Bulk Nanocrystalline Materials Research Articles

Shock-induced plastic deformation of nanopowder Ti during consolidation and spallation.

Consolidating nanopowder metals via impact loading is a potentially significant method for synthesizing and processing bulk nanocrystalline materials. However, until now, the microstructural features, plastic deformation during consolidation, and corresponding mechanisms have been seldom revealed. Using molecular dynamics (MD) simulations, we have studied the plastic deformation, densification, spallation, and micro-jetting in nanopowder titanium (np-Ti) during shock. Upon impact, np-Ti undergoes a transition from heterogeneous plasticity, including basal stacking faults (SFs) and {101̄2} twinning, to homogeneous disordering, as the impact velocity increases. Then the nanopowder structure evolves into a bulk nanostructure after the final densification, contributed by pore collapse. The subsequent detwinning arises during the release and tension stage, conducing to a partial structural recovery. When the impact velocity up ≥ 1.0 km s-1, the spallation is following, prompted via GB-sliding and disordering. Upon shock impact, it also facilitates micro-jetting owing to the presence of nanopores, contributing to the pressure gradient and transverse velocity gradient.

Ultrastrong Nanocrystalline Inconel 718 Fabricated by Powder High‐Pressure Torsion and Annealing

In this study, bulk nanocrystalline material is fabricated from Inconel 718 powder through a process of powder high‐pressure torsion (HPT) and subsequent annealing heat treatment. Numerous dislocations generated during the powder HPT process successfully produced bulk Inconel 718 with nanocrystalline grains during the post‐annealing treatment. In addition, the Inconel 718 exhibited additional strengthening contributions such as dislocation strengthening by high pre‐existing dislocation density and precipitation strengthening from randomly distributed precipitates. As a result of these strengthening effects, the Inconel 718 showed ultrastrong yield strength of 1.40 GPa and ultimate tensile strength of 1.53 GPa. Although the elongation of 12.0% did not indicate good ductility, the proposed Inconel 718 boasted improved mechanical properties compared to previous studies of Inconel 718 fabricated by various processing methods. The prepared Inconel 718 and its fabrication technique suggest the potential for manufacturing ultrahigh‐strength bulk nanocrystalline materials not only for Inconel 718 but also for various metallic materials using powder metallurgy.

Scalable synthesis of a bulk nanocrystalline material with a multitude of divergent properties through a traditional manufacturing process

A long-standing endeavor for materials scientists is to develop structural materials with a multitude of extreme properties. Toward this, nanocrystalline materials have enhanced mechanical strength and unique deformation mechanisms that provide potential for various structural applications. However, for the production of structural materials to be more viable, the scalability of nanocrystalline materials to bulk scale utilizing industrially based practices that are reliable and stable is as critical as some of the unique properties of such materials. The work presented here highlights an important step forward in the developmental process of nanocrystalline-based structural materials with a multitude of optimized properties and scalability for various engineering applications. By utilizing common thermo-mechanical processing routes such as hot isostatic pressing (HIP) in combination with open die forging on alloys that form extremely stable nanostructures, a single, fully dense 7.5 lbs ingot of nanocrystalline copper tantalum (Cu-Ta) was produced. Microstructural evolution during processing as well as the comparative mechanical response is reported. This work builds on the tremendous progress that has been made in the field of nanostructured materials in the last two decades to produce volumes of material that can be measured in centimeters and kilograms rather than microns and milligrams.

Surface Nanocrystallization and Numerical Modeling of 316L Stainless Steel during Ultrasonic Shot Peening Process

Surface nanocrystallization of metals and alloys via high-frequency ultrasonic shot peening (USP) can significantly increase the mechanical properties of the materials. However, the relationship between the external process parameters and the internal microstructure of the materials is still unclear and an accurate numerical model to simulate the USP process is urgently required for better control of the grain refinement process. In this study, we successfully realized surface nanocrystallization of 316L stainless steel using USP with an ultrasonic frequency and amplitude of 20 kHz and 50 μm, respectively. The microstructure evaluation of 316L stainless steel during USP was revealed. We established a finite element numerical model to simulate the high-frequency USP process and calculated the plastic strain and stress distribution of 316L stainless steel during the grain refinement process. We investigated the effects of the ultrasonic frequency, working distance, and ultrasonic amplitude on the plastic strain and stress distribution on the materials using the finite element simulation method. The dynamic behavior of the shot during the USP process was studied using a high-speed camera, and the FE simulation results agreed well with the experimental results. We also investigated the impact of multiple shots during the USP process by the high-speed camera observation and FE simulation. Research results indicate that high-frequency USP is an effective method to obtain large-scale bulk nanocrystalline materials and the finite element simulation can help materials scientists and engineers to better understand the relationship between the process parameters and microstructure evaluation of 316L stainless steel.

A comparative study of Nd15Fe78B7 and Nd15Co78B7 systems: phase formations and coercivity mechanisms

Both Nd-Fe-B and Nd-Co-B nanocrystalline magnets exhibit coercivity above 1 T. While the microstructure and mechanism of the coercive force of Nd-Fe-B magnets are well known, the Nd-Co-B system is much less studied, and the microstructure and origin of the high coercive state in this material is not well understood. In this work, we selected Nd15Fe78B7 and Nd15Co78B7 alloys with simple and stable stoichiometries to reveal the similarities and differences between these two systems. For this study, melt-spun nanocrystalline ribbons, hot-compacted magnets and bulk materials crystallized from the amorphous precursor were used. The coercivity of Nd15Fe18B7 alloy is in the range of 0.9–1.7 T, while for Nd15Co18B7 it was of 0.5–1.0 T. Electron microscopy and atom probe tomography analyses indicate that, in contrast to conventional Nd15Fe78B7, where the ferromagnetic Nd2Fe14B phase is the dominant matrix phase and the Nd-rich phase distributes along the grain boundaries providing magnetic decoupling, the Nd15Co18B7 alloys comprise of three randomly redistributed exchange-coupled ferromagnetic phases: Nd2Co14B, NdCo5 and NdCo4B. Single crystals of all constituent phases were grown and their spontaneous magnetization Ms and anisotropy constant K1 were determined. Using these measured intrinsic parameters and real nanostructure of Nd15Fe78B7 and Nd15Co78B7 alloys, micromagnetic simulations were carried out in order to reveal the differences in the coercivity mechanisms in these two distinctly different systems.

Size effects and failure regimes in notched micro-cantilever beam fracture

Fracture tests using notched micro-cantilever (MC) specimens are increasingly being used to measure the fracture toughness of materials at the micro-scale. Detailed finite element analyses (FEAs) of loading of self-similar micro- and bulk cantilever beam fracture specimens using isotropic, elastoplastic constitutive models, are conducted to critically examine the validity of the toughness data obtained using MC specimens. From the simulated normalized load versus load point displacement and the corresponding normalized energy release rate versus load curves, the transition of the failure regimes from crack propagation to plastic collapse of the uncracked notch ligament are identified. While the crack propagation regime allows for the estimation of valid fracture toughness of the probed material, the plastic collapse regime does not. The effects of specimen aspect ratios, material hardening, and yield criteria on the master curves and transition point are examined. A method to interpret the failure regimes, assess the validity and size effects in micro-cantilever fracture tests is proposed. An expression is derived for the minimum size requirements of MC fracture specimens in order to get a valid fracture test. The data reported in the literature from fracture tests on bulk metallic glasses, nano-crystalline materials and ultra-fine grained materials are assessed using the proposed methodology. The present work has important implications for specimen design, interpretation of failure regimes, and potential size effects in MC fracture tests.

Combined scanning probe microscopy and electron microscopy study of microstructure evolution in copper processed by equal channel angular pressing

Abstract We produced submicrocrystalline Cu using the equal channel angular pressing technique with four passes. The as-deformed samples exhibited a high yield strength > 400 MPa and a poor elongation before failure < 20%. The majority of the grain boundaries after equal channel angular pressing were of the low-angle type. A series of heat treatments in the temperature range 220 – 300 °C was performed and partly recrystallized microstructures with different fractions of recrystallized grains were obtained. The microstructures were analyzed by a combination of transmission electron microscopy, orientation image microscopy, light microscopy and scanning probe microscopy. The increase in ductility in partly recrystallized samples was accompanied by a decrease in the yield strength. It was suggested that to obtain a bulk nanocrystalline material which is both strong and ductile the majority of the grain boundaries should be of the high-angle type. It is shown that the scanning probe microscopy technique allows one to determine the microstructure of partly and fully recrystallized samples and to draw qualitative conclusions about the defects density and their distribution in the sample.

A novel route to superhard nanocrystalline cubic boron nitride: Emulsion detonation and high-pressure high-temperature transformation-assisted consolidation

Synthesizing bulk nanocrystalline materials is challenging since grain growth should be suppressed whereas densification promoted. Here, we demonstrate a novel route to synthesize superhard bulk nanocrystalline cubic boron nitride (cBN), which combines the use of emulsion detonation and high-pressure high-temperature transformation-assisted consolidation. The emulsion detonation process activates BN to possess unique structure and chemistry, i.e. wurtzitic BN nanograins in hexagonal BN matrix with enhanced structural disordering and oxygen impurity, a combination that enhances the nucleation rate of cBN and its densification leading to the formation of bulk nanocrystalline cBN at reduced conditions. The cBN, synthesized at 7.5 GPa and 1800 °C, displayed Vickers hardness values of 50−62 GPa for 5−20 N loads. The findings in the study suggest a feasible solution to synthesize bulk nanocrystalline cBN in a more scalable way, while also providing design insights on how to refine grain growth while enhancing densification to synthesize bulk nanocrystalline materials.

High-Pressure Torsion of Non-Equilibrium Hydrogen Storage Materials: A Review

As the most abundant element in the world, hydrogen is a promising energy carrier and has received continuously growing attention in the last couple of decades. At the very moment, hydrogen fuel is imagined as the part of a sustainable and eco-friendly energy system, the “hydrogen grand challenge”. Among the large number of storage solutions, solid-state hydrogen storage is considered to be the safest and most efficient route for on-board applications via fuel cell devices. Notwithstanding the various advantages, storing hydrogen in a lightweight and compact form still presents a barrier towards the wide-spread commercialization of hydrogen technology. In this review paper we summarize the latest findings on solid-state storage solutions of different non-equilibrium systems which have been synthesized by mechanical routes based on severe plastic deformation. Among these deformation techniques, high-pressure torsion is proved to be a proficient method due to the extremely high applied shear strain that develops in bulk nanocrystalline and amorphous materials.

A novel nano Cu/Ti–Si3N4 ceramic substrates fabricated by spark plasma sintering and its bonding mechanism

The nanocrystalline materials have prominent advantages, but the fabrication of nanocrystalline bulk materials, especially nanocrystalline structure parts, is still difficult. In the practical application of nanocrystalline materials, it is very necessary to join them to other structural materials. In this study, a novel nano-Cu/Ti–Si3N4 ceramic substrates is first fabricated using SPS with the addition of Ti transition layers. Microstructural evolution in the bonding zone of nano Cu/Ti–Si3N4 ceramic substrates is investigated to determine the interfacial bonding mechanism, which is based on the microstructures obtained at different holding times. The analysis of microstructural evolution reveals that the diffusion of O, Ti and Cu atoms under the action of pulse current promotes three important interfacial reactions, which in turn promotes the microstructural evolution of the bonding zone. The joint with a two-layer structure, which consist of a Ti2O layer and a Cu layer with dispersed Ti4Cu2O (Ti3Cu3O), exhibits has the best bonding properties.

Thermal processes under explosive loading of amorphous alloy powders

The article deals with the state of production of bulk amorphous and nanocrystalline materials. Conditions for the preservation of the “X-ray” amorphous state in compact materials obtained by explosive pressing of amorphous alloy powders are formulated. The two-front Stefan problem is solved and it is shown that the cooling rate of liquid layers on the contact surfaces of powder particles under explosive loading corresponds to the critical quenching rate of the formation of amorphous structures in metal alloys.

The photocatalytic properties of nanocrystalline materials in different forms

Nanocrystalline materials have strong light absorption ability and photocatalysis performance. Different forms of nanocrystalline materials have different photocatalysis performance levels. Therefore, in this paper the photocatalytic properties of nanocrystalline materials in different forms are studied by experiments. In the experiment, the same materials were used, and three different kinds of samples – block, tube and particle – were made according to different manufacturing processes and processing methods. The results show that the bulk and coarse nanocrystalline materials have the largest specific surface area, the largest value of light absorption efficiency and the strongest photocatalytic ability. The specific surface area of the tubular nanocrystalline materials is moderate, the value of light absorption rate is moderate and the photocatalytic capacity is general. The nanocrystalline materials with particle morphology have the smallest specific surface area, the smallest value of light absorption rate and the weakest photocatalytic ability.

Elastic properties of nanocrystalline materials of hexagonal symmetry: The core-shell model and atomistic estimates

Anisotropic core-shell model of a nano-grained polycrystal is extended to estimate the effective elastic stiffness of several metals of hexagonal crystal lattice symmetry. In the approach the bulk nanocrystalline material is described as a two-phase medium with different properties for a grain boundary zone and a grain core. While the grain core is anisotropic, the boundary zone is isotropic and has a thickness defined by the cutoff radius of a corresponding atomistic potential for the considered metal. The predictions of the proposed mean-field model are verified with respect to simulations performed with the use of the Large-scale Atomic/Molecular Massively Parallel Simulator, the Embedded Atom Model, and the molecular statics method. The effect of the grain size on the overall elastic moduli of nanocrystalline material with random distribution of orientations is analyzed.

Microstructural Changes Influencing the Magnetoresistive Behavior of Bulk Nanocrystalline Materials

Bulk nanocrystalline materials of small and medium ferromagnetic content were produced using severe plastic deformation by high-pressure torsion at room temperature. Giant magnetoresistive behavior was found for as-deformed materials, which was further improved by adjusting the microstructure with thermal treatments. The adequate range of annealing temperatures was assessed with in-situ synchrotron diffraction measurements. Thermally treated Cu–Co materials show larger giant magnetoresistance after annealing for 1 h at 300 °C, while for Cu-Fe this annealing temperature is too high and decreases the magnetoresistive properties. The improvement of magnetoresistivity by thermal treatments is discussed with respect to the microstructural evolution as observed by electron microscopy and ex-situ synchrotron diffraction measurements.

Microstructure and thermoelectric property of (Bi<sub>1–x</sub>Tb<sub>x</sub>)<sub>2</sub>(Te<sub>0.9</sub>Se<sub>0.1</sub>)<sub>3</sub> fabricated by high pressure sintering technique

Nanocrystalline bulk materials n-type (Bi<sub>1–<i>x</i></sub>Tb<i><sub>x</sub></i>)<sub>2</sub>(Te<sub>0.9</sub>Se<sub>0.1</sub>)<sub>3</sub> (<i>x</i> = 0, 0.002, 0.004, 0.008) are fabricated by high pressure sintering (HPS) technique. The HPS samples are then annealed for 36 h in a vacuum at 633 K. The phase compositions and crystal structure of HPS sample are analyzed by X-ray diffraction. The microscopic morphology of HPS sample is observed by field-emission scanning electron microscopy. The electric conductivity, Seebeck coefficient, and thermal conductivity of the HPS sample and annealed sample are measured in a temperature range from room temperature to 473 K. The effects of Tb content on crystal structure and thermoelectric properties of the sample are systematically studied. The results show that HPS sample consists of nanoparticles. With the increase of content of Tb, the cell volume increases. Besides, the power factor increases but thermal conductivity decreases through doping Tb, thus the optimal figure of merit (<i>ZT</i>) value increases. The Tb doping amount of <i>x</i> = 0.004 is an optimal doping amount. At this doping amount, the maximum <i>ZT</i> of 0.29 is achieved, which is enhanced by 32% compared with the <i>ZT</i> value of undoped sample. The thermoelectric performance can be improved significantly by annealing. The thermal conductivity of the annealed sample with <i>x</i> = 0.004 is 0.9 W·m<sup>–1</sup>·K<sup>–1</sup> at 373 K, decreased by 23% compared with the thermal conductivity of HPS sample. Consequently, the <i>ZT</i> value of annealed sample is significantly higher than that of HPS sample. The maximum thermoelectric <i>ZT</i> of 0.99 is achieved for annealed sample with <i>x</i> =0.004 at 373 K. Furthermore, it is worthwhile to note that this annealed sample possesses a <i>ZT</i> value larger than 0.8 when the temperature is higher than 323 K.

Atomic resolution analyses on defects in nanocrystalline Cu-based alloys generated by severe plastic deformation

Bulk nanocrystalline (NC) materials possess high volume fraction (up to 50%) of grain boundaries (GBs), which contributes significantly to their mechanical and physical properties. Nevertheless, high density of dislocations at GBs and the associated strain fields complicate the analyses on the fine GBs structures of NC materials. Here, taking NC Cu-based alloys as model systems, the atomic-scale configurations of dislocations and typical GBs triggered by severe plastic deformation are well investigated using aberration-corrected high-resolution transmission electron microscopy and geometric phase analysis. A periodic array of extended dislocations with a spacing of 2.2 nm is found to form low-angle GBs (LAGBs) in NC Cu-based alloys. The interaction between dislocations and twins play a crucial role in forming the GBs of NC. Twin boundaries (TBs) in NC Cu-based alloys can serve as barriers for trapping dislocations, which in turn tune the TBs directly evolving into LAGBs or high-angle GBs. In addition, a symmetric 6° [001] tilt LAGB is also analyzed to be composed of a/2〈100〉 edge dislocations, which are activated by the a/2〈110〉 full dislocation dissociation or simple shear deformation of {100} planes. Our study offers insights into the atomic resolution structures of dislocations and GBs in NC materials.

An atomistic simulation study of nanoscale sintering: The role of grain boundary misorientation

Sintering is a processing technique used to produce bulk materials from powder compacts. Recently, sintering has been the subject of active research for its relevance to a wide range of applications, such as additive manufacturing and fabrication of bulk nanocrystalline materials. Of particular interest is the role of grain boundaries (GBs) on sintering mechanisms, cooperative mass transport, and pore shrinkage rates. Herein, atomistic simulations are leveraged to investigate sintering kinetics and densification rates of nanoscale particles as a function of GB misorientation. The two-particle geometry is used to examine particle neck growth rates and crystallographic re-orientation events, and report relative GB diffusion rates as a function of GB misorientation. For the three-particle configuration, simulation results reveal a plethora of pore shrinkage profiles ranging from complete shrinkage to stagnant response depending on the GBs present in the system. This is the first atomistic study that systematically examines the role of GB misorientation on pore shrinkage rates. Our results highlight the need to revisit continuum sintering treatments in order to account for the anisotropy in GB properties.

The factors leading to abnormal grain growth during sintering of hard alloy

This work investigated the sintering of the nanocrystalline tungsten carbide and cobalt composite powder. Vacuum sintering and spark plasma sintering (SPS) were used to consolidate powders into bulk nanocrystalline materials. In addition, two methods of the cobalt introduction were developed for making WC/Co compositions. The first one uses mechanical mixing in ball milling machine. The second one is based on a precipitation and reduction of the cobalt salt on the WC particle surface. The oxygen was always present in the produced powders. Therefore, a control of common carbon is necessary to produce bulk materials with correct chemistry. When vacuum sintering or SPS process are used for consolidation, the addition of grain growth inhibitor is necessary to minimize grain growth. Sintering hard alloys using the nanocrystalline WC-Co composite powders has unique properties and potential for commercial applications.

An Insight into Machining of Thermally Stable Bulk Nanocrystalline Metals

Recent advances have enabled the production of nano‐grained metallic alloys with a thermally stable microstructure. Consequently, the production of bulk samples and subsequent machining and testing of standardized test specimens is now a real possibility. Therefore, for the first time, the authors report on the in‐depth characterization and machinability of bulk nanocrystalline materials. In particular, this study addresses the feasibility of machining and to what extent, if any, the microstructure is altered due to the high stresses and temperature incurred during machining. Toward that goal, a series of copper–tantalum nanocrystalline threaded cylindrical tensile samples are machined from extruded rods. Advanced characterization techniques, such as transmission electron microscopy, are employed which indicated that the grain size of the Cu–Ta alloy was further reduced by approximately one‐third. This reduction in grain size is quite noteworthy given an estimated total strain of 260% and moderate temperature increase resulting from the machining operation. This unexpected grain refinement is attributed to tantalum‐based nanoclusters dispersed through the matrix which limit grain growth in the initial microstructure during machining. Overall, the authors observe a continuous chip formation and machinability of bulk nanocrystalline materials, which stems from stable nano‐grains and having a near elastically perfectly plastic material behavior.

Precipitation-Strengthened and Microlayered Bulk Copper- and Molybdenum-Based Nanocrystalline Materials Produced by High-Speed Electron-Beam Evaporation–Condensation in Vacuum: Structure and Phase Composition

The phase composition and structurization of three types of copper and molybdenum composite materials, 0.8 to 5 mm thick, which were condensed from the vapor phase at substrate temperatures 700 and 900°C, are considered: precipitation-strengthened composites, microlayered composites with alternating copper and molybdenum layers 1 to 10 μm thick, and bulk nanocrystalline composites with alternating layers thinner than 0.5 μm. Standard precipitation-strengthened Cu- and Mo-based materials condensed from the vapor phase at substrate temperatures 700–900°C can be produced over a relatively narrow composition range of the strengthening phase (0.1–3 wt.% Mo). When Mo content is 3–5 wt.%, the molybdenum particles change their shape from round to acicular and become discontinuous chains oriented perpendicularly to the vapor flow. If there is more than 5 wt.% of the second phase, the condensed composite materials (CCMs) show a layered structure. The layered structure can be observed in other CCM types (Cu–W, Cu–Cr, NiCrAlTi–Al2O3). Layered copper and molybdenum CCMs, 6 mm thick, produced on a rotating substrate heated to 700 ± 30°C have been experimentally confirmed to belong to bulk nanocrystalline materials.