Nanoporous microtubes of a nickel-copper alloy were obtained from a Cu-44Ni-1Mn (wt%) commercial wire (200 μm diameter). A new synthesis method was established through three steps: 1) partial oxidation of the wire at 1173 K in air, 2) removal of the inner unoxidized core by chemical etching, 3) reduction in 10 bar hydrogen atmosphere. During oxidation, the segregation of Cu and Ni occurred because of their different diffusion coefficients in the corresponding oxides. As a consequence, pores were formed by Kirkendall effect and due to selective chemical etching of the different oxides. Additional porosity formed because of volume contraction during reduction with hydrogen. After reduction, the microtube shows a composition gradient from the inner wall (almost pure nickel) to the outer wall (almost pure copper). The process allowed to obtain microtubes with tuneable wall thickness and inner pores around 180 ± 80 nm. The morphological features developed suggest improved capillarity properties for applications in MEMS.

Ag-20 wt.%Cu (wt%) hypoeutectic alloy has been rapidly solidified by means of planar flow casting technique. Two fcc solid solutions have been identified by X-ray diffraction. Microstructures have been observed by electron microscopy. A refinement of the eutectic microstructure, as well as of the Ag-rich primary phase, has been observed for high quenching rates, leading to a hardness value up to 235 Vickers. The lattice parameter and phase fraction of the Ag-rich solid solution increase as a function of quenching rates (i.e. wheel speed). The solidification processes occurring during rapid quenching have been described on the basis of thermodynamic and kinetic arguments. (C) 2014 Elsevier B.V. All rights reserved.

In this work, the effect of cold rolling on structure, microstructure and mechanical properties of rapidly solidified Ag73.2Cu17.1Zr9.7 (at.%) amorphous/crystalline composite, containing 80 wt.% Ag, is studied. The as quenched composite is mainly characterized by small amorphous droplets, rich in Cu and Zr, uniformly dispersed in a Ag-rich f.c.c. matrix, as a consequence of the miscibility gap in the liquid. After cold rolling, no phase transformation is detected, whereas a microstructural refinement of the f.c.c. Ag-rich matrix is observed. No evidence of plastic deformation or fracture of the amorphous droplets embedded in the Ag-rich matrix is visible, suggesting that strain is mainly sustained by the crystalline matrix. Microhardness values of both as quenched and rolled ribbons range around 240 HVN, indicating that no significant strain hardening occurs. Experimental values for hardness are successfully fitted considering Orowan and load bearing models. The additional contribution from Hall-Petch effect in the crystalline matrix brings to an overestimation of the calculated values with respect to the experimental ones. Interparticle free distance is smaller than crystallites size, indicating that a fine dispersion of small hard amorphous droplets inhibits dislocation motion in the matrix more effectively than grain boundaries. (C) 2013 Elsevier B.V. All rights reserved.

Pure silver has been rapidly quenched (RQ) in ribbon form by means of planar flow casting with different cooling rates, obtained with different velocities of the cooling wheel. Ingots of pure silver have been subjected to severe plastic deformation (SPD) using constrained groove pressing and equal-channel angular pressing. The grain size of RQ and SPD samples has been analysed using electron microscopy, showing that the higher the solidification rate and plastic deformation, the finer the microstructure. Preferred orientations and quenched-in defect concentration have been determined from X-ray diffraction analysis, showing a correlation between density of defects and cooling rate. Higher defect concentration was found in the samples processed via SPD. Vickers hardness numbers range from 135 for SPD, 70 for RQ and 45 for annealed samples. Differential scanning calorimetry analysis revealed multi-step coarsening of the microstructure at various temperatures for all samples.

A quantitative description of the Mg-rare earth EV31 alloy during the first stages of the precipitation sequence using in situ small-angle X-ray scattering (SAXS) is presented. In situ evolutions of the size, volume fraction and number density of precipitates formed at 150°C and 200°C were obtained. A kinetic mechanism suggests that the precursor nanoparticles are nucleated at the beginning of the artificial ageing and, at 200°C, these particles grow mainly by accretion of the solute from the matrix without further nucleation. The particles grow within two regimes: (i) at the beginning of ageing, the growth is associated with solute diffusion with an apparent activation energies of 0.78eV (diffusion assisted by vacancies); (ii) further growth is associated with solute diffusion with an apparent activation energies of 1.16eV (bare solute diffusion). After about 2h at 200°C, corresponding to the condition of maximum hardness for this alloy, the present results indicate a volume fraction of about 1.5% occupied by particles with an average Guinier radius of 2nm. The evolution of the volume fraction at 150°C, studied for a similar time interval, is weaker than the one found at 200°C.