson-type model are performed to reveal the mechanisms underlying the low ductility and strength–ductility trade-off of NC metals. Low GB strength and nonmonotonic variation of dislocation storage ability with respect to the grain size are revealed as the dominant factors responsible for the low ductility and the strength–ductility trade-off in NC metals. The speculation that there exists a most brittle grain size for NC metals is confirmed. It is also found that the ductility of NC metals is governed by the GB damage and the ability of intragranular dislocation storage at high GB strength. Moreover, it is shown that the failure tensile strain for cube-textured NC copper at a certain grain size could be more than twice the failure tensile strain for untextured one at high GB strength.

Titanium-protein nanocomposites as new biomaterials produced by high-pressure torsion

The development of new biomaterials with outstanding mechanical properties and high biocompatibility has been a significant challenge in the last decades. Nanocrystalline metals have provided new opportunities in producing high-strength biomaterials, but the biocompatibility of these nanometals needs to be improved. In this study, we introduce metal-protein nanocomposites as high-strength biomaterials with superior biocompatibility. Small proportions of bovine serum albumin (2 and 5 vol%), an abundant protein in the mammalian body, are added to titanium, and two nanocomposites are synthesized using a severe plastic deformation process of high-pressure torsion. These new biomaterials show not only a high hardness similar to nanocrystalline pure titanium but also exhibit better biocompatibility (including cellular metabolic activity, cell cycle parameters and DNA fragmentation profile) compared to nano-titanium. These results introduce a pathway to design new biocompatible composites by employing compounds from the human body.

Synthesis of Nanocrystalline Metal Tungstate NiWO4/CoWO4 Heterojunction for UV-Light-Assisted Degradation of Paracetamol

The discharge of pharma products such as paracetamol (PCT) into water has resulted in great harm to humans and emerged as a potential threat requiring a solution. Therefore, the development of smart and efficient materials as photocatalysts has become imperative in order to treat PCT in wastewater. The present study demonstrates the synthesis of pristine NiWO4 and CoWO4 and a heterojunction nanostructure, NiWO4/CoWO4, through a hydrothermal process using a Teflon-lined autoclave at 180 °C for 18 h. Various spectroscopic techniques, such as X-ray diffraction (XRD), Fourier transform infrared (FTIR), ultraviolet–visible (UV–Vis), transmission electron microscopy (TEM), scanning electron microscopy–energy dispersive X-ray (SEM–EDX), and X-ray photoelectron spectroscopy (XPS) were utilised to determine the lattice, structural, optical, and morphological information of the solid nanomaterial upon heterojunction formation. The synthesised nanomaterials were exploited for the photocatalytic degradation of paracetamol (PCT) under UV light irradiation. Photocatalytic experiments were performed for the optimization of various reaction parameters, such as irradiation time, pH, catalyst dose, and PCT concentration at room temperature. The results obtained suggested that the heterojunction nanocomposite NiWO4/CoWO4 exhibited enhanced photocatalytic efficiency (97.42%) with PCT as compared to its precursors—96.50% for NiWO4 and 97.12% for CoWO4. The photocatalytic data were best defined by the Langmuir–Hinshelwood (L–H) model of pseudo-first-order kinetics, with apparent rates constant at 0.015 min−1 for NiWO4, 0.017 min−1 for CoWO4, and 0.019 min−1 for NiWO4/CoWO4 NC. It was observed that NiWO4/CoWO4 NC with enhanced optical properties effected a higher rate of PCT degradation due to the improved bandgap energy upon heterojunction formation. The scavenger test revealed the involvement of •OH radicals as reactive oxidant species (ROS) in PCT degradation. The material was found to be highly stable and reusable for the degradation of PCT at optimized reaction conditions.

Equal channel angular extrusion (ECAE): Die design, affecting variables and development of mechanical behavior and properties of bio-metals

There are numerous practices for producing nanocrystalline (NC) and ultrafine-grained (UFG) metals, composites, polymers and alloys; amongst which Equal Channel Angular Extrusion (ECAE) is one of the renowned techniques in metal forming processes under the severe plastic deformation process (SPD) in which a massive plastic strain is enforced on a bulk material. The novelty of this work is the design and fabrication of a split type of die specifically for carrying out ECAE on difficult-to-deform bio-metals. The die is designed for a channel angle of 120°. The study provides a detailed discussion of the pre and post-extrusion tests, process flowchart, and operational variables. This work offers a valuable opportunity to investigate the implications of the ECAE process for other challenging-to-deform metals. Additionally, the ultrafine-grained materials produced through ECAE have potential applications in industries such as automobile, power, and aerospace.