P. Mengucci

Early ageing mechanisms in a high-copper AlCuMg alloy

Abstract Early stages of transformation of a metastable Al4.4Cu1.7Mg alloy have been studied by DSC, microhardness and TEM, during natural ageing. The progressive disappearance of the first exothermal signal of the as-quenched material, the corresponding initial hardness increase, and TEM/SAD observations, suggest the formation of GP+GPB zones.

Further results on cerium fluoride crystals

Abstract A systematic investigation of the properties of cerium fluoride monocrystals has been performed by the “Crystal Clear” collaboration in view of a p

Structural characterization of biomedical Co-Cr-Mo components produced by direct metal laser sintering.

Direct metal laser sintering (DMLS) is a technique to manufacture complex functional mechanical parts from a computer-aided design (CAD) model. Usually, the mechanical components produced by this procedure show higher residual porosity and poorer mechanical properties than those obtained by conventional manufacturing techniques. In this work, a Co-Cr-Mo alloy produced by DMLS with a composition suitable for biomedical applications was submitted to hardness measurements and structural characterization. The alloy showed a hardness value remarkably higher than those commonly obtained for the same cast or wrought alloys. In order to clarify the origin of this unexpected result, the sample microstructure was investigated by X-ray diffraction (XRD), electron microscopy (SEM and TEM) and energy dispersive microanalysis (EDX). For the first time, a homogeneous microstructure comprised of an intricate network of thin ε (hcp)-lamellae distributed inside a γ (fcc) phase was observed. The ε-lamellae grown on the {111}γ planes limit the dislocation slip inside the γ (fcc) phase, causing the measured hardness increase. The results suggest possible innovative applications of the DMLS technique to the production of mechanical parts in the medical and dental fields.

Hydrogen kinetics in magnesium hydride: On different catalytic effects of niobium

The hydrogen desorption kinetics from pure and Nb-doped MgH2 samples was studied as function of the Nb concentration (6×10−4<[Nb∕Mg]<5×10−2). Structural and kinetics analyses indicate that Nb acts as catalyst both when Nb atoms aggregate forming NbH clusters dispersed in the MgH2 and also when Nb is contained as atomic impurity. It is suggested that the local atomic environment around the Nb atom acts as seed for h-Mg phase nucleation which constitutes the rate limiting step for the hydrogen kinetics while, when the Nb concentration exceeds about 1at.%, the rate limiting step is hydrogen diffusion.

Laser-reactive ablation deposition of silicon-nitride films

Silicon-nitride films were deposited on silicon waters by XeCl (308 nm) excimer-laser ablation of silicon in low-pressure (0.05–5 mbar) ammonia atmospheres. Series of 10 000 pulses at the repetition rate of 8 Hz were directed to the target surface. The fluence was set at about 5 J/cm2. Pulse duration was about 30 ns. The deposited films were characterized by different techniques (X-ray diffraction, X-ray photoelectron spectroscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, scanning electron microscopy, profilometry). Silicon-nitride films with thickness close to 1 μm were obtained under specific experimental conditions.

Oxidation interference in direct laser nitridation of titanium: relative merits of various ambient gases

Abstract A comparison is performed between the relative merits of using an ambient NH 3 atmosphere or a stoichiometrically equivalent N 2 -H 2 (1:3) mixture for the direct laser nitridation of titanium samples by multipulse excimer laser ( λ = 308nm) irradiation. It is shown that the N 2 -H 2 mixture proves more effecient in inhabiting the unwanted interference of the oxidation process and that the whole process proceeds through the action of an erosion plasma on a molten layer covering the laser-irradiated area.

Hardening nanostructures in an AlZnMg alloy

The formation of nanoscale and sub-nanoscale solute aggregates (clusters, Guinier–Preston zones and precipitates) in an AlZnMg alloy (Al–2.1 at.% Zn–1.5 at.% Mg) has been followed by a combination of experimental techniques with the aim of correlating the properties of the aggregates with their thermal history. The choice of thermal treatments was guided by the results of mechanical and calorimetric characterizations, supported by transmission electron microscopy for the identification of the morphology of the aggregates. Positron annihilation spectroscopy (using two variants of this technique, coincidence Doppler broadening and lifetime spectroscopy) was adopted for determining the local chemistry in the proximity of open volume defects. The geometrical parameters of the distribution (size, volume fraction, numerical density of the solute aggregates) were obtained by small-angle X-ray scattering. The results of the investigation provide new information regarding: two families of vacancy-rich clusters formed during or immediately after quenching; Guinier–Preston zones formed at 95°C after room-temperature pre-ageing; growth of η′ and η phases at 150°C; solute clusters formed at room-temperature in conditions of secondary ageing after preliminary heating at 150°C.

Characterization of C-N thin films deposited by reactive excimer laser ablation of graphite targets in nitrogen atmosphere

Abstract Carbon nitride films were deposited at room temperature on 〈111〉 Si substrates by XeCl laser ablation of graphite in low pressure (1–50 Pa) N2 atmosphere at a fluence of 12 J/cm2. N/C atomic ratios up to 0.5 were inferred from Rutherford backscattering measurements. Different diagnostic techniques (ARXPS, FTIR transmission spectroscopy, EDS, SEM and XRD) were used to characterize the deposited films. XRD spectra indicate a polycrystalline structure of the films.

Nb clusters formation in Nb-doped magnesium hydride

Extended x-ray absorption fine structure spectroscopy, x-ray diffraction and transmission electron microscopy were used to analyze the Nb coordination and clustering in Nb-doped (5 at. %) h-Mg film samples deposited by rf magnetron sputtering. Results show that the catalytic effect of the Nb doping in the H2 absorption and desorption kinetics is connected with the formation of Nb nanoclusters dispersed in the host matrix. The H2 desorption from β-MgH2 is favored by local elastic stresses produced by β-NbH0.89 clusters on the MgH2 matrix that reduces the stability of the hydride phase and by preferential paths in the nanocomposite hydride.

A study of the high-temperature ageing of Al-Cu-Mg-Ag alloy 201

Abstract The microstructural evolution of Al-Cu-Mg-Ag alloy 201 during ageing at 190°C after solution treatment, quenching and room-temperature annealing has been investigated by a joint differential scanning calorimetry and transmission electron microscopy (TEM) analysis, with the aim of clarifying the early stages of ω-CuAl2 precipitation. After room-temperature ageing, the TEM observations show the presence of both copper Guinier-Preston zones (GPZs) and a phase exhibiting a hexagonal structure with lattice parameters a = 0.507nm and c = 0.692 nm, here called Ω′. At this stage, the Al zone axis electron diffraction pattern exhibits a structure completly different from those reported in literature when the ω phase is present. This pattern can be interpreted by taking into account the GPZ and ω structures. During ageing at 190°C the copper GPZ formed at room temperature immediately disappear while the well known precipitation sequence Θ″→Θ′ occurs. At the same time, ω‘slowly disappears while ω preci...