E. Bonetti

The influence of grain size on the mechanical properties of nanocrystalline aluminium

Abstract Uniaxial tensile tests were performed at room temperature on nanocrystalline aluminium (n-Al) prepared by mechanical attrition and cold consolidation with average grain size in the 20–40 nm range. The stress-strain curves analysis shows an enhanced tensile strength and a reduced ductility of n-Al with respect to the coarse-grained material. Anyway, the strength increase is much lower than predicted by extrapolation of the Hall-Petch relation to nanometer sized grains. Moreover, the strengthening rate (Hall Petch coefficient) is strongly reduced in comparison with coarse-grained Al. The observed behaviour is discussed in connection with microstructure evolution during mechanical attrition.

A study of nanocrystalline iron and aluminium metals and Fe3Al intermetallic by mechanical alloying

Pure iron and aluminium powders and a mixture of composition Fe75Al25 were treated mechanically in a high-energy mill for up to 40 h. X-ray diffraction and analytical transmission electron microscopy were coupled to elastic energy dissipation and dynamic Youngs modulus measurements to study the structural transformation of the specimens induced by the mechanical treatment. A quantitative comparison between the structural behaviour of the pure elements and of the mixture was carried out. The role of the parameters such as the composition, the grain size and the activation energy during the process was examined in relation to the competing mechanisms of plastic deformation and recovery.

Synthesis by ball milling and characterization of nanocrystalline Fe3O4 and Fe/Fe3O4 composite system

Nanocrystalline Fe3O4 and a composite system constituted by nanocrystalline Fe and Fe3O4 have been synthesized by ball-milling commercial magnetite and an equimolar mixture of iron and magnetite powders. The physical parameters governing the milling process have been strictly controlled so as to achieve the nanocrystalline state of the precursor material and to avoid chemical reactions. X-ray diffraction and Mossbauer spectroscopy measurements have been carried out both on as-milled powders and on samples previously subjected to annealing treatments in the 100–600 °C temperature range. The results, providing information on the structural and compositional features of the produced samples, are discussed in terms of structural disorder which is healed by subsequent annealing. In the case of the composite system, this analysis indicates that a high mixing degree between the constituent phases has been reached. In particular, the presence of a sextet with anomalous hyperfine parameters in the Mossbauer spectr...

Hydrogen sorption in Pd-decorated Mg―MgO core-shell nanoparticles

Mg nanoparticles with metal-oxide core-shell morphology were synthesized by inert-gas condensation and decorated by in situ Pd deposition. Transmission electron microscopy and x-ray diffraction underline the formation of a noncontinuous layer with Pd clusters on top of the MgO shell. Even in the presence of a thick MgO interlayer, a modest (2 at. %) Pd decoration deeply enhances the hydrogen sorption properties: previously inert nanoparticles exhibit metal-hydride transformation with fast kinetics and gravimetric capacity above 5 wt %.

Metal-hydride transformation kinetics in Mg nanoparticles

The hydrogen sorption kinetics of magnesium nanoparticles prepared by inert gas condensation and coated by a magnesium oxide layer were investigated by a volumetric apparatus. The metal-hydride transformation was studied by transmission electron microscopy of the nanoparticles both in the as-prepared state and after hydrogen cycling. In small nanoparticles (≈35 nm) hydride formation proceeds by one-dimensional growth controlled by diffusion through the hydride, while the reverse transformation to metal involves interface-controlled three-dimensional growth of nuclei formed at constant rate. Large nanoparticles (≈450 nm) exhibit very low reactivity attributed to reduced probability of hydrogen dissociation/recombination and nucleation at the particle surface.

Microstructure-related anelastic and magnetoelastic behavior of nanocrystalline nickel

Nanocrystalline nickel was prepared by a planetary ball milling apparatus working in a vacuum of 10−4 Pa in the 150–300 K temperature range. The kinetic of the milling process and the microstructure evolution upon annealing were followed by x-ray diffraction and mechanical spectroscopy measurements. It was observed that thermal annealing up to 600 K induces a strong reduction of the internal strains without significant grain growth. Measurements of elastic energy dissipation and dynamic elastic modulus as a function of temperature showed that in the nanocrystalline samples, anelastic relaxation processes occur, with the activation energy of grain boundary diffusion. A systematic study of the magnetic field dependence of the dynamic modulus (ΔE effect) revealed a correlation between the ΔE magnitude and the strain values obtained by x-ray diffraction analysis.

Hydrogen storage and phase transformations in Mg–Pd nanoparticles

Microstructure refinement and synergic coupling among different phases are currently explored strategies to improve the hydrogen storage properties of traditional materials. In this work, we apply a combination of these methods and synthesize Mg–Pd composite nanoparticles by inert gas condensation of Mg vapors followed by vacuum evaporation of Pd clusters. Irreversible formation of the Mg6Pd intermetallic phase takes place upon vacuum annealing, resulting in Mg/Mg6Pd composite nanoparticles. Their hydrogen storage properties are investigated and connected to the undergoing phase transformations by gas-volumetric techniques and in situ synchrotron radiation powder x-ray diffraction. Mg6Pd transforms reversibly into different Mg–Pd intermetallic compounds upon hydrogen absorption, depending on temperature and pressure. In particular, at 573 K and 1 MPa hydrogen pressure, the metal-hydride transition leads to the formation of Mg3Pd and Mg5Pd2 phases. By increasing the pressure to 5 MPa, the Pd-richer MgPd in...

Structural evolution of mechanical alloyed Fe-Al powders after consolidation and thermal ageing

Elastic energy dissipation and dynamic Young’s modulus measurements coupled with x‐ray and energy‐dispersive analyses were employed to follow the structural transformation of bulk samples prepared by mechanical alloying Fe‐Al powders mixed in the atomic ratio Fe/Al=3. The results show that it is possible to synthesize a nanocrystalline bulk Fe3Al intermetallic phase by properly combining mechanical treatments of the powders with suitable temperature thermal aging. Driving mechanisms and transformation paths leading to this stable phase, not otherwise observed in the simply thermally aged Fe‐Al crystalline powders, are examined and discussed.

Mechanical behaviour of nanocrystalline iron and nickel in the quasi-static and low frequency anelastic regime

Abstract In this research, we made use of mechanical spectroscopy to study the anelastic behaviour of nanocrystalline Fe and Ni in quasi-static, low-frequency (0.01–10 Hz) regime. The elastic energy dissipation coefficient (Q−1) and the stress relaxation have been measured as a function of frequency and temperature, in a range of temperatures where appreciable grain growth is not expected to occur. The use of such low frequency probes puts into evidence a very strong change in the material response, induced by low temperature annealing (T

Structural and elastic behavior of Fe50Al50 nanocrystalline alloys

Pure iron and aluminum powders were mixed in the equiatomic ratio and mechanically alloyed in a high‐energy ball mill for different times. Structure refinement of x‐ray powder diffraction data was performed to study the structural transformations induced by mechanical and subsequent thermal annealing treatments. The mechanical alloying (MA) process induces a progressive dissolution of aluminum phase into the bcc iron phase. After 32 h of MA a single‐phase Fe(Al) bcc extended solid solution, with lattice parameter a0=2.891 A, average coherent domain size 〈D〉≊50 A, and lattice strain 0.5%, was observed. The annealing of the specimens after MA up to 8 h favored the aluminum dissolution in α‐iron and the precipitation of the Al5Fe2 phase, whereas a nanostructured B2 FeAl intermetallic compound was observed in the annealed samples which were previously milled for 8, 16, and 32 h. In the same specimens a minority cubic phase Fe3AlCX, anti‐isomorphous with perovskite, derived from contamination of ethanol and in...