Papers by Author: O.A. Lambri

Abstract: The behavior of internal friction Q^-1 and dynamic shear modulus has been studied in polypropylene charged with either different volume fraction or size of magnetite (Fe₃O₄) particles, as a function of the applied magnetic field at 318 K. An increase of the alternating (AC) magnetic field oscillating with 50 Hz, leads to an increase of the internal friction. In addition, during the subsequently decreasing alternating magnetic field, the internal friction decreases, but a hysteretic behavior appeared. In fact, the internal friction of the decreasing part of magnetic field amplitude is found to be smaller than during the previously increasing amplitude part of the treatment with the alternating magnetic field. Subsequent magnetic treatment cycles, lead to successively decreasing internal friction. In contrast, during the increase of a direct (DC) magnetic field, the internal friction decreases and the elastic modulus increases. The behavior of the internal friction and the elastic modulus during the application of an oscillating magnetic field (AC) is discussed on the basis of the development of both, a new zone with different rheological characteristics than the matrix but of the same material (self-inclusion), and/or a deteriorated or damaged zone (chain’s cuts) of the polymer matrix in the neighborhood of the magnetite inclusion. These effects are promoted by the movement or small relative rotation of the magnetite particles related to the surrounding matrix controlled by the oscillating field. The behavior of the internal friction and elastic modulus during the application of a direct (DC) magnetic field is discussed on the basis of the increase of the internal stresses into the polymer matrix due to the promotion of the magnetomechanical stresses.

Abstract: 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.

Abstract: In this work we present a novel procedure, involving linear viscoelastic analysis, to discriminate the two possible contributions of the observed damping peak which appears around 840 K – 1050 K in mechanically deformed high purity single-crystalline molybdenum. An interesting feature of the procedure is that, for low damping samples, it can efficiently resolve experimental peaks that result from the superposition of different processes independently of the ratio between their relaxation strengths. This allows us to confirm that two different relaxation processes appear in molybdenum in the temperature range about 0.3 Tm, one around 840 K, and the other one near 1050 K. These can be related to diffusion and to a coupled mechanism involving creation and diffusion of vacancies, respectively.