E. Sampaolesi

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.

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.

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

Anelastic and structural behavior of ball milled nanostructured iron

Abstract Mechanical spectroscopy and XRD analysis were employed to study the anelastic behaviour and structural evolution of nanostructured iron prepared by mechanical attrition under different conditions. The results show that the thermally induced structural relaxation, without extensive grain growth, leads to strong reductions of the microstrain, dynamic modulus recovery and significant modifications of the anelastic relaxation spectra. These results are compared with those obtained by other structure sensitive techniques. The role of the interfaces and of their structural evolution in determining the observed behaviour of different physical properties is discussed.

Thermal evolution of ball milled nanocrystalline iron

Abstract Mechanical spectroscopy and X-ray diffraction measurements were employed to investigate the thermally induced structural evolution of nanocrystalline iron, prepared by mechanical attrition through different ball milling equipment in different structural states. The evolution of the elastic modulus, grain size and microstrain as a Junction of temperature and after selected isothermal treatments indicates that the rearrangement of structural configuration at the interfaces may stabilise the nanostructure against grain growth.

A combined study of nanocrystalline aluminium by X-ray diffraction and mechanical spectroscopy

Abstract Nanocrystalline aluminium was prepared by ball milling in different conditions. The milled powders were characterized by X-ray diffraction in order to determine accurately the crystal size and the internal strains. Mechanical spectroscopy measurements in the 300–700 K temperature range were performed with a torsion pendulum on consolidated nanocrystalline powders. The anelastic spectrum is characterized by a broad internal friction peak and an exponential background. These data are compared with those obtained on a coarse grained sample and correlated with the information derived from X-ray diffraction analysis.

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.

Upward modulus trend in NiAl and NiFeAl single crystals

Abstract Dynamic Young modulus, torsion modulus and elastic energy dissipation were measured in Ni 56.2 Fe 0.3 Al 43.1 Ta 0.3 Mo 0.1 (NiAl crystal) and Ni 56.6 Fe 12.3 Al 30.2 Hf 0.6 (Al 2 O 3 ) 0.3 (NiFeAl crystal). The elastic constant C ′=( C 11 − C 12 )/2 has in both alloys a positive temperature derivative: (d C ′/d T )>0 from 150 K up to 1300 K. The elastic energy dissipation exhibits a Debye-like peak whose activation energy is 2.36±0.05 eV in NiAl and 1.98±0.09 eV in NiFeAl. The peak is described as a Zener’s pair reorientation.

Anelastic Behaviour of Nanocrystalline Aluminium Prepared by Mechanical Attrition

Nanocrystalline aluminium was prepared by mechanical attrition with different equipment and in different thermodynamic conditions. The milled powders were characterized by X-ray diffraction and mechanical spectroscopy measurements. A coarse grained aluminium sample was measured as well. An internal friction peak, attributed to a grain boundary sliding mechanism, was observed in all samples. The results indicate that the activation energy for grain boundary sliding is lowered in nanocrystalline samples, due to the enhanced grain boundary diffusion. A significant decrease of the high temperature elastic energy dissipation was observed too. This is a consequence of the reduced dislocation density and activity in the nanograins.