O.A. Lambri

Mechanical dynamical spectroscopy in CuLi alloys produced by electrodeposition

Abstract Cu-Li alloys obtained by electrodeposition were studied by means of the mechanical dynamical spectroscopy (MDS) technique (also called mechanical spectroscopy), which consists of the simultaneous measurement of the damping and the elastic modulus as a function of temperature. In this work the MDS measurements were performed as a function of temperature. Also the strain amplitude dependent damping behaviour was studied. The damping spectrum of pure copper as a function of temperature shows low and the intermediate temperature grain boundary damping peaks at approximately 500 and 750 K respectively. It should be pointed out that, in the Cu-Li alloys a new peak at 625 K was found. This new peak could be the result of two physical effects, which are overlapped. The first effect would be a peak owing to the melting of the Cu 18 at.% Li crystals and the second effect would be a peak owing to the Zeners pair reorientation mechanism. The influence of the thermomechanical state of the samples before being electrodeposited was investigated, and the changes in the MDS spectra, measured during heating and cooling were also investigated.

Study of non-linear effects related to the snoek-köster relaxation in Nb

Abstract The variation of mechanical loss (internal friction) with the amplitude of oscillation, e, was studied in NbO with various oxygen contents (100–1200 at.ppm) by applying an inverted torsion pendulum ( f ≅ 3 Hz ). The Snoek—Koster maximum was analysed with respect to various non-linear effects: amplitude-dependent damping, peak temperature shift, peak width and asymmetry. The asymmetry was judged from the low- and high-temperature halfwidth of the maximum, r LT and r HT . In all specimens we observe an increase in the peak heights and r HT with increasing e, whereas the peak temperatures and r LT decrease with e. The non-linear effects are discussed with respect to the theory of the SK relaxation involving kink-pair generation in screw dislocations in the presence of interstitial foreign atoms.

Structural Changes in EPDM Subjected to Ageing in High Voltage Transmission Lines

This work presents a study of the structural changes in aged EPDM (ethylene-propylene-diene monomer) from outdoor high voltage transmission lines. Samples were taken from failed and non-failed insulators after up to ten years in service on a 33 kV line. Infrared absorption, hydrophobicity index, dynamical mechanical analysis (DMA) and dielectric relaxation (DR) were employed as experimental techniques. The same experimental techniques were also applied to new samples of EPDM that were subjected to high electric fields (up to 14 kV/mm) for several hours. Results from DMA and DR techniques are in good agreement and, together with previous results from neutron irradiation techniques, suggest that the experimental data obtained from outdoor-aged and laboratory tested samples may be explained by a decrease in the degree of crystallinity due to the interaction between electrical and mechanical effects in the dielectric. The proposed model is based on the disentanglement of the crystallites by forces originated in the interaction of the electric field with the polar groups located in the amorphous phase of the polymer matrix. An alternative explanation, based on a relaxation process appearing at the interfaces between the filler (ATH) and the polymer matrix conflicts with several experimental results presented in this work.

Study of the phases in a copper cathode during an electrodeposition process for obtaining Cu–Li alloys

Abstract The evolution of Cu–18at%Li crystals, Cu2O, Li2CO3, and CuO phases, which nucleate and grow during the electrodeposition process in a copper cathode, was examined. The electrodeposition process was mainly responsible for promoting the formation of copper oxides and lithium-carbonates in the cathode. The evolution of both the quantity of Cu–Li crystals formed at the surface of the cathode and the total lithium concentration of the sample as a function of the electrodeposition time was studied.

Effect of the oxygen in the evolution of the microstructure in a Cu-18 at.% Li alloy

Abstract The evolution of the microstructure in a Cu–18 at.% Li alloy as a function of temperature has been studied through thermogravimetry (TGA), neutron diffraction (ND) and X-ray diffraction (XRD) studies. The studied alloy was in the form of powders obtained by an electrodeposition process. The alloy oxidation during heating in air plays a very interesting behaviour giving rise to the decrease in the lithium content in solid solution in the alloy. In fact, the oxidation of the alloy from about 600 K leads simultaneously to the reduction in the lithium content in solid solution and to the appearance of lithium dioxide, Li2O2, and copper oxides, Cu2O, in the alloy.

On the precipitation processes in commercial QE22 magnesium alloy

Abstract Electrical resistivity measurements, differential thermal analysis, mechanical spectroscopy and small angle neutron scattering studies have been performed on commercial QE22 magnesium alloy. The aim of this work is to resolve the appearance of Guinier – Preston zones and also to survey the small angle neutron scattering technique for various temperatures in this alloy. The coupling of all the employed experimental techniques in this work is crucial for detecting the precipitation of the Guinier – Preston zones. This is due to the strong changes in the microstructure produced by the main precipitation process, which starts at about 440 K and ends at about 640 K. We found that the temperature at which Guinier – Preston zones start to grow is about 473 K and that the dissolution temperature is 537 K.

Study of the δ reversion process in 8090 alloys

Al-Li based alloys have been widely studied for their potential applications in the aerospace industry, owing to the fact that they exhibit a higher Young`s modulus and lower density than the conventional Al-based alloys. In this work, the microstructure evolution between room temperature (RT) aging of the as-quenched sample, and the microstructure associated with high temperature treatments in a 8090 alloy has been studied. Thermoelectric-power measurements (TEP), X-rays diffraction (XRD) and Vickers micro hardness test (VMH) were employed in this work in order to know simultaneously the microstructural state and the mechanical behavior. Due to the precision of the TEP measurements, the present study shows clearly the kinetics of the {delta}{prime} reversion process. Therefore, the best reversion treatment in order to reduce the hardness of the as-quenched sample after a solid solution treatment can be accurately determined.

Internal stresses in the electrostriction phenomenon viewed through dynamic mechanical analysis studies conducted under electric field

A model for studying the mechanical internal stresses into dielectric materials is developed in the present work. The model takes into account the formalism of inclusions in continuous media. The dielectric material is assumed to be partitioned in different cubes which form a sizeable bulk material in such a way that a given cube can be of the dipolar phase and its neighbor can be of the same phase or the matrix. The behavior of the internal stresses promoted by the electrostriction phenomenon can be monitored by studying the behavior of both the misfit coefficient related to the strain misfit and the transfer of elastic energy process. The equations obtained through the here presented model are calculable using magnitudes from mechanical tests, in particular dynamic mechanical analysis, which is very sensitive to changes in the microstructure. Dynamic mechanical analysis as a function of the electric field is reported, perhaps for the first time in literature, for the study of dielectric materials, giving rise to a useful tool for studying the behavior of internal stresses in dielectric materials.

Mechanical Spectroscopy and Neutron Diffraction Studies in Fe-Al-Si Alloys

Mechanical spectroscopy and neutron diffraction studies were performed on several samples with compositions Fe - 25 at. % (Al + Si) and Fe - 15 at. % (Al + Si). It was found that the solute grain boundary relaxation is strongly dependent on the degree of order in the sample. A decrease in the orderdegree allows the development of a relaxation peak at around 1000 K during cooling from 1200 K. In contrast, if the order degree is not decreased, the grain boundaries remain locked and consequently the relaxation peak does not appear. The magnetic response both in the asquenched and after thermal treatment states was also explored and correlated to the microstructural state.

Magnetic memory effect in magnetite charged polypropylene composite

Abstract The behaviour of damping and dynamic shear modulus in polypropylene charged with either different volume fraction or size of magnetite (Fe3O4) particles, as a function of the applied magnetic field at 318, 353 and 403 K; has been studied. An increase of the alternating magnetic field oscillating with 50 Hz, leads to an increase of the damping. In addition, during the subsequently decreasing alternating magnetic field, the damping decreases, but a hysteretic behaviour appeared. The behaviour of the damping and the elastic modulus under the application of an alternating magnetic field was explained by the development of a magnetic fatigue damage occurring around the particle interface due to oscillation of magnetite particles. In contrast, during the increase of a direct magnetic field, the damping decreases and the elastic modulus increases. Measurements performed at 353 and 403 K allowed observing the interaction process among the particles of magnetite in the polymer matrix. After the decrease in the direct magnetic field, from the maximum reached value, damping and modulus remain smaller and higher, respectively; giving rise to a memory effect. In addition, a mesoscopic description of magnetite filled polymer composite materials has been performed in the continuous media by considering the interaction between magnetic and mechanical forces. Theoretical predictions of here developed model were qualitatively applied with good success for explaining the memory effect in magnetite filled polypropylene under the application of a direct magnetic field.