Nanocrystalline Aluminum Research Articles

Magnetic properties of nano-sized 5 at.%Fe–Al systems

High energy ball milling is a promising materials processing technique that is widely used to produce nanocrystalline structures. However, when stainless steel or hardened steel containers and balls are used for milling, contamination from the milling medium can influence the material properties of the final nanostructured products due to intercalation of iron (Fe) as an impurity. This study reports the effect of iron contamination on nanocrystalline aluminum (Al) powder. 57Fe Mossbauer spectroscopy and bulk magnetization studies using a vibrating sample magnetometer show that pure Al powder milled in hard steel media is strongly ferromagnetic at room temperature due to Fe contamination of about 5 at.% from the milling medium. TEM studies indicate that the system consists mainly of nano-sized Fe interspersed in Al with average crystallite sizes of ~2 and ~5 nm for Fe and Al, respectively. A comparative study of this system made with a mechanically alloyed Fe–Al system with the same percentage of Fe mixed with pure Al and mechanically alloyed using tungsten carbide vials and balls shows that the saturation magnetization, coercivity, Curie temperature, and low temperature behavior (field cooled–zero field cooled) are very different in the two cases. The different magnetic properties of the two systems can be attributed to the presence of magnetic and non-magnetic phases present.

Molecular dynamics investigation of shock front in nanocrystalline aluminum: grain boundary effects

The shock front structure and the plastic deformation of nanocrystalline aluminum under shock loading are investigated by using molecular dynamics simulations. The simulation results show that: after the elastic wave was generated, the grain boundary sliding and deformation dominated the early plastic deformation mechanisms, then the partial dislocations were nucleated at the deformed grain boundaries and spread within the grains, finally the process of stacking faults, deformation twins and full dislocation formation in the grain dominated the latter stage of the plastic deformation. The structural characteristics after the shock front swept over is that the stacking faults and the deformation twins are left in grains, and the majority of the full dislocations are annihilated at the opposite grain boundaries. It is reported for the first time that the shock front structure reflects the time sequence of two different plastic deformation mechanisms in nanocrystalline aluminum.

Density Functional Theory Calculations of Properties of the Grain Boundaries in Aluminum

ABSTRACTThe Density Functional Theory has been used to analyze an inter-granular segregation of Cu and Mg. The stability of Cu and Mg atoms in the aluminum matrix, intermetallic phases and symmetric twist grain boundaries has been compared. The quantitative description of solubility of Cu and Mg atoms in the nano-crystalline aluminum has been proposed. The calculations have been carried out to investigate the properties of symmetric twist boundaries in aluminum with and without Cu/Mg atoms. The phenomena of are discussed and its effect on the stability of precipitates containing these elements.

Phase and microstructural evolution of yttrium-doped nanocrystalline alumina: A contribution of advanced microscopy techniques

Well-dispersed nano-crystalline transition alumina suspensions were mixed with yttrium chloride aqueous solutions, with the aim of producing Al 2O 3–Y 3Al 5O 12 (YAG) composite powders. DTA analysis allowed to highlight the role of yttrium on the α-phase crystallization path. Systematic XRD and HRTEM analyses were carried out in parallel on powders calcined in a wide temperature range (600–1300 °C) in order to follow phase and microstructural evolution. A thin, homogeneous yttrium-rich layer was yielded on the alumina particles surface; yttrium diffusion into the alumina matrix was negligible up to 1150 °C whereas, starting from 1200 °C, aggregates of partially sintered alumina particles appeared, stuck together by yttrium-rich thin films. Moreover, in the yttrium-richer zones, such as alumina grain boundaries and triple joints, yttrium-aluminates precipitated at alumina particles surface. Finally, at 1300 °C, alumina–YAG composite powders were produced, in which YAG was homogenously distributed among the alumina grains.

Luminescence studies on swift heavy ion irradiated nanocrystalline aluminum oxide

Pellets of nanocrystalline aluminum oxide synthesized by a combustion technique are irradiated with 120MeV Au9+ ions for fluence in the range 5×1011–1×1013ionscm−2. Two photoluminescence (PL) emissions, a prominent one with peak at ∼525nm and a shoulder at ∼465nm are observed in heat treated and Au9+ ion irradiated aluminum oxide. The 525nm emission is attributed to F22+-centers. The PL intensity at 525nm is found to increase with increase in ion fluence up to 1×1012ionscm−2 and decreases beyond this fluence. Thermoluminescence (TL) of heat-treated and swift heavy ion (SHI) irradiated aluminum oxide gives a strong and broad TL glow with peak at ∼610K along with a weak shoulder at 500K. The TL intensity is found to increase with Au9+ ion fluence up to 1×1013ionscm−2 and decreases beyond this fluence.

On the Contribution of Friction to the Contact Damping in Atomic Force Acoustic Microscopy

In atomic force acoustic microscopy (AFAM) one exploits the contact resonances of the micro-fabricated cantilever, either as an imaging method where the contrast arises from the local variation of elastic and anelastic properties, or for local quantitative measurements. In this work we used AFAM in its spectroscopic mode, where contact-resonance curves are recorded as a function of the loading force P. From such curves the local contact stiffness and the contact damping were determined. The contact damping was found to have a power dependence on the load P when measured on the two different samples quartz-glass and nanocrystalline aluminum. We attribute this effect to micro-sliding of the sensor tip on the sample described by Mindlin for macroscopic contacts. We calculate the friction coefficient from the dissipated energy per cycle in the contact area to be 0.1, while the amplitude of the oscillatory sliding velocity was in the range of 0.05 m/s.

Atom probe crystallography: Characterization of grain boundary orientation relationships in nanocrystalline aluminium

Spatial Distribution Maps (SDM) in their various forms have previously been used to identify and characterize crystallographic structure within APT reconstructions. Importantly, it has been shown that such SDM analyses can also provide the crystallographic orientation of the specimen with respect to the direction of the detector in the original experiment. In this study, we investigate the application of SDMs to the analysis of APT reconstruction of a nanocrystalline Al film. We demonstrate that significant intra-granular crystallographic information is retained in the reconstruction, even in the x-y plane perpendicular to the direction of the detector. Further, the crystallographic orientation of the grains can be characterized highly accurately not only with respect to the bulk specimen but also their misorientation with respect to neighbouring grains.

Sintering behaviour of nano-crystalline γ-Al 2O 3 powder without additives at 2–7 GPa

Al 2O 3 ceramics were fabricated without additives under high pressure (2–7 GPa) at different temperatures (600–1200 °C) using nanocrystalline alumina powder with metastable γ-Al 2O 3 phase as the starting material. It was shown that high pressure increases the nucleation rate while reducing the growth rate of the transformed α phase so that its grain size decreases and nano-scale grains in the sintered structure can be achieved. On the other hand the sintered samples at 7 GPa and high temperature (1000 °C) have shown micron-scale large grain sizes compared to those sintered at lower pressures, for the same temperature and sintering time. This could be attributed to the higher input energy in the system at high pressure and high temperature conditions, thereby reaching the final stage in sintering more quickly. In this work, the best combination of grain size (∼200 nm) and density (98.0% TD) was obtained under the sintering condition of 1000 °C at 7 GPa with a holding time of 1 min. Thus for high pressure/high temperature conditions, the sintering time should be reduced to prevent grain growth.

A novel low temperature synthesis technique of sol–gel derived nanocrystalline alumina abrasive

The effect of ball milling process, co-doped seed and two step sintering technique on the properties of sol–gel derived alumina abrasive sintered at low temperature was investigated. The results showed that ball milling time with 10 h can be effective in enhancing the activity of the precursor and the microstructural uniformity of sintered alumina abrasive. A small amount of Al2O3–(NH4)3AlF6 co-doped seed addition had potential synergistic effects for reducing α-Al2O3 phase transformation temperature and improving the mechanical property of alumina abrasive. A remarkable suppression of grain growth was achieved by controlling sintering temperature with two-step sintering method. Therefore, by using ball milling process, co-doping α-Al2O3–(NH4)3AlF6 seed and two-step sintering technique, the sol–gel derived uniform nanocrystalline alumina abrasive is easily achieved at low temperature. Nanocrystalline alumina abrasive prepared at these conditions exhibited excellent mechanical properties and wear resistance compared to fused corundum abrasive and those sol–gel derived corundum abrasive with conventional sintering methods.

Nanostructured Alumina from Freeze-Dried Precursors

Nanocrystalline alumina has been obtained on the 100 g scale by thermal decomposition of precursors resulting from the freeze-drying of aqueous solutions of different aluminum-containing products, namely aluminum acetate and aluminum L-lactate. Samples prepared at different temperatures (from 873 to 1573 K in steps of 100 K) were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and surface area measurements. In the acetate case, the transformation sequence involves the formation of θ-Al 2 O 3 as an intermediate phase between γ-Al 2 O 3 and α-Al 2 O 3 , whereas this θ phase is not observed in the lactate case. TEM and SEM images show the nanoparticulate character of the aluminas obtained at relatively low temperatures, with typical particle size in the 5-10 nm range. Progressive grain growth occurs as temperature increases. Otherwise, the precursor characteristics have a clear influence on the microstructure of the resulting aluminas, as reflected also by the measured BET surface area values. Whereas long aluminum acetate fibers results in open arrays of low aggregated alumina particles, large aluminum lactate sheets lead to comparatively compact alumina microstructures. Nanostructured alumina obtained from the lactate precursor has been reconstituted in a granulated powder with sufficient consistence and flowability to allow it to be thermal sprayed and deposited on a stainless steel substrate. XRD data show that γ-Al 2 O 3 is the major phase in the coating, which includes also α-Al 2 O 3 particles. SEM results offer evidences on the nanostructured character of the coating.

A study of the structure and properties of nanostructured ZrO2-Y2O3-Al2O3 composite powders

The correlation between temperature treatment conditions and the ratio of components in nanostructured fibrous powders with a composition of ZrO2-Y2O3-Al2O3 and their porous crystal structure and physicochemical properties is studied. The dependences of the ratio between zirconia tetragonal and monoclynic phases on the treatment temperature and the alumina content are found to have a nonmonotonic character. The growth of zirconia crystallite size is suppressed by introduced nanocrystalline alumina in a temperature range of 600–1200°C, which is caused by the processes of ternary solid solution formation. The bulk and picnometric density values of materials are proportional to the temperature of heat treatment. The temperature dependence of the specific surface and the size of oxide grain particles has an inversely proportional character. With increasing alumina content in the powders, the specific surface increases, while the picnometric and bulk densities decrease.

Role of nano-size reinforcement and milling on the synthesis of nano-crystalline aluminium alloy composites by mechanical alloying

High-energy wet ball milling was successfully employed to synthesize nano-crystalline Al 6063 alloy powders reinforced with 1.3 vol.%Al 2O 3, 1.3 vol.%Y 2O 3 and 0.65 vol.%Al 2O 3/0.65 vol.%Y 2O 3 at nano-size level. In the present study, the crystallite size of the matrix powder particle was affected by the addition of different types of nanoceramic particles separately and in combination keeping total volume percentages of such addition as constant. The nano-composite powders were characterized by SEM, FESEM, HRTEM, XRD, particle size analyzer, EDAX and DTA. Using Williamson–Hall equation, crystallite size and lattice strain of various aluminium composite powders were estimated with broadening of XRD peaks. XRD results showed that the crystallite size of aluminium reached 53 and 37 nm, respectively, after 40 h milling in case of pure Al 6063 and Al 6063/0.65 vol.%Al 2O 3/0.65 vol.%Y 2O 3 nano-composite powder with uniform particle size distribution. HRTEM observation confirmed the nano-crystalline nature of Al 6063/0.65 vol.%Al 2O 3/0.65 vol.%Y 2O 3 milled powder.

Structure and properties of mechanochemically synthesised aluminium hydroxide fluoride phases AlF x(OH) 3- x· nH 2O

A successful mechanochemical synthesis of nanocrystalline aluminium hydroxide fluoride samples AlF x (OH) 3- x ·nH 2O with pyrochlore structure is described for the first time. Optimal conditions for the synthesis can be adjusted using γ-Al(OH) 3 and β-AlF 3·3H 2O as educts applying an Al:F ratio of 1:1.5. Under these circumstances, the reaction proceeds almost stoichiometrically. On the other hand a classical solid state chemical reaction at elevated temperature is not successful with the same educts. Strongly disordered or amorphous products are obtained with differing Al:F molar ratios at the beginning. It was found that structure and water content of the educts are playing a crucial role for the product composition.

Electrodeposition of nanocrystalline aluminium from a chloroaluminate ionic liquid

In this letter we show that nanocrystalline aluminium can be electrodeposited in the Lewis acidic ionic liquid based on AlCl 3 (60 mol%) and 1-(2-methoxyethyl)-3-methylimidazolium chloride ([MoeMIm]Cl) (40 mol%). The study comprised cyclic voltammetry, potentiostatic polarization, and SEM and XRD measurements. The methoxy group in the side chain of the imidazolium cation significantly influences the electrodeposition pathway of Al in comparison to [EMIm]Cl/AlCl 3. Cyclic voltammetry shows a significant current loop attributed to nucleation. Shiny Al layers are obtained with an average crystallite size of about 40 nm.

Structural changes during annealing of submicro- and nanocrystalline aluminum alloys

The annealing-induced evolution of the structure and microhardness of submicro-and nanocrystalline Al—3% Mg and Al 1570 alloys produced by torsional severe plastic deformation are studied. Annealing of the Al-3% Mg alloy at 373–423 K and annealing of the Al 1570 alloy at 373–473 K are shown to result in the relaxation of internal stresses and subsequent normal grain growth. As the annealing temperature increases, the microhardness decreases. At higher temperatures (473 K for the Al—3% Mg alloy and 573 K for the Al 1570 alloy), anomalous grain growth takes place. This growth is accompanied by the appearance of numerous grains with a high dislocation density, a high concentration of impurity atoms in grain boundaries, and an increase in the microhardness. These effects are explained.

Creep in nanocrystalline materials: Role of stress assisted grain growth

To date, only a limited number of creep studies have been carried out on nanocrystalline materials. These studies have remained largely inconclusive in establishing the creep mechanisms in nanocrystalline materials. The stress exponent and activation energy values obtained for nanocrystalline materials do not correlate well with conventional, well established creep models. Furthermore, discrepancy between experimentally determined deformation rates and theoretical predictions suggests that an entirely new mechanism of creep may be operational in these materials. Thus, this work aims to develop an understanding of the creep behavior of nanocrystalline materials by considering a stress assisted grain growth mechanism that has been recently identified in these materials. A model that provides an understanding of some observations made in creep literature has been developed. Impression creep data obtained from tests on nanocrystalline aluminum agree with model predictions.

Influence of the grain size and structural state of grain boundaries on the parameter of low-temperature and high-rate superplasticity of nanocrystalline and microcrystalline alloys

A model has been proposed for calculating the grain size optimum for the deformation of nanocrystalline and microcrystalline materials under superplasticity conditions. The model is based on the concepts of the theory of nonequilibrium grain boundaries in metals. It has been demonstrated that the optimum grain size dopt can be calculated as the size at which a high level of nonequilibrium of grain boundaries is combined with a high intensity of the accommodation of grain boundary sliding. The dependences of the quantity dopt on the rate and temperature of the strain and the thermodynamic parameters of the material have been derived. The results obtained have been compared with the experimental data on the superplasticity of nanocrystalline and microcrystalline aluminum and magnesium alloys.

Numerical Simulations on the Growth of Thin Oxide Films on Aluminum Substrates

We investigated the oxidation of nanocrystalline aluminum surfaces by using variable charge molecular dynamics at 600 K under three oxygen pressures: 1, 10 and 20 atm. The interaction potential was described by the electrostatic plus (Es+) model that allows dynamical charge transfer among atoms. We mainly focused on the effect of the oxygen pressure on the oxidation kinetic, the chemical composition and the microstructure of the oxide films formed. The results show that oxidation kinetics as well as chemical composition and microstructure depend on the applied oxygen pressure. The oxide film thickness tends to a limiting value equal to ~3 nm. Finally, we obtained a partially crystalline oxide films for all oxygen pressures and we observed that the degree of crystallinity increases with time.

Combustion Synthesis of Nanocrystalline Al<sub>2</sub>O<sub>3</sub> Powders Using Urea as Fuel: Influence of Different Combustion Aids

The present article reports the results of studies related to the synthesis of nano-crystalline alumina powder by combustion process using different combustion aids. Aluminium nitrate as oxidant and urea as fuel were used as reagents. Starch and ammonium nitrate were compared as combustion aids. The results showed that employment of starting fuel with combustion aids resulted in synthesizing nano-crystalline alumina powder with small crystalline size (<20 nm) and fine agglomerates. By using combustion aid, the energetics of the combustion reaction has been changed. The different selection of combustion aid determines the characteristics of the as-synthesized powders.

Shock consolidation of nanocrystalline 6061-T6 aluminum powders

Fully and partially nanocrystalline aluminum alloy powders made by cryomilling and plane-strain machining are dynamically consolidated into bulk compacts using a three-capsule gas-gun compaction geometry. Compacts with 98–99% theoretical mass density are produced, while retaining unique features of their initial microstructures. The microstructure of the fully nanocrystalline cryomilled powder compacts consists of 50–150 nm-thick elongated laths and 10–50 nm equiaxed grains. Partially nanocrystalline plane-strain machined powder compacts retain their bimodal microstructure composed of nanoscale grains near the particle surfaces and larger microscale grains with high dislocation densities in the particle interior. In this paper, the powder processing and compaction approach, and their effects on the physical, microstructural, and mechanical properties of the final compacts are described.