G. Scipione

X-ray powder diffraction and Mossbauer study )of nanocrystalline Fe-Al prepared by mechanical alloying

Abstract Iron-aluminium alloys of composition Fe 50 Al 50 and Fe 75 Al 25 were produced by mechanical alloying (MA) of the pure elemental powders. A structural refinement of X-ray powder data on the mechanically alloyed products according to the Rietveld method has detailed the progressive dissolution of aluminium into the lattice of α-iron as a function of MA time. With respect to pure iron, a volume expansion of ≈ 3% is measured in both compositions mechanically alloyed for 32 h. In the iron and aluminium phases, the Debye-Waller static disorder increases as a function of MA time and the intrinsic shape of the peak profiles becomes predominantly Cauchy. These changes are accompanied by an increase in the average microstrain and by a reduction in the average crystallite size (which includes also the effect of dislocations). The Mossbauer spectra show that in the equiatomic case the initial sharp magnetic sextet of α-iron is progressively reduced and is replaced by a doublet, while for the Fe 75 Al 25 composition a broad magnetic sextet is eventually obtained. Thermal scans at 600°C of the specimens mechanically alloyed for 2, 4 and 8 h precipitate essentially the Al 5 Fe 2 phase. In the case of the Fe 50 Al 50 specimens, annealing of the powders mechanically alloyed for 16 and 32 h precipitates mainly the partially ordered FeAl intermetallic compound, whilst no ordering is obtained in the Fe 75 Al 25 case.

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.

Grain growth and phase stability in a nanocrystalline ZrO2-15w% Al2O3 ceramic

Abstract Nanocrystalline ZrO2-15w% Al2O3 ceramics were sintered in air and in vacuum between 700°C and 1200°C. The nanocrystalline composite was-prepared by dispersing n-ZrO2 and n-Al2O3 powders produced by Inert-Gas Condensation and Chemical Vapor Condensation, respectively, in methanol. Grain growth, phase transformations and pore structures of the composite were investigated and compared to results obtained for pure n-ZrO2. Pure n-ZrO2 was sintered to 95% relative density with a maximum grain size of 63 nm. In the nanocrystalline composite, the ZrO2 grains were smaller than in n-ZrO2 for all temperatures, with a maximum size of 32 nm after sintering at 1200°C. The relative density reached a maximum value of 97%. While the n-ZrO2 was completely monoclinic at all sintering temperatures, the nanocrystalline composite contained a large fraction of tetragonal phase, with a maximum of about 80% after sintering at 1100°C in vacuum.

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.

Anelasticity and structural stability of nanostructured metals and compounds

Abstract Internal friction and dynamic elasticity moduli on thin reeds of nanostructured Al, Fe, FeAl and Fe 3 Al intermetallics prepared by ball milling have been measured. A relaxational damping peak in pure metals at 350–500 K and a modulus increase at 450–500 K without appreciable grain growth has been detected. Moreover, magnetoelastic coupling dependent on grain size has been observed in iron. In the nanophase intermetallic aluminides a strong relaxational damping peak was observed in the 700–800 K range. These results are briefly discussed with reference to the anelastic behaviour of similar coarse grained materials.

Low-temperature deformation behavior of nanocrystalline 5 mol% yttria stabilized zirconia under tensile stresses

Abstract Nanocrystalline 5 mol% yttria stabilized zirconia ceramics (n-5Y-TZP) with relative densities of about 80 % and grain sizes between 30 and 60 nm were deformed plastically in tension at 1050 °C ( m ). Total strains exceeding 0.2 and strain rates as high as 10 −5 s −1 without failure were observed. Porosity was not found to be critical for the ability to deform without cracking. A preliminary estimate of the grain size exponent in the constitutive equation for superplastic flow yields a value of 1.3. Furthermore, the stress exponent is determined to be 2.7 and related to a deformation model based on grain boundary sliding as the rate controlling process.

Magnetoelasticity and internal strains in nanocrystalline nickel

Abstract The magnetic field dependence of the dynamic elastic modulus (ΔE effect) was studied in ball-milled nanocrystalline Nickel, as a function of the milling times and annealing treatments. The analysis of the crystallite size and internal strains has been performed as well by Fourier analysis of the X-ray spectra. A correlation between ΔE and the lattice strain calculated by X-ray analysis is observed, suggesting that the magnetic anisotropy is mainly determined by the internal strains.

Mechanical behaviour of NiAl and Ni3Al ordered compounds entering the nano-grain size regime

Abstract The elastic energy dissipation and dynamic modulus of several intermetallic compounds with the NiAl (B2) and Ni 2 Al (L1 2 ) composition and different grain sizes were measured. The grain dimensions were deduced by XRD data analysis. These alloys exhibit a temperature increasing background damping which raises faster as the grain size enter the nanometric regime. The elastic modulus of out of stoichiometry single crystals increases up to 20 % with temperature in a reproducible manner.

Anelastic properties and solid state reactivity of Fe-Al nanostructured intermetallic compounds

Abstract Iron and Aluminium elemental powders mixed in the atomic ratio Fe50Al50 and Fe75Al25 have been alloyed using an high energy mill system. The structural transformation of the milled products has been followed using XRD techniques. Samples obtained from cold-consolidation of the milled powders have been used for Elastic Energy Dissipation measurements (IF) and analysed in the temperature range 300–980 K. The XRD spectra corresponding to specimens milled for long times reveal the formation of the FeAl and Fe3Al nanostructured intermetallic compounds. The IF spectra show two relaxational damping peaks P1,P2 in the temperature range 600–980 K. The peak evolution for different milling treatments indicates a Zener and a grain-boundary relaxation mechanisms for P1 and P2 respectively.