Papers by Author: Vicente Recarte

Abstract: The wide damping maximum which is reported to appear in bones, involving both cortical and cancellous parts, between around 280 K and 420 K; has been determined to be a composition of different processes taking place at different temperatures in cancellous and cortical parts. In fact, in the present work the mechanical response of cow ribs bones has been analysed by coupling mechanical spectroscopy, differential scanning calorimetry, thermogravimetry and scanning electron microscopy studies. Cancellous part develops two damping maxima at around 320 K and 350 K. Cortical part exhibits a wide maximum in damping between around 310 K and 410 K and another damping relaxation between 390 K and 410 K. The physical-chemical driving force giving rise to the above relaxation processes are discussed.

Abstract: This chapter presents a review of the most recent and systematic works performed on the study of the effect of atomic order on the structural and magnetic properties of Ni-Mn-Ga ferromagnetic shape memory alloys. It is shown that a correlation between long-range atomic order and the martensitic and Curie temperatures can be established from the analysis of its evolution under high and low temperature thermal treatments. In particular, it is demonstrated that, irrespectively of the thermal treatment, both transformation temperatures increase with the increasing L21 atomic order degree, showing practically the same linear dependence on the nextnearest- neighbors atomic order parameter, in such a way that the effect of the atomic order on the relative stability between austenite and martensite can be quantitatively determined. In this respect, it is shown that the effect of atomic order on the martensitic (and also the premartensitic) transformation is directly related to the variation of the magnetic exchange coupling.

Abstract: The effect of the partial substitution of Fe by Co and Mn on the structural and magnetic properties of Fe-Pd alloys has been investigated by means of calorimetric, magnetic and neutron diffraction measurements. The addition of Mn increases both the reversible and irreversible MT temperatures whereas the addition of Co causes the opposite effect, which points out that that the compositional dependence of the transformation temperatures of the Fe-Pd alloys is not related to the Pd amount but to the e/a concentration. Furthermore, it is shown that the appearance of the BCT phase is directly related to the tetragonality of the FCT phase. On the other hand, the amount of FCT martensite that irreversibly transforms into the undesirable BCT phase is considerably reduced by both the Co and Mn doping. The substitution of Fe by Co seems to be especially interesting for magnetic field-induce strains applications since both the magnetic anisotropy and the saturation magnetization of the Fe-Pd FCT martensite are considerably increased with the Co addition.

Abstract: We have studied by means of positron lifetime measurements the role that vacancy type defects play in the martensitic transformation in magnetic Ni-Mn-Ga shape memory alloys. The measurements presented in this work have been performed in two ternary alloys transforming to modulate and non-modulated martensitic phases. Positron experiments have been realized at room temperature after subsequent isochronal heating at different temperatures up to a maximum temperature of 600°C. Positron results show a large variation of the average lifetime value with the isochronal annealing temperature in non-modulated samples. However, the response in the modulated samples is quite different. These results obtained in both samples are discussed in term of different type of positrons trapping defects and their evolution with the annealing temperature. They have also been compared with Differential Scanning Calorimetry experiments performed in the same samples. The work states the clear influence of the vacancy concentration on the martensitic transformations of these alloys.

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.