Nanostructured Materials最新文献

The fatigue behaviour of the fine-grain 5056 Al-alloy processed by equal-channel angular pressing (ECAP) is explored. This material exhibits a slightly enhanced fatigue life at low stress amplitudes. However, no improvement in the fatigue limit is observed. Fatigue performance is discussed in terms of fatigue life, crack nucleation and propagation. Structural changes during fatigue are investigated by transmission electron microscopy. It is shown that the fine structure achieved during processing is unstable and tends to relax with cycling, resulting in local recovery of the pre-deformed material. Structure relaxation during fatigue is supposed to provoke notable cyclic softening which is particularly pronounced at higher applied stresses. It is found that the crack growth rate is greater in the fine-grain ECAP material than in its coarse-grain counterpart. The latter is attributed to the roughness-induced crack closure and crack deflections which is more significant in conventional alloy. The improvement of fatigue properties at low-cyclic regime is believed to be due to a higher resistance to crack nucleation in the fine-grained material having a larger yield stress value.

The present work studied the effect of heat treatment on tensile properties of bulk nanocrystalline Al (nano-Al) cold-compacted from nano-Al powder. The nano-Al powder synthesized by the active H plasma evaporation was compacted under 1.42 GPa pressure at room temperature into disks with 25 mm diameter and 2 mm thickness. The cold-compacted disks were heat-treated at temperature range from 540°C to 635°C, then machined into the tensile specimens with 25 mm total length. The results indicated that the grain size of the cold-compacted nano-Al heat-treated at 635°C is basically consistent with the primary powder. The tensile specimens heat-treated at temperatures lower than 600°C are broken prematurely in the elastic deformation region during the tensile test process with a brittle intergranular fracture. The specimens heat-treated at temperatures higher than 600°C give the complete stress-strain curves, the yield strengths σ_0.2 and tensile strengths σ_b are 12–16 times and 5–6 times those of annealed coarse-grained Al respectively, the elongations-to-failure are all 4%, and the mechanism of failure is transgranular shear fracture with a ductile dimples fracture.

The infrared spectra of the additives adsorbed on nano-zirconia powder surfaces were measured by using diffuse reflectance Fourier transform (DRIFT) spectroscopy. The nature of the structural information has been studied by preparing zirconia suspensions at various pH and different concentrations of polyacrylate addition. These DRIFT spectra can provide useful information about polyacrylate existence on zirconia surface including the ionization degree and its configuration. The addition of polyacrylate into zirconia aqueous suspension could completely change the surface chemistry and interaction of zirconia particles.

Mössbauer spectroscopy, X-rays diffraction and differential scanning calorimetry are used to study a fcc Fe 45at.% Cu nanocrystalline solid solution obtained by mechanical alloying. These results supported by molecular dynamics simulations are used to discuss some models in mechanical alloying. The kinetic energy ou even a function of it appears to be the parameter driving the reactions inside the mill. Impact frequency and powder mass seem to contribute only on the time scale. A capilarity pressure is shown to exist in order to alloy the nanocrystalline grains. Despite this, an extra energy, provided by the shocks, is shown to be necessary to start up the process. Final product appears to be a function of strains/stress fields and material response.

Packing characteristics and sintering behavior of the bimodal mixtures of nanocrystalline (75 nm) and conventional (1 μm) alumina powders were investigated. Green density of the pure nanocrystalline compact was lower than that of the pure coarse particles. Increase in the packing density was observed at 70% coarse particle composition. The powders shrinkage characteristics were determined by dilatometry. The cold pressed compacts were sintered within the temperature range of 1300°C to 1600°C. The green density followed the packing rules giving rise to higher packings in the bimodal compacts. At temperatures below 1400°C, the relative density kept this pattern. Above 1400°C, this advantageous densification was lost. The changes in densification of the compacts versus powder composition and sintering temperature were discussed with respect to phase transition and sintering in the compacts.

Nanosized CeO₂, and CeO₂-SnO₂, 100–500 nm thick, films on glass substrate were prepared using sol-gel dip-coating method procedure. The average grain size <R>, obtained by SAXS (small-angle X-ray scattering), varied with the number of dips for the CeO₂-SnO₂ samples. For the CeO₂ films, obtained by dipping it 8 times, <R> increased compared to CeO₂-SnO₂ films, which were obtained by the same number of dips, from 4.4 to 5.3 nm. Specific surface areas of both these films were also determined and varied from 0.18 × 10⁷ to 0.51 × 10⁷ cm⁻¹. SAXS measurements also revealed the layered structure of CeO₂ and CeO₂-SnO₂ films.

A new metallic glass alloy characterized by high amplitude of magnetoelastic waves was synthesized. X-ray spectra evidenced the amorphous structure in the as-cast material, while differential thermal analysis confirmed the metastable initial phase and the occurrence of several crystallization peaks. The alloy shows a high difference between the glass transition and crystallization temperatures. Several heat treatments were performed and the structural relaxation effects were shown by means of the behavior of magnetization, Curie temperature and the resonant magneto-elastic wave amplitude. The strong increase of the last parameter, induced by the occurrence of the amorphous structure, is the most important result for application. Magnetic and magneto-elastic properties deteriorate if crystallization occurs, but in the optimal condition a partial nanocrystallisation may be present.

The Al₈₆Y₅Ni₅Fe₄ amorphous and partially nanocrystalline alloys were investigated by means of differential scanning calorimetry (DSC), temperature coefficient of resistivity (TCR) and Transmission Electron Microscopy. The resistivity vs. temperature isochronal relation was used to calculate the transformed volume fraction, x(T), in the first stage of transformation. The comparison between these results and the DSC ones show that the TCR method is more correct for the nanocrystallization process analysis.

The Fe_xNi_100-x (x = 50, 65 and 80) alloys were synthesized in a conventional horizontal low energy ball mill. The X-ray diffraction was used to identify and characterise various phases during the milling process of the Fe₈₀Ni₂₀ alloy exhibiting a bcc structure whereas for x = 65 and 50 the fcc structures are found. The steady state grain size is about 10 nm. Magnetisation measurements after various milling periods allow to monitor a rate at which Ni atoms dissolve in the iron lattice. The room temperature values of the effective magnetic moment raise with the increasing milling period. All the alloys studied exhibit the ferromagnetic ordering. The magnitude of the magnetic interactions is moderately suppressed at prolonged milling as revealed by the Curie temperatures reduced down to 950 K. Such variations are caused by the deviations in the interatomic arrangements of atoms especially in the intergrain regions. The Moessbauer spectroscopy confirmed the ferromagnetic ordering and was used to calculate the distribution of hyperfine magnetic fields. The mean hyperfine fields are 33.8 T for Fe₈₀Ni₂₀ and correspond to the one to two Ni atoms in nearest neighbourhood. In the remaining alloys, at most, five Ni atoms are located in a neighbourhood of the Fe atom.

Off-axis electron holography has been applied in order to investigate quantitatively the remanent magnetization state of single ferromagnetic Co₉₃Cu₇ and Ni nanowires, respectively. The magnetization reversal of an individual Co₉₃Cu₇ nanowire has been followed by observing a series of remanent states, obtained ex-situ by applying different external magnetic field sweeps parallel to the nanowire axis. The relation between mis-oriented crystal grains and non-uniform magnetization states has been studied by the combination of electron holography and conventional transmission electron microscopy. Nickel nanowires have been heated in-situ to temperatures above the Curie temperature and cooled down again. The magnetic state during the heating experiment was followed by a series of electron holograms. A single Ni nanowire was examined in detail. It was observed to be uniformly magnetized before the heating cycle. The ferromagnetic state spontaneously formed upon cooling from above the Curie temperature, exhibited a magnetic state with two antiparallel magnetic domains.