Nanocrystalline Aluminum Research Articles

In Situ Microstructural Control and Mechanical Testing Inside the Transmission Electron Microscope at Elevated Temperatures

With atomic-scale imaging and analytical capabilities such as electron diffraction and energy-loss spectroscopy, the transmission electron microscope has allowed access to the internal microstructure of materials like no other microscopy. It has been mostly a passive or post-mortem analysis tool, but that trend is changing with in situ straining, heating and electrical biasing. In this study, we design and demonstrate a multi-functional microchip that integrates actuators, sensors, heaters and electrodes with freestanding electron transparent specimens. In addition to mechanical testing at elevated temperatures, the chip can actively control microstructures (grain growth and phase change) of the specimen material. Using nano-crystalline aluminum, nickel and zirconium as specimen materials, we demonstrate these novel capabilities inside the microscope. Our approach of active microstructural control and quantitative testing with real-time visualization can influence mechanistic modeling by providing direct and accurate evidence of the fundamental mechanisms behind materials behavior.

Electrochemical Corrosion of Bulk Nanocrystalline Aluminum in Acidic Sodium Sulfate Solutions at Room Temperature

The corrosion properties of metallic materials are usually associated with the compactness of the oxide film and electrochemical parameters, such as corrosion potential, corrosion current density, pitting potential, etc. The valence electron configurations of metallic materials can also affect their corrosion properties because the nature of their corrosion processes involves the exchange and transportation of valence electrons between metallic atoms and ions. In the present work, the electrochemical corrosion of bulk nanocrystalline aluminum (BN-Al) produced by severe rolling technique and its conventional polycrystalline aluminum (CP-Al) counterpart in 0.01, 0.05, and 0.1 M HCl + 0.25 M Na2SO4 solutions at room temperature was studied by potentiodynamic polarization, electrochemical impedance spectroscopy, and scanning electron microscopy. The uniform and pitting corrosion resistances of BN-Al were simultaneously enhanced in comparison with those of CP-Al. The ion adsorption in oxide films along depth p...

A high surface area nanocrystalline alumina with tailoring texture by mixed template

We report a facile synthesis of porous alumina (PA) which enables control of pore structure using a mixture of fatty alcohol polyoxyethylene ether (AEO-7) and poly (alkylene oxide) triblock copolymers (P123) as the template via an evaporation-induced self-assembly (EISA) pathway associated with thermal treatment. N2 adsorption–desorption results show that using over 80wt% AEO-7, the obtained materials are super-microporous (pore size in the range of 1–2nm), then if further decrease in the AEO-7 content will transform to mesopore (pore size >2nm). When the mass fraction of P123 is less than 20wt%, the material has a large surface area (more than 650m2/g) and nanocrystalline phase is obtained at 400°C. Even after calcination at 750°C for 1h, the sample of mass fraction 20wt% P123 still exhibits a high specific area of 450m2/g with large microporosity.

Aluminum Matrix Composites Strengthened with CuZrAgAl Amorphous Atomized Powder Particles

The Al-matrix composites were prepared by hot pressing in vacuum of an aluminum powder with 20 and 40 wt.% addition of the amorphous Cu43Zr43Ag7Al7 alloy (numbers indicate at.%) obtained using gas atomization method. The amorphous structure of the powder was confirmed using x-ray diffraction, DSC, and TEM. The average size of mostly spherical particles was 100 μm, so the powder was sieved to obtain maximum size of 60 μm. The composites were prepared using uniaxial cold pressing in vacuum and at a temperature of 400 °C. The composites of hardness from 43 to 53 HV were obtained for both additions of the amorphous phase. They reached compression strength of 150 MPa for 20% of amorphous phase and 250 MPa for the higher content. The modest hardening effect was caused by crack initiation at Al/amorphous interfaces. The amorphous phase was only partially crystallized in the hot-pressed composites, what did not cause hardness decrease. The application of nanocrystalline aluminum powders obtained by high-energy ball milling for the matrix of composites allowed obtaining nanocrystalline aluminum matrix composites of size near 150 nm, strengthened with the amorphous powders, whose compression strength was near 550 MPa for the composite containing 40% of the amorphous phase and slightly lower for the composite containing 20% of the phase. They showed much higher ductility of 23% in comparison with 7% for the composite containing 40% amorphous phase. The distribution of the strengthening phase in the nanocrystalline matrix was not homogeneous; the amorphous particles formed bands, where majority of cracks nucleated during compression test.

Mechanical strain mediated carrier scattering and its role in charge and thermal transport in freestanding nanocrystalline aluminum thin films

In bulk metals, mechanical strain is known not to influence electrical and thermal transport. However, fundamentally different deformation mechanisms and strain localization at the grain boundaries may influence electron or phonon scattering in nanocrystalline materials. To investigate this hypothesis, the authors developed an experimental approach, where the authors performed thermal and electrical conductivity measurements on 100 nm thick freestanding nanocrystalline aluminum films with average grain size of 50 nm in situ inside a transmission electron microscope (TEM). The authors present experimental evidence of decrease in thermal conductivity and increase in electrical resistivity as a function of uniaxial tensile strain. In-situ TEM observations suggest that grain rotation induced by grain boundary diffusion, and not dislocation-based plasticity, is the dominant deformation mechanism in these thin films. The authors propose that diffusion causes rise in oxygen concentration resulting in increased defects at grain boundaries. Presence of oxygen only at the grain boundaries is confirmed by energy dispersive spectroscopy. Increased defect concentration by mechanical strain at grain boundary causes the change in thermal and charge transport.

Molecular dynamics studies of the roles of microstructure and thermal effects in spallation of aluminum

Spallation behaviors of nanocrystalline aluminum under shock loading are studied by nonequilibrium molecular dynamics simulations. Simulations on single-crystal aluminum are conducted for comparison. Both classical spallation and micro-spallation are studied. Our simulations show that the shock front structure of the nanocrystalline aluminum sample apparently displays two stages plasticity –– the grain boundary mediated stage and the dislocation mediated stage. The spallation mechanism is dominated by cavitation, i.e., nucleation, growth, and coalescence of voids. It is found that void nucleation mechanism is different in single-crystal aluminum and nanocrystalline aluminum. Void nucleation is induced by dislocation activities in single-crystal aluminum, while is induced by mechanical separation and sliding of grain boundaries in nanocrystalline aluminum. Thermal dissipation during cavitation is studied, and the mutual promotion between cavitation and melting is discovered. Thermal dissipation during cavitation leads to temperature arising in the vicinity of voids and promotes melting around voids. On the other hand, melting of materials leads to dropping of spall strength and thus facilitates the cavitation process. Some quantitative discussions on spall strength and thermal dissipation rate are proposed. The spall strength of single-crystal aluminum first increases and then decreases as shock intensity increases, which is attributed to the competition mechanism between strain rate hardening and temperature softening. Grain boundary effect on spall strength is more remarkable in cases of lower shock intensity. The thermal dissipation rate in void nucleation stage is much larger than in void growth–coalescence stage.

Formation mechanism of bulk nanocrystalline aluminium with multiply twinned grains by liquid quenching: A molecular dynamics simulation study

The formation mechanism of bulk nanocrystalline aluminium with multiply twinned grains has been investigated by a large-scale molecular dynamics simulation. The results show that bulk nanocrystalline aluminium can be obtained directly by quenching liquid at an appropriate cooling rate window. Most nanograins do not merge with each other in the coarsening stage due to the fast cooling rate, but are separated by high-angle grain boundaries. The nanograins exhibit a narrow grain-size range with an average diameter of 6.4nm and random crystallographic orientations. These microstructure features are consistent with some of the bulk nanocrystalline alloys prepared by melt casting in experiments. The nanograins display various twinned morphologies, and they can be described by three twin elements of parallel, cross and fivefold twinning in varying amounts. The multiply twinned nanograins come from the successive formation of twin HCP planes during the layer-by-layer growth of FCC stacking blocks, and the growth kinetics plays a critical role in the grain morphology. This formation mechanism is different from that of fivefold deformation and annealing twins in the nanocrystalline metals.

Fracture toughness testing of nanocrystalline alumina and fused quartz using chevron-notched microbeams

We show that chevron-notched samples offer an attractive approach to the measurement of fracture toughness in micron-scale samples of brittle materials and use the method to characterize quartz and nanocrystalline alumina. Focused ion beam milling is used to carve bend bars of rectangular cross-section a few micrometres wide and containing a notch with a triangular ligament. Load-controlled testing is conducted using a nanoindentation apparatus. If the notch is appropriately machined, cracks nucleate and propagate in a stable fashion before becoming unstable. Sample dimensions are measured using a scanning electron microscope, and are used as input in finite element simulations of the bars’ elastic deformation for various crack lengths. The calculated compliance calibration curve and the measured peak load then give the local fracture toughness of the material. Advantages of the method include a low sensitivity to environmental subcritical crack growth, and the fact that it measures toughness at the tip of a sharp crack situated in material unaffected by ion-milling. The approach is demonstrated on two materials, namely, monolithic fused quartz and nanocrystalline alumina Nextel™ 610 fibres; results for the latter give the intrinsic grain boundary toughness of alumina, free of grain bridging effects.

Microstructure and tensile properties of bulk nanostructured aluminum/graphene composites prepared via cryomilling

In order to develop high strength metal–matrix composites with acceptable ductility, bulk nanostructured aluminum–matrix composites reinforced with graphene nanoflakes were fabricated by cryomilling and hot extrusion processes. Microstructure and mechanical properties were characterized and determined using transmission electron microscopy, electron dispersion spectroscopy, as well as static tensile tests. The results show that, with an addition of only 0.5wt% graphene nanoflakes, the bulk nanostructured aluminum/graphene composite exhibited increased strength and unsubdued ductility over pure aluminum. Besides, the mechanical properties of the composites with higher content of graphene nanoflakes were also measured and investigated. Above 1.0wt% of graphene nanoflakes, however, this strengthening effect sharply dropped due to the clustering of graphene nanoflakes. Furthermore, the optimal addition of graphene nanoflakes into the nanocrystalline aluminum matrix was calculated and discussed.

Thermally Stable Nanocrystalline Aluminum Alloys Processed by Mechanical Alloying and High Frequency Induction Heat Sintering

Thermally Stable Nanocrystalline Aluminum Alloys Processed by Mechanical Alloying and High Frequency Induction Heat Sintering

Inorganic Insulated Metal Substrates

Insulated metal substrates (IMS) are gaining ground in electronics applications thanks to their high thermal conductivity and low cost. However, the organic dielectrics (such as epoxy or polyimide based material) traditionally used on IMS are limited by their maximum operating temperature, generally to below 150–200 °C. Thus in high temperature applications manufacturers are restricted to using expensive inorganic substrates such as alumina (Al2O3), aluminium nitride (AlN) or silicon nitride (Si3N4). A cost-effective IMS with an inorganic dielectric would be an attractive alternative for high temperature electronics. Cambridge Nanotherm has developed an electrochemical process for building inorganic dielectric ceramic onto a metal base. Nanotherm ceramic material is nanocrystalline alumina with a grain size of 20 to 60 nanometres. This grain size plays a critical role in providing the dielectric layer with its unique combination of properties such as high thermal conductivity (6–7 W/mK), high dielectric strength (>50 V/um) and formability when applied on thin, foil-type substrates. The Nanoceramic layer can be built from 3 to 50 microns thick, depending on the breakdown voltage required. This avoids excessive dielectric thickness that unnecessarily increases the thermal resistance of the system. The electric circuit is built onto the ceramic surface using either PVD metal sputtering followed by galvanic metal build up or conventional thick film processing. The result is a cost-effective, easy to process and use inorganic substrate with a thermal conductivity around 150 W/mK and a maximum working temperature above 350 °C. This paper will present an overview of the key electrical and thermal properties of Nanoceramic aluminium substrates and their manufacturing process. Potential use in the thermal management of high temperature electronic devices will be discussed with reference to some applications.

Effect of Processing on Mechanically Alloyed and Spark Plasma Sintered Al‐Al2O3 Nanocomposites

Metal matrix nanocomposites are advanced materials developed using ceramic nanoreinforcements and nanocrystalline metal matrices. These composites have outstanding properties and high potential for large number of functional and structural applications. In this work, nanocrystalline aluminium and Al‐Al2O3 nanocomposites were synthesised using mechanical alloying and consolidated through spark plasma sintering technique. Scanning electron microscopy, X‐ray diffraction, and mapping were used to characterize the powders and sintered samples. Density and hardness of sintered samples were measured using densimeter and hardness tester, respectively. It was found that milling of pure aluminium for 24 h reduced its crystallite size to less than 100 nm. For Al‐Al2O3 nanocomposites, milling for 24 h decreased the crystallite size of the aluminium phase and resulted in uniform dispersion of the reinforcement. Sintering of the synthesised powders led to grain growth. Al2O3 contributed to growth inhibition when samples were sintered for 20 minutes and improved the hardness but reduced densification. The Al‐10 vol.% Al2O3 nanocomposite had the highest Vickers hardness value of 1460 MPa.

A Comparative Study on the Electrochemical Corrosion Behavior of Microcrystalline and Nanocrystalline Aluminum in Natural Seawater

A Comparative Study on the Electrochemical Corrosion Behavior of Microcrystalline and Nanocrystalline Aluminum in Natural Seawater

Reaction kinetics for the solid state synthesis of the AlH3/MgCl2 nano-composite by mechanical milling.

The process of mechanical milling has been proved to be a cost-effective way to synthesize the AlH3/MgCl2 nano-composite by using MgH2 and AlCl3 as reagents. However, so far there is no comprehensive knowledge of the kinetics of this process. In an effort to predict the reaction progress and optimize the milling parameters, the kinetics of the synthesis of the AlH3/MgCl2 nano-composite by mechanical milling of MgH2 and AlCl3 is experimentally investigated in the present work. The reaction progress or the transformation fraction upon milling for different times is evaluated using the isothermal hydrogen desorption test of the as-milled samples at 220 °C, which is much lower than the threshold temperature for the de-hydriding of the reagent MgH2 but enough for the de-hydriding of the as-synthesized nano-sized AlH3. The effects of milling parameters on the reaction kinetics as well as the underlying mechanism are discussed by referring to the mechanical energy input intensity, the vial temperature and the Gibbs free energy change for the reaction. Furthermore, it is found that the Johnson-Mehl-Avrami (JMA) model can well describe the kinetics theoretically. By fitting the experimental data with the JMA expression, the theoretical kinetics expressions, the equation parameters, and the activation energy are obtained.

A Comparative Study of Solvothermal and Sol‐Gel‐Derived Nanocrystalline Alumina Catalysts for Ethanol Dehydration

The ethanol dehydration to ethylene over alumina catalysts prepared by solvothermal and sol‐gel methods was investigated. Also, a commercial alumina was used for comparison purposes. The results showed that the catalytic activity depends on the properties of catalyst derived from different preparation methods and reaction temperature. The alumina synthesized by solvothermal method exhibited the highest activity. This can be attributed to the higher surface area and larger amount of acid site, especially the ratio of weak/strong acid strength as determined by N2 physisorption and NH3‐TPD studies. The solvothermal‐derived catalyst exhibited an excellent performance with complete ethanol conversion and 100% selectivity to ethylene at 350°C in comparison with other ones. In addition, we further studied the catalytic dehydration of alumina catalyst modified with Fe. The presence of 10 wt.% Fe decreased both conversion and ethylene selectivity. However, the acetaldehyde selectivity apparently increased. It was related to the dehydrogenation pathway that takes place on Fe species.

A low cost approach to synthesize sand like AlOOH nanoarchitecture (SANA) and its application in defluoridation of water

This paper describes a chitosan template assisted synthesis of nanocrystalline aluminium oxyhydroxide (γ-AlOOH) at an ambient temperature (30±2°C) and atmospheric pressure. High ability of the composite in removing fluoride from water has been demonstrated. The structural, morphological and compositional characteristics of the composite were studied using various spectroscopic and microscopic techniques. The results showed that the composite has nanoarchitecture comprising a number of γ-AlOOH nanoparticles of size less than 10nm attached to the fibrils of chitosan. Batch and continuous flow adsorption experiments were conducted to assess the parameters that influence the adsorption process. The parameters investigated include contact time, initial fluoride concentration, adsorbent dose, pH of the solution, co-existing ions in water, and size of the adsorbent granules. The composite showed good affinity to fluoride and the maximum uptake capacity was 13.47 (mg/g) (mg/L)−1/n as described by Freundlich isotherm model. The kinetics of adsorption was affected by the size of the granules and the kinetic data followed pseudo-second-order rate equation. Except bicarbonate and sulfate, other competing ions studied did not affect the uptake of fluoride significantly. The high affinity to fluoride, simple and eco-friendly synthesis approach demonstrate the utility of the material for defluoridation of water.

Optimization of diamond coated microdrills in aluminum alloy 7075 machining: A case study

Aluminum alloy 7075 is widely used for producing micro-scale heat sinks, micro-fluidic devices, micro-propellers and so on. This paper deals with optimizing microstructure and thickness of diamond coatings on microdrills used in 7075 aluminum alloy machining. Firstly, the friction tests between microcrystalline diamond (MCD), nanocrystalline diamond (NCD) films and aluminum alloy reveal that the stable coefficient of friction (COF) of MCD–aluminum alloy working pair is 0.240, much higher than that of NCD–aluminum alloy working pair (0.072). The decrease of COF is mainly attributed to the lower roughness of NCD films and the presence of more graphite or the non-diamond phases in NCD coatings. Afterwards, comparative cutting tests involving MCD, NCD, diamond-like coating (DLC) and TiAlN coated microdrills show that after drilling 200 holes, NCD coated microdrills exhibit the best cutting performance. Furthermore, NCD coated microdrills with coating thicknesses of 1μm, 2μm, 4.5μm and 7μm are fabricated and their cutting performance is studied in aluminum alloy machining. The cutting experiments show that the NCD coated microdrill with coating thickness of 4.5μm shows the best cutting performance, exhibiting not only lowest flank wear and no tool tipping or chipping on the main cutting edges but also the highest quality of drilled holes because of the outstanding adhesive strength and wear resistance of the NCD coating.

Strain rate sensitivity of bulk multi-phase nanocrystalline Al–W-based alloy

High-energy ball milling of conventional coarse-grained aluminium and nanocrystalline W in an Al-10 at.%W composition results in the formation of a two-phase mixture of Al and W with nanocrystalline features. Subsequent compaction of these powders using spark plasma sintering (SPS) at 748 K resulted in the formation of an Al12W phase in the nanocrystalline aluminium matrix. It is suggested that the mere attainment of nanocrystallinity was not enough to trigger a reaction between Al and W to form Al12W but that sufficient thermal activation was also required, as supplied during SPS. The second-phase particles (~175 nm in size) are uniformly distributed in the nanocrystalline Al matrix having a grain size of ~40 nm. The nanocomposite possessed a high hardness of 5.42 ± 0.33 GPa and an elastic modulus of 145 ± 5 GPa, both measured using depth-sensing nanoindentation. At room temperature, this novel nanocomposite exhibited a strain rate sensitivity (SRS) of 0.024 ± 0.001 and an activation volume in the range of 3.78–3.88 b3. Interfacial regions, viz. grain boundaries and triple junctions in the matrix and the reinforcement, matrix/particle boundaries, etc. could be influential factors in deciding the SRS and the activation volume. A scanning probe microscope image of the nanoindent shows a plastic flow region around the periphery of the indent.

Effect of sintering temperature on microstructures and mechanical properties of spark plasma sintered nanocrystalline aluminum

Organic-coated aluminum nano-powders were consolidated by spark plasma sintering technique with low initial pressure of 1MPa and high holding pressure of 300MPa at different sintering temperature. The effect of sintering temperature on microstructures and mechanical properties of the compact bulks was investigated. The results indicate that both the density and the strain of the nanocrystalline aluminum increase with an increase in sintering temperature. However, the micro-hardness, compressive strength and tensile stress of the compact bulks increase initially and then decrease with increasing sintering temperature. The nanocrystalline aluminum sintered at 773K has the highest micro-hardness of 3.06GPa, the best compressive strength of 665MPa and the supreme tensile stress of 282MPa. A rapid grain growth of nanocrystalline aluminum sintered at 823K leads to a decrease in micro-hardness, compressive strength and tensile stress. After annealing, a remarkable increase in strain and a slight rise in strength were obtained due to the relief of the residual stress in nanocrystalline Al and the formation of composite structure.

Grain Boundary Internal Friction Peak in Nanocrystalline Aluminum Studied by Continuously Changing-Temperature

The bulk nanocrystalline aluminum are fabricated by cryomilling and hot-press sintering. The internal friction (IF) and relative dynamic modulus measurements in the nanocrystalline aluminum (Al) have been made using a multifunction internal friction apparatus (MFIFA) at low frequencies (0.2-3.0 Hz) over the wide temperature ranges from room temperature to 450 °C, while continuously changing-temperature forced vibration measurement. In the temperature spectrums of IF during heating process and cooling process, a distinct IF peak is found. Corresponding to the IF peak, the relative dynamic modulus decreases rapidly. The peak temperature of the IF peak shifts towards higher temperature with increasing frequency, i.e., the IF peak seem due to the thermal activated relaxation process. The IF peak is a grain boundary IF peak, which is associated with the diffusive grain boundary of Al/Al. Its activation energy has been calculated to be 2.21±0.04×10-19 J and the pre-exponential factor is 10-14 s in IF measurements.