A. Somoza

Yield and internal stresses in aluminum filled epoxy resin. A compression test and positron annihilation analysis

The influence of the filler content on the mechanical properties of an epoxy resin composite filled with aluminum powder was investigated. Compressive tests were performed at room temperature and at different strain rates. The response of the composites was also studied by positron annihilation lifetime spectroscopy. The dependence of the yield stress on the filler content is shown. The results are discussed in terms of a proposed model that takes into account the contribution of the filler powder. To this purpose information from positron spectroscopy is important since it allows to correctly evaluate the internal stresses introduced in the composite epoxy lattice by the metal filler.

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.

A comparative study of the post-quench behaviour of Cu–Al–Be and Cu–Zn–Al shape memory alloys

Abstract This paper reports a comparative investigation of the effect of quenching on the Cu–Al–Be and Cu–Zn–Al shape memory alloys by the use of several experimental techniques. In a first stage, the order–disorder transitions in these alloys have been characterized by means of modulated calorimetry. Results have proved that the A2⇋DO3 transition in Cu–Al–Be is first order with a latent heat of 1160 J/mol; the B2⇋L21 transition in Cu–Zn–Al is second order, and a peak in the specific-heat vs temperature curve has been observed. Secondly, the post-quench behaviour of these alloys, when subjected to some of the typical heat treatments used to stabilize the β phase, has also been studied by means of neutron diffraction, positron annihilation and highly sensitive calorimetry. A different post-quench time evolution of the martensitic transition temperatures has been found for the two alloys. For Cu–Al–Be, this evolution has been shown to be correlated with positron annihilation data, while, for Cu–Zn–Al, a correlation with neutron diffraction data has been established. These results show that the measured shifts in the transition temperatures induced by a quench are mostly due to an excess of vacancies in the case of Cu–Al–Be, and to an incomplete degree of L21 atomic order in Cu–Zn–Al.

Quenched-in defects and martensitic transformation in CuAlBe shape memory alloys

Abstract From positron lifetime and calorimetric measurements, the concentration and nature of the defects introduced by quenching from different Tq temperatures, and their influence on the martensitic transformation undergone by a CuAlBe shape memory alloy are studied. The main effect of quenches is to modify the concentration of single vacancies in the system. The obtained dependence of the thermodynamic properties (transition temperatures and latent heat) of the martensitic transition upon Tq, is explained in terms of the effective relative change of Be concentration due to quenched-in vacancies. Results are compared with published data for other Cu-based shape memory alloys.

Pre-precipitation study in the 7012 Al-Zn-Mg-Cu alloy by electrical resistivity

Abstract The early stages of pre-aging at near room temperature in an Al–Zn–Mg–Cu based commercial alloy were studied by electrical resistivity. The resistivity changes can be adequately described in the same terms of a Johnson–Mehl–Avrami (JMA) type function for the volume fraction growth of the Guinier–Preston zones or pre-precipitate solute clusters formed. For one specific case, resistivity results were correlated with those obtained using synchrotron-radiation small-angle X-ray scattering (SR-SAXS). Based on the analysis of the parameters of the JMA equation, the existence of a complex diffusion process during pre-precipitation is deduced. An apparent activation energy of 0.69±0.04 eV for the formation of the above mentioned particles is found. From this energy value, the migration of Mg-vacancy complexes is identified as the main microstructural mechanism responsible for the pre-precipitation.

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.

Current positron studies of structural modifications in age-hardenable metallic systems

The sensitivity of positron annihilation characteristics to the structural changes occurring in supersaturated metallic systems has been demonstrated and explained since the early 1980s. This knowledge is currently applied for positron annihilation spectroscopy (PAS) investigations regarding the main phenomena that govern the structural evolution of a supersaturated alloy when various thermal treatments are applied: (a) kinetics of vacancy-type defects retained after quenching; (b) solute migration; (c) formation of solute clusters and/or GP zones; (d) precipitation. The analysis of PAS data gives information on the chemical composition, the defect structure and the density of decomposition products of nanometric size. The present article, that includes an exhaustive repertory of works published since 1987, discusses the value of the contribution that PAS studies can give to physical metallurgy in the field of age-hardenable systems. Most examples are taken from the direct experience of the authors.

Secondary ageing in Al-Cu-Mg

Secondary ageing, that is microstructural evolution occurring at room temperature after short heating at temperatures above the metastable phase boundary of Guinier-Preston zones, has been studied for an Al-Cu-Mg alloy with a high Cu-to-Mg ratio. Combined data from positron annihilation spectroscopy, Vickers microhardness measurements and differential scanning calorimetry show that, on secondary ageing after 5 or 7min at 190°C, firstly, hardening takes place at a rate nine to 16 times slower than natural age hardening; secondly, vacancies slowly released by Cu-rich aggregates formed during the heat treatment at 190°C promote further formation of solute aggregates, with a time-dependent chemical composition; thirdly, the thermal stability of the structures formed during secondary ageing increases with increasing dwell time at room temperature; and, fourthly, solute aggregates formed at 190°C undergo a structural reorganization and possibly a change in the composition, leading to species with a different thermal stability. The slow release of vacancies from Cu-rich aggregates is proposed as one of the limiting factors of the hardening rate.

On the two-step ageing of a commercial Al - Zn - Mg alloy; a study by positron lifetime spectroscopy

Formation of Guinier - Preston zones and precipitation, induced by two-step ageing thermal treatments in an age-hardenable commercial Al - Zn - Mg-based alloy, have been studied by positron lifetime spectroscopy and Vickers microhardness measurements. In particular, an initial softening of the specimens, occurring in the early stages of the second ageing step (artificial ageing at ), appears to be correlated with a strong decrease in the positron lifetime, which indicates a partial dissolution of the GP zones. If the treatment at is interrupted at this stage, and the ageing continues at a lower temperature (either room temperature or ), the positron lifetime and the microhardness recover to the values reached at the end of the first ageing step (natural ageing at room temperature for five days). The kinetics of this process is discussed in terms of reconstruction of GP zones; the activation energy value of this process is also obtained.

Positron lifetime spectroscopy and decomposition processes in commercial Al-Zn-Mg-based alloys

Positron lifetime spectroscopy and Vickers microhardness measurements were used for studying the decomposition sequence of Al-Zn-Mg systems. Various microstructural changes (formation, dissolution and recuperation of GPII zones, formation of -particles) were induced by using isochronal annealing and multiple-step ageing thermal treatments. The experimental results give the following information: (a) changing the quenching temperature used for the initial homogenization treatment over the interval from -15 to has no apparent effect on the subsequent behaviour of the alloy; (b) the hardening effect of natural ageing is positively correlated with the density of small and uniformly distributed GPII zones existing in the alloy at the end of the treatment; (c) the interruption in the reversion stage of artificial ageing at is followed, at room temperature and at , by a recovery that takes place with the same characteristic time constant as governs GPII formation after the homogenization treatment; (d) the dependence of on the temperature agrees with activation energies for the migration of the solute that depend on the concentrations of Mg and of the quaternary additions (Cu, Mn).