Bulk Nanocrystalline Aluminum Research Articles

Smart manufacturing approach to manufacture bulk nanocrystalline aluminum for lightweight applications

Smart manufacturing approach to manufacture bulk nanocrystalline aluminum for lightweight applications

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

Processing and Thermal Conductivity of Bulk Nanocrystalline Aluminum Nitride.

Producing bulk AlN with grain sizes in the nano regime and measuring its thermal conductivity is an important milestone in the development of materials for high energy optical applications. We present the synthesis and subsequent densification of nano-AlN powder to produce bulk nanocrystalline AlN. The nanopowder is synthesized by converting transition alumina (δ-Al2O3) with <40 nm grain size to AlN using a carbon free reduction/nitridation process. We consolidated the nano-AlN powder using current activated pressure assisted densification (CAPAD) and achieved a relative density of 98% at 1300 °C with average grain size, ~125 nm. By contrast, high quality commercially available AlN powder yields densities ~75% under the same CAPAD conditions. We used the 3-ω method to measure the thermal conductivity, κ of two nanocrystalline samples, 91% dense, = 110 nm and 99% dense, = 220 nm, respectively. The dense sample with 220 nm grains has a measured κ = 43 W/(m·K) at room temperature, which is relatively high for a nanocrystalline ceramic, but still low compared to single crystal and large grain sized polycrystalline AlN which can exceed 300 W/(m·K). The reduction in κ in both samples is understood as a combination of grain boundary scattering and porosity effects. We believe that these are finest reported in bulk dense AlN and is the first report of thermal conductivity for AlN with ≤220 nm grain size. The obtained κ values are higher than the vast majority of conventional optical materials, demonstrating the advantage of AlN for high-energy optical applications.

Artifact-free bulk nanocrystalline Al-Li alloys with multiple deformation mechanisms and improved tensile properties

An artifact-free bulk nanocrystalline (NC) aluminum (Al)-2% lithium (Li) alloy was synthesized in this study using in-situ consolidation via a combination of cryogenic and room temperature milling. The mechanical behavior of this alloy was investigated by tensile testing and microhardness measurements, and it was compared with coarse-grained (CG) Al alloys and a commercially pure NC Al synthesized using the same method. The transmission electron microscopy (TEM) analysis revealed that the grain size of the NC Al-2% Li alloy and NC pure Al are 18 nm and 29 nm, respectively. The NC Al-Li alloy showed extremely high yield and ultimate tensile strength values of 440 MPa and 556 MPa, respectively. In addition, a high tensile ductility of 14% was achieved in the NC Al-Li alloy along with a relatively high strain hardening exponent (0.11). The high-resolution TEM investigations indicate a dependency of these extraordinarily tensile properties on multiple deformation mechanisms such as dislocation slip and pile-up and deformation twinning.

Electrochemical Corrosion Behavior of Bulk Nanocrystalline Aluminum in Formic Acid Solutions at Ambient Temperature

Electrochemical Corrosion Behavior of Bulk Nanocrystalline Aluminum in Formic Acid Solutions at Ambient Temperature

Microstructure characterization and mechanical properties of bulk nanocrystalline aluminium prepared by SPS and followed by high-temperature extruded techniques

The bulk nc Al was successfully fabricated by SPS and high-temperature extrusion process in this study. Microstructure and mechanical properties of sample were also investigated. Results show that the dislocation densities are 1.847×1014m−2 and 1.673×1014m−2 corresponding to the SPSed and extruded Al, respectively. After SPSed Al was extruded at 450°C, the grains increased in size by a factor of 2.35 relative to the initial microstructure. Also, the mechanical properties and physical properties of extruded Al are all higher than that of SPSed Al except that SPSed Al has the highest hardness and lower electrical conductivity. The enhancement of mechanical and physical properties of extruded sample is mainly to densification behavior during extruded process.

Effect of Sintering Temperature on Microstructure and Mechanical Properties of Nanocrystalline Aluminum 5083 Alloy

The bulk nanocrystalline (NC) aluminum (Al) 5083 was synthesized by spark plasma sintering (SPS) technique with low initial pressure of 1 MPa, high holding pressure of 300 MPa and holding time of 4 min at different sintering temperatures, using surface passivated nanopowders. The effect of sintering temperature on microstructure and mechanical properties of the bulk NC Al 5083 were investigated. Results indicate that the density, grain size, the hardness and the compressive strength of the bulk NC Al 5083 increase with an increase in sintering temperature. The mechanical properties of the material are greatly improved due to the fine grain size. The bulk NC Al 5083 sintered at 723 K has the highest micro-hardness of 2.37 GPa and the best compressive strength of 845 MPa.

Atomic dynamics of grain boundaries in bulk nanocrystalline aluminium: A molecular dynamics simulation study

Dynamics of grain boundary (GB) atoms in the bulk nanocrystalline aluminium is investigated by means of a large-scale molecular dynamics simulation. It is found that the GB atoms in the nanocrystalline aluminium display the glassy dynamics. Their dynamic features are, on one hand, similar to those in the glass systems and bicrystal GBs, in terms of the time-correlation functions of mean-square displacement (MSD) and non-Gaussian parameter (NGP). But on the other hand, these GB atoms also demonstrate some different dynamic behaviors due to the more complicated microstructure. In particular, the immobile GB atoms are localized into their equilibrium positions due to the strong cage effect, and they are mainly at the surfaces of grains, especially the large grains, while the mobile GB atoms are active and can easily hop away from the cage around them. These mobile GB atoms gather together at the surfaces of small grains and the triple junction regions of GBs, and they form some abnormally big clusters. The size distribution of these clusters significantly deviates from the power law.

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...

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.

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.

Local atomic structures in grain boundaries of bulk nanocrystalline aluminium: A molecular dynamics simulation study

The microstructures of grain boundaries (GBs) in bulk nanocrystalline aluminium have been investigated by a large-scale molecular dynamics method. Bulk nanocrystalline aluminium is obtained directly by liquid quenching at an appropriate cooling rate, which has narrow grain-size distribution and high-angle GBs. It is found that up to 89.75% GB atoms (named as GB1 atoms) are located at the nearest-neighbour coordination shell around nanograins; others (named as GB2 atoms) are mainly at triple junctions. Local atomic structures in the GBs are quantified in terms of a recently developed method, in which the neighbours of an atom are identified with a parameter-free topological criterion rather than a fixed cut-off distance rc. The results demonstrate that though there are a large number of different cluster types in both the GB1 and the GB2 regions, only a few ones with FCC-like order appear with high frequency in the GB1 region and play a crucial role in the microstructural feature of the GB1. The GB1 region displays short-to-long range order. The GB2 region presents ICO- and BCC-like short-range orders whose degrees are in between the liquid and amorphous, but the medium-range order at the cluster-scale is very weak.

The electrochemical corrosion characterization of bulk nanocrystalline aluminium by X-ray photoelectron spectroscopy and ultra-violet photoelectron spectroscopy

The electrochemical corrosion of bulk nanocrystalline aluminium (BN-Al) produced by severe rolling technique and its conventional polycrystalline aluminium counterpart (CP-Al) was studied by weight loss and scanning electron microscope (SEM). The uniform and localized corrosion resistances of BN-Al were simultaneously enhanced in comparison with CP-Al in 0.25mol/L HCl solution at room temperature due to the thinner, more compact and stable oxide film on BN-Al. The valence electron configurations of BN-Al and CP-Al were characterized by ultra-violet photoelectron spectroscopy (UPS). The electronic structures of oxide films on BN-Al and CP-Al were studied by X-ray photoelectron spectroscopy (XPS). The larger work function of BN-Al, the larger binding energies of valence electrons of BN-Al, the weaker Cl− adsorption on BN-Al, and the larger binding energies of Al3+2p3/2 and Cl−2p3/2 in the oxide film on BN-Al could explain the enhanced corrosion resistance of BN-Al and the inconsistent corrosion results of nanocrystalline materials at atomic scale.

Strain rate sensitivity studies on bulk nanocrystalline aluminium by nanoindentation

Abstract Nanocrystalline aluminium powder synthesized using high energy ball milling process was characterized by X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). The studies indicated the powder having an average grain size of ∼42 nm. The consolidation of the powder was carried out by high-pressure compaction using a uni-axial press at room temperature by applying a pressure of 1.5 GPa. The cold compacted bulk sample having a density of ∼98% was subjected to nanoindentation which showed an average hardness and elastic modulus values of 1.67 ± 0.09 GPa and 83 ± 8 GPa respectively at a peak force of 8000 μN and a strain rate of 10 −2 s −1 . Achieving good strength along with good ductility is challenging in nanocrystalline metals. When enough sample sizes are not available to measure ductility and other mechanical properties as per ASTM standards, as is the case with nanocrystalline materials, nanoindentation is a very promising technique to evaluate strain rate sensitivity. Strain rate sensitivity is a good measure of ductility and in the present work it is measured by performing indentation at various loads with varying loading rates. Strain rate sensitivity values of 0.024–0.054 are obtained for nanocrystalline Al which are high over conventional coarse grained Al. In addition, Scanning Probe Microscopy (SPM) image of the indent shows that there is some plastically flown region around the indent suggesting that this nanocrystalline aluminium is ductile.

Characterization of nanocrystalline Al-based alloy produced by mechanical milling followed by cold-pressing consolidation

In the present study high energy mechanical milling followed by high-pressing consolidation has been used to obtain bulk nanocrystalline aluminum based alloy. Fully dense disks with homogenous microstructure were obtained. X-ray analysis indicates that high strain imposed by mechanical milling and severe plastic deformation (SPD) processing causes microstructure showing a mixture of a significant lower value of the crystallite size (around 60 nm) and the creation of a large number of linear defects which induces microstrains (around 0.7%). In addition, the elevated value of hardness which was obtained for the consolidated disks and the favorable comparison with the value measured for the same as-received alloy confirm the potential of the SPD technique when it is collected with high energy mechanical milling for producing higher performance materials. On the basis of calorimetric measurements, as a function of consolidation stresses, we can say that the thermal behaviour was characterized by a significant heat release and grain growth during the heating cycle. Moreover, the noticed broad endothermic peak can be attributed to some precipitants containing magnesium.

Tensile behavior of bulk nanocrystalline aluminum synthesized by hot extrusion of ball-milled powders

The deformation behavior of bulk nanocrystalline aluminum down to 48 nm in grain size, produced by hot extrusion of ball-milled powders, has been evaluated by compression and tension tests. Isotropic yield stress increases following the Hall–Petch relationship with decreasing grain size down to 70 nm. However, a positive deviation in the relationship is found for grain sizes below 70 nm, and aluminum with a grain size of 48 nm exhibits a superior yield stress of 500 MPa with ductile failure.

Bulk nanocrystalline aluminum 5083 alloy fabricated by a novel technique: Cryomilling and spark plasma sintering

Dense, bulk nanocrystalline aluminum 5083 alloy was fabricatedvia a combined technique: cryomilling (mechanical milling at cryogenic temperature) to achieve the nanocrystalline Al 5083 powder and spark plasma sintering (SPS) to consolidate the cryomilled powder. The results of X-ray diffraction analysis indicate that the average grain size in the SPS consolidated material is 51 nm, one of the smallest grain sizes ever reported in bulk Al alloys produced by powder metallurgy derived methods. In contrast, transmission electron microscopy (TEM) analysis revealed a bimodal grain size distribution, with an average grain size of 47 nm in the fine-grained regions and approximately 300 nm in the coarse-grained regions. Nanoindentation was used to evaluate the mechanical properties and the uniformity of the consolidated nanocrystalline Al 5083. The hardness of the material is greatly improved over that of the conventional equivalent, due to the fine grain size. The mechanisms for spark plasma sintering and the microstructural evolution are discussed on the basis of the experimental findings.