Nanocrystalline Aluminum Research Articles

Atomic-scale insights into the effect of layer thickness ratio on the tensile properties of heterostructured nano-aluminum

Gradient-structured alloys with lamellar microstructures show extraordinary potential in breaking the strength-ductility trade-off dilemma. When partial grain size exceeds the Hall-Petch limit, the deformation mechanism is not fully understood. In this study, the competing relationship between grain boundary (GB) softening and hetero-deformation-induced (HDI) strengthening in laminated nano-grained aluminum was investigated under uniaxial tensile loading. In the soft domains, the high activity of grain boundaries results in pronounced GB softening. However, when the volume fraction of the soft layer exceeds 50 %, the flow stress surpasses the predicted values of the rule of mixtures (ROM), demonstrating a significant HDI strengthening. A detailed analysis of microstructural evolution is conducted, including strain gradients, GB migration, and dislocation distribution. The effects of mechanical incompatibility between soft and hard layers on the distribution of plastic strain and overall material strength are elucidated in gradient structures. The study provides critical theoretical insights into the design and development of high-performance gradient nanocrystalline aluminum alloys, particularly in applications where a balance between strength and ductility is of paramount importance.

Smart manufacturing approach to manufacture bulk nanocrystalline aluminum for lightweight applications

Smart manufacturing approach to manufacture bulk nanocrystalline aluminum for lightweight applications

Effect of alumina source on the retention of rhenium during low-activity waste feed conversion to glass

Volatility of technetium poses a challenge during the vitrification of low-activity waste (LAW) feed to glass. Although this is typically resolved through recycle loops, this can lead to other problems, such as sulfate phase formation. Thus, ensuring high single-pass retention of Tc (or Re, its non-radioactive surrogate) is required for enabling high waste-loading formulations. In our previous study, we observed significantly higher Re retention in final waste glass produced from a LAW melter feed containing gibbsite as the alumina source compared to a compositionally similar feed containing kyanite. To investigate this effect, we prepared representative LAW melter feeds with kyanite and gibbsite as alumina sources and measured the Re retention in the produced glasses. We found that the Re retention is consistently higher in glasses produced with gibbsite, by up to 20 %. We attribute this result to the formation of nanocrystalline alumina in melter feeds containing gibbsite. Possessing a high surface area, nanocrystalline alumina can adsorb the perrhenate-containing molten salt, increasing the rate at which Re is incorporated into the alkali-alumino-boro-silicate glass-forming melt. In addition, we demonstrate that replacing kyanite with gibbsite in LAW melter feeds has no adverse effects on their processing during vitrification.

Two-phase model for inverse Hall–Petch effect in nanocrystalline thin film: Atomistic simulation study

This work presents two methods to improve the understanding of the mechanical behavior of nanocrystalline thin films. Firstly, a simple two-phase model is proposed to explain the inverse Hall–Petch effect. The model suggests that the nanocrystalline material consists of the grain-boundary and the grain interior with different mechanical properties. Secondly, a computational method is developed to create more realistic grain boundaries in simulations of polycrystals. Traditionally created grain boundaries by Voronoi tessellation are too dense and do not accurately reflect the reality and the experimental results. The strength of the nanocrystalline aluminum thin film is simulated using molecular dynamics. The grain size dependence of the elastic modulus, ultimate tensile stress, and engineering yield strength is demonstrated. The experimental values of modulus and strength are approximately 6 times smaller probably due to the presence of porosity in the real sample. An approach is developed to introduce porosity in the simulated samples at the grain boundaries and in the grain interior to reduce this discrepancy. The modeled value of modulus for a grain size of 40 nm without porosity is 67GPa, whereas, with 50% grain-boundary porosity and 20% intra-granular porosity it is 30GPa. For the ultimate tensile strength, we get 3GPa and 1.4GPa for samples without porosity and with porosity, respectively. The simulated values of modulus with porosity are still 4 times higher and the values of strength are about 2 times higher than the experimental ones. The amount of porosity in our method can be adjusted to fit the experimental values; however, high values of porosity cause the mechanical instability of the simulated samples.

Grain boundary-mediated plasticity in aluminum films unraveled by a statistical approach combining nano-DIC and ACOM-TEM

Nanomechanical on-chip testing is combined with nanoscale in situ digital image correlation and automated crystal orientation mapping in TEM to deliver novel statistically representative quantitative data about the deformation mechanisms in nanocrystalline aluminum films. The films are very ductile, with a rare stable multiple necking process with local strains reaching up to 0.45 and macroscopic elongation up to 0.17. The strain fields with resolution below 100 nm are related to the underlying microstructure and crystallographic orientation maps. This reveals nanoscopic shear bands forming preferentially along GB with high misorientations, tilted at +/− 45° with respect to loading direction. The analysis of these data prove that the strong strain delocalization process is promoted by GB migration and grain rotation, leading to large strain rate sensitivity. The distribution of misorientation angles between grains evolve during deformation. The GBs with misorientation between 20° and 40°, which are the GBs with highest energy, involve the largest strains.

Exploring the Influence of Nanocrystalline Structure and Aluminum Content on High-Temperature Oxidation Behavior of Fe-Cr-Al Alloys.

The present study examines the high-temperature (500-800 °C) oxidation behavior of Fe-10Cr-(3,5) Al alloys and studies the effect of nanocrystalline structure and Al content on their resistance to oxidation. The nanocrystalline (NC) alloy powder was synthesized via planetary ball milling. The prepared NC alloy powder was consolidated using spark plasma sintering to form NC alloys. Subsequently, an annealing of the NC alloys was performed to transform them into microcrystalline (MC) alloys. It was observed that the NC alloys exhibit superior resistance to oxidation compared to their MC counterparts at high temperatures. The superior resistance to oxidation of the NC alloys is attributed to their considerably finer grain size, which enhances the diffusion of those elements to the metal-oxide interface that forms the protective oxide layer. Conversely, the coarser grain size in MC alloys limits the diffusion of the oxide-forming components. Furthermore, the Fe-10Cr-5Al alloy showed greater resistance to oxidation than the Fe-10Cr-3Al alloy.

Synergy of Spall Strength and Toughness in Nanograined Metals.

Under shock loading, the spall strength of nanocrystals exhibits intricate grain-size effects due to the presence of abundant grain boundary and dislocation activities. However, the influence of size on spall toughness and void evolution has been largely overlooked. This study employs molecular dynamics simulations to investigate the damage accumulation characteristics of nanocrystalline aluminum across various grain sizes. Unlike the trade-off observed in quasi-static loading conditions, our study reveals a consistency in which grain size governs both nanovoid nucleation and coalescence, yielding a novel spall strength-toughness synergy. These insights highlight grain sizes that are particularly susceptible to spall fracture, offering a crucial understanding of nanocrystal failure mechanisms in extreme environments.

Thermal conductivity of nanocrystalline alumina films fabricated by aerosol deposition

Films fabricated by aerosol deposition (AD) are expected to be used as electrical insulating coatings. High thermal conductivity is required in addition to electrical insulation properties for electrical insulation substrate applications, but the thermal conductivity of AD films has not been reported. AD films are nanocrystalline films consisting of crystals several tens of nanometers in size, and it is unclear what effect the small particle size has on thermal conductivity. In this study, the thermal conductivity of an AD alumina film was measured by the light flash method as 15–31 W m−1 K−1 at room temperature and was large despite the particle size of several tens of nanometers. Analysis was performed using a model of grain size effects on thermal conductivity.

Molecular dynamics simulations of tensile and creep-ratcheting behaviour of CNT reinforced columnar nanocrystalline Al nanocomposites

High-temperature cyclic loading is a major factor affecting the life of a structural component. In this study, the tensile and creep-ratcheting behaviour of three volume fractions of CNT-reinforced columnar nanocrystalline aluminium has been studied using hybrid potentials (EAM, AIREBO and LJ potentials) at two different stress ratios (R = 0.4, 0.6) and three different temperatures (T = 300 K, 467 K and 653 K). The aim is to determine the stress-strain curve, strain-time curve, overall dislocation density, dislocation types, and mainly the underlying deformation mechanisms of CNT-based NC Al matrix nanocomposites. Increasing the CNT volume fraction in the matrix has shown improved UTS values, enhanced dislocation annihilation rate, dissipation of the hysteresis loop energy, lower strain accumulation and higher time to fracture. The specimen fails quickly at higher temperatures which is depicted by the widening of the hysteresis loop. The predominant deformation mechanisms like grain boundary diffusion, widening, merging, sliding and rotations have been observed. Grain boundary diffusion drives the creep-ratcheting deformation behaviour under three temperatures and the misalignment has been noticed at the interaction of CNT-NC Al nanocomposite during the creep-ratcheting deformation at high temperature. The Shockley-partial and perfect dislocations have been determined to control the underlying deformation mechanism. The current work can serve as a keystone in the creep-ratcheting analysis of structural materials employed in high-temperature loading environments.

A multiscale FEM-MD coupling method for investigation into atomistic-scale deformation mechanisms of nanocrystalline metals under continuum-scale deformation

This study aims to develop a multiscale bridging method for investigating nanocrystalline metals based on macro-scale deformation. For this purpose, we propose a hierarchical multiscale computational method that can focus on some of the elements in a finite element model for scale bridging to atomistic-scale models. This method assumes that atomistic-scale nanocrystalline models are related to the integration points in a finite element and deform based on the macro-scale deformation. Nanocrystalline aluminum was chosen for the validation of the multiscale method. The finite element method (FEM) and the molecular dynamics (MD) method were used for continuum-scale and atomistic-scale simulations, respectively. We utilized the notion of the CauchyBorn rule (CBR) for communicating deformation information from the continuum scale to the atomistic scale. We studied three different cases with two nanocrystalline models and two loading cases to compare differences resulting from crystal structures and loading. Based on the crystal structure change during relaxation, nonequilibrium grain boundaries (NEGBs) were shown to play a role as deformation mechanisms in the plastic regime and induce the onset and migration of crystal defects, including deformation twins, as reported in the experiment. Furthermore, the crystal orientation dependence of the onset of crystal defects was confirmed by the comparison of the results from the two different nanocrystalline models. The qualitative agreement of the results with experimental observations is also confirmed. The proposed ‘FEM-MD’ method can bridge a large-scale gap, for example, from a nano-scale to a continuum-scale such that an MD model can be coupled to a millimeter or centimeter scale compared to other embedding methods. The present method is ideal for investigating the dislocation behavior of nanocrystalline materials, which contain multi-grained nanostructure at finite temperature, undergoing various loading scenarios at the macro-scale.

Enhancement in Mechanical Properties of Bulk Nanocrystalline Aluminum by Grain Boundary Strengthening Mechanism

Enhancement in Mechanical Properties of Bulk Nanocrystalline Aluminum by Grain Boundary Strengthening Mechanism

Enhanced properties of nanocrystalline alumina ceramic with compressive prestress and coherently aligned nanograins

AbstractPrestress strategy is effective to inhibit crack advancement and toughen structural materials such as concrete and glass. However, it is difficult to be utilized in ceramic because of the solid‐state processing. In this work, nanocrystalline alumina ceramic with high compressive prestress and coherently aligned nanograins were successfully prepared by the ultra‐high pressure sintering technology. The as‐prepared ceramics exhibited remarkably enhanced mechanical properties, including hardness of 25.9 GPa and fracture toughness of 3.2 MPa·m1/2, which were respectively 36% and 51% higher than corresponding parameters of sapphire. Ultra‐high pressure generated compressive prestress as large as several hundred megapascal, and coherent nanograins in the dimensions of 10–15 nm, which markedly contributed to the simultaneously enhancement in toughness and hardness for ceramic materials. The prestressing strategy provides a new opportunity for fabricating advanced ceramics with high failure resistance.

Gallium Nanoparticle Formation and Doping of Nanocrystalline Alumina from a Ga–Al Liquid Metal Hydrogen Generating Reaction

Gallium Nanoparticle Formation and Doping of Nanocrystalline Alumina from a Ga–Al Liquid Metal Hydrogen Generating Reaction

Molecular Dynamics Simulation on Shocked Nanocrystalline Aluminum

The characteristics of shocked nanocrystalline aluminum are investigated by using molecular dynamics method based on the embedded atom method potential function. The result presents the particle velocity profile and the width of shock front in detail. The simulated Hugoniot relations are basically consistent with the experimental data and other molecular dynamics results. The width of shock front decreases with the particle velocity exponentially.

Development of high-performance nanostructured aluminum and its constitutive modeling

A new technique, an in-situ hot-extrusion-based synthesizing process, is proposed to develop high-performance nanocrystalline aluminum (nc-Al) with an optimally tuned strength-to-ductility ratio suitable for various technologically relevant applications. Comprehensive investigations are conducted by characterizing mechanical and microstructural properties to realize the influence of various synthesizing variables on the properties of the bulk nc-Al. Furthermore, a continuum-scale constitutive modeling approach is proposed based on dominant microstructural mechanisms of plastic deformation and implemented into a finite element solver using a user-defined material interface. It is shown that the proposed theory can provide a versatile platform to predict the nanocrystalline aluminum mechanical response quite well.

TEM Study of a Layered Composite Structure Produced by Ion-Plasma Treatment of Aluminum Coating on the Ti-6Al-4V Alloy

Using the methods of transmission electron microscopy and energy-dispersive spectroscopy, we study the microstructure and phase composition of the coating and modified intermetallic layers obtained in a Ti-6Al-4V alloy by the deposition of the Al coating and subsequent processing in low-pressure non-self-sustained arc discharge plasma (CIPT—complex ion-plasma treatment). The deposition of the aluminum coating on the Ti-6Al-4V alloy is accompanied by the formation of a layered and a gradient microstructure: nanocrystalline near the “coating/substrate” interface and ultrafine-grained in the outer part of the aluminum coating, with α-stabilized region of ≈5 µm thick in the surface layer in base titanium alloy. After the CIPT, the coating and the surface of the base titanium alloy have a layered morphology: each of the layers possesses different grain structure and composition. In the direction from the outer surface of the specimen to the base material, the following phase sequence has been confirmed by diffraction and elemental analysis: TiAl3 → TiAl3 + nc-(Al(Ti) + α-Ti) → nc-(Al(Ti) + α-Ti) → TiAl3 → TiAl3 + TiAl → TiAl → Ti3Al → α-Ti alloy → (α + β)-Ti alloy. The nanocrystalline aluminum layer, which has been formed during the deposition of the aluminum coating, does not undergo phase transformation and recrystallization under the CIPT. Nanocrystalline structure can favor the interdiffusion of the elements between the coating and base material, and stimulate phase transformation in coarser grains situated under and over it.

Effect of Copper Segregation at Low-Angle Grain Boundaries on the Mechanisms of Plastic Relaxation in Nanocrystalline Aluminum: An Atomistic Study.

The paper studies the mechanisms of plastic relaxation and mechanical response depending on the concentration of Cu atoms at grain boundaries (GBs) in nanocrystalline aluminum with molecular dynamics simulations. A nonmonotonic dependence of the critical resolved shear stress on the Cu content at GBs is shown. This nonmonotonic dependence is related to the change in plastic relaxation mechanisms at GBs. At a low Cu content, GBs slip as dislocation walls, whereas an increase in Cu content involves a dislocation emission from GBs and grain rotation with GB sliding.

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.

Effect of Magnesium Dopant on the Grain Boundary Stability of Nanocrystalline Aluminum Powders during Cryomilling

In this investigation, pure aluminum (Al) powders were cryomilled with and without magnesium dopants to study (a) the effect of cryomilling time on the crystallite size and (b) the effect of magnesium dopant on Al to achieve grain boundary stability. The cryomilling process was carried out using liquid nitrogen for different durations. The characterization of the cryomilled powders was carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) to understand the particle morphology, crystallite size, and elemental composition. The results demonstrated that the size of the crystallites in both Al and Mg-doped Al powders reduces as the cryomilling duration increases. The results also indicated that the preferential segregation of Mg dopant at the grain boundaries of Al provides stability to the cryomilled powders at elevated temperatures. This article discusses the mechanism for the changes in crystallite size and the effect of the Mg dopant on the grain boundary stability in Al powders.

Molecular dynamics investigation of loading orientation effect on dynamic behaviors of void in aluminum

The micro-damage mechanisms in ductile metals play a very important role in the spallation process. Nonequilibrium molecular dynamics (NEMD) is used to examine the effect of loading orientation on void evolution in single crystal (SC) and nanocrystalline (NC) aluminum under isotropic tensions. Besides void nucleation, growth, and coalescence, a brand-new phenomenon, void collapse, is also discovered in the late growth stage, and the collapse behavior is confirmed by the atomic trajectory tracing approach (ATTA). The loading orientation has a significant impact on the maximum void number, maximum void surface area, and void collapse velocity in the SC model but not in the NC model; while the void volume and its fraction that can capture the void collapse behavior are unaffected by the loading direction in the SC and NC models. In the SC model, the loading orientations [−110][001][110] and [100][010][001] make void collapse hardest and simplest. Meanwhile, with the help of the slicing method, it is discovered that the damage mechanism in the NC model is dominated by intracrystalline, intercrystalline, and transcrystalline fractures, with the second being the directional fracture along the grain boundary, whereas it belongs to the intracrystalline fracture in the SC model. In addition, the material experiences the hardening and softening phases during the nucleation stage, which are distinguished by the maximal tensile stress.