C. Macchi

Secondary precipitation in Al-Zn-Mg-(Ag) alloys

Secondary ageing of age-hardenable aluminium alloys occurs at temperatures below the solvus of GP zones after a preliminary ageing at a higher temperature. The phenomenon has technological interest, as it may be included in heat treatments giving a substantial benefit on the mechanical properties. In the present work, positron annihilation lifetime spectroscopy (PALS) is applied in combination with Vickers hardness measurements for an investigation on secondary ageing of Al–4wt.%Zn–3wt.%Mg–xAg, where x=0, 0.1, 0.2, 0.3, 0.5 wt.%. Ageing regimes have been characterised by the substantially different evolutions that are observed. The results shed light on the interplay between the formation of coherent solute aggregates (clusters or GP zones) and the precipitation of semi-coherent or incoherent precipitates, which are in competition to control the hardening effects. PALS data show that secondary ageing in the ternary Al–Zn–Mg alloys produces coherent aggregates even in the presence of a well-developed stage of semi-coherent or incoherent precipitation that is obtained if the alloys are first aged to peak hardness. In the presence of Ag, on the contrary, the effects of coherent aggregation during secondary ageing are observed only if the preliminary ageing is interrupted well before reaching peak hardness.

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.

CO2 Laser irradiation of GeO2 planar waveguide fabricated by rf-sputtering

GeO2 transparent glass ceramic planar waveguides were fabricated by a RF-sputtering technique and then irradiated by a pulsed CO2 laser. The effects of CO2 laser processing on the optical and structural properties of the waveguides were evaluated by different techniques including m-line, micro-Raman spectroscopy, atomic force microscopy, and positron annihilation spectroscopy. After laser annealing, an increase of the refractive index of approximately 0.04 at 1.5 µm and a decrease of the attenuation coefficient from 0.9 to 0.5 db/cm at 1.5 µm was observed. Raman spectroscopy and microscopy results put in evidence that the system embeds GeO2 nanocrystals and their phase varies with the irradiation time. Moreover, positron annihilation spectroscopy was used to study the depth profiling of the as prepared and laser annealed samples. The obtained results yielded information on the structural changes produced after the irradiation process inside the waveguiding films of approximately 1 µm thickness. In addition, a density value of the amorphous GeO2 samples was evaluated.

Chitosan-graft-poly(n-butyl acrylate) copolymer: Synthesis and characterization of a natural/synthetic hybrid material.

Two chitosan polymers with different deacetylation degree and molecular weight were subjected to grafting reactions with the aim to enhance the properties of these bio-based materials. Specifically, n-butyl acrylate in different proportions was grafted onto two different deacetylation degree (DD%) chitosan using radical initiation in a surfactant free emulsion system. Infrared spectroscopy was used to confirm grafting and products grafting percentage and efficiency were evaluated against acrylate/chitosan ratio and DD%. Thermal and structural properties and the behavior against water of the raw and grafted biopolymers were studied using several experimental techniques: differential scanning calorimetry, transmission electron microscopy, dynamic light scattering, water swelling, contact angle and positron annihilation lifetime spectroscopy. The influence of the grafting process on the morphological and physicochemical properties of the prepared natural/synthetic hybrid materials is discussed.

Positron trapping in BaTiO3 perovskite

Positron lifetime spectra in BaTiO3 single crystals were measured at temperatures up to 873 K, also at room temperature after quenching from temperatures up to 473 K. The explored temperature range includes the ferroelectric to paraelectric phase transition. The material displays an irreversible behaviour when heated, with anomalies around TC. The temperature dependence of the positron lifetime becomes reversible only after annealing above 550 K. The results suggest modifications of the charge state of non-equilibrium vacancies occurring at moderate temperature and microstructural changes taking place at high temperature. The reversible behaviour of the annealed crystal is consistent with thermally activated positron trap formation.

Early Solute Clustering in an AlZnMg Alloy

A. Dupasquier, R. Ferragut, M. M. Iglesias, C. E. Macchi, M. Massazza, P. Mengucci,G. Riontino, A. Somoza 1 Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci 32, I-20133 Milano, Italy (alfredo.dupasquier@polimi.it) 2 Istituto Nazionale di Fisica della Materia, Italy 3 IFIMAT, UNCentro and CICPBA, Pinto 399, B7000GHG Tandil, Argentina 4 Dipartimento di Chimica IFM, Università di Torino, Via P. Giuria 9, I-10125 Torino, Italy Dipartimento di Fisica e Ingegneria dei Materiali e del Territorio, Università Politecnica delle Marche, Via Brecce Bianche,I-60131 Ancona, Italy

Vacancy clustering in pure metals: some first principle calculations of positron lifetimes and momentum distributions

First results of a systematic study on the vacancy clustering process in Al, Cu, Mg and Nb are presented. To this aim first principle calculation of positron lifetimes and positron-electron momentum distributions were performed. We test the reliability of the computational scheme used by comparing some of the calculated results with experimental ones.

Vacancy-Solute Aggregates in Al-Zn-Mg-(Cu, Ag)

Positron annihilation spectroscopy in two versions (lifetime and coincidence Doppler broadening) has been applied to investigate solute/vacancy interactions when minor amounts (<1wt.%) of Ag or Cu are added to the alloy Al-4Zn-3Mg (wt.%) during ageing at 150°C. The results show early clustering of vacancies with Zn (and with Cu, if present). Ag displays a strong interaction with vacancies in competition with Mg and forms clusters that may help further aggregation of the other alloying elements during artificial ageing. High Mg concentration is observed at the misfit interfaces of semi-coherent or incoherent precipitates.

GeO2 glass ceramic planar waveguides fabricated by RF-sputtering

GeO2 transparent glass ceramic planar waveguides were fabricated by a RF-sputtering technique and then irradiated by a pulsed CO2 laser. Different techniques like m-line, micro-Raman spectroscopy, atomic force microscopy, and positronbannihilation spectroscopy were employed to evaluate the effects of CO2 laser processing on the optical and structuralbproperties of the waveguides. The GeO2 planar waveguide after 2h of CO2 laser irradiation exhibits an increase of 0.04 inbthe refractive index, measured at 1542 nm. Moreover, the technique of laser annealing is demonstrated to significantlybreduce propagation loss in GeO2 planar waveguides due to the reduction of the scattering. Upon irradiation of the surfacebthe roughness decreases from 1.1 to 0.7 nm, as measured by AFM. Attenuation coefficients of 0.7 and 0.5 dB/cm at 1319 and 1542 nm, respectively, were measured after irradiation. Micro-Raman measurements evidence that the system embeds GeO2 nanocrystals and their phase varies with the irradiation time. Moreover, positron annihilation spectroscopy was used to study the depth profiling of the as prepared and laser annealed samples. The obtained results yielded information on the structural changes produced after the irradiation process inside the waveguiding films of approximately 1 μm thickness.

Vacancy–solute interactions during ageing of a cold-worked Mg–RE-based alloy

The vacancy–solute interactions during artificial ageing at 250○C of cold worked samples of a commercial magnesium alloy WE54 (Mg–RE based) were studied by coincidence Doppler broadening of positron annihilation radiation and positron annihilation lifetime spectroscopy. The results show that, in the as-cold-worked state, the vacancies are associated with dislocations that are generated by the cold work and that, after artificial ageing at 250○C, the vacancies are associated with solute elements and help the formation of precipitate precursors. This mechanism accelerates the formation of hardening precipitates without any apparent changes in the precipitation sequence and in the products of the decomposition of the supersaturated solid solution. The present study demonstrates that the stronger hardening response achieved in the cold-worked samples originates from the presence of a higher concentration of vacancies that is introduced by the cold work and is retained in the first few minutes of ageing.