Materials Science Forum Vols. 475-479

Abstract: One of the characteristics not to be ignored of the magnetostrictive material used in active vibration control is its hysteresis. In this paper this characteristic has been studied. At first a mathematical model of the hysteresis loop has been obtained on the basis of experiment. This model depends on the frequency and the amplitude of the alternating current inputted to magnetostrictive actuator. Then further modeling of the magnetostrictive actuator has been put forward.

Abstract: Damage characterization was carried out by comparing single mode sensor signals collected before and after the damage event. By subtracting the signals of both conditions from each other, a scatter signal is produced which can be used for damage localization. By using Gabor wavelet to analyze single mode Lamb wave recorded before damage and scatter signal, the difference of time-of-flight can be obtained. Combining with the ellipse technique, the localization experiment on fiberglass plate of dimension 100×100×2mm is completed.

Abstract: The defect distributions have been investigated using positron lifetime spectroscopy on amorphous and nanocrystalline Pr2Fe14B samples, produced by melt-spinning and nanocrystallization route. The main two components can be concluded that were ascribed to vacancy-like defects in the intergranular layers or the interfaces, and microvoids or large free volumes with size compared to several missing atoms at the interactions of the atomic aggregates or the crystallites. The remarkable changes in the positron lifetimes from the amorphous structure to the nanocrystalline with varied sizes can be interpreted, indicating that the structural transformation and the grain growth induce the defect distribution changes occurring at the interfaces with different shape and size.

Abstract: Nanocomposite Pr7.5Dy1Fe71Co15Nb1B4.5 ribbons were prepared by melt-spinning and subsequent annealing. Interaction domains were imaged using magnetic force microscopy (MFM) because of strong exchange coupling between hard and soft magnetic grains. Coercivity was determined by exchange coupling pinning field. But the magnetization reversal of nanocomposite magnets were different from that of traditional single phase permanent magnets.

Abstract: Annealing effect on the magnetic properties of Fe-Pt nanoparticle was investigated. Fe-Pt nanoparticles were synthesized by the chemical reduction of H2PtCl6 and the decomposition of Fe(CO)5 in the presence of PVP polymers. The samples were annealed at several temperatures from 350°C to 600°C and the structural and magnetic properties were studied by TEM, XRD and magnetization measurements. FePt nanoparticles as-synthesized showed super-paramagnetism and annealed samples at temperatures more than 400°C, ferromagnetism. The crystal structure of as-synthesized sample was disordered fcc, but chemically ordered fct, when it was annealed at more than 400°C. The annealing effects are the concatenation or agglomeration of nanoparticles accompanied with the increase in particle diameter and with the ordering of the FePt superlattice. The critical diameter for emergence of ferromagnetism of FePt particle has been considered to be 3.6 to 5.6 nm in mean diameter, under the existence of the ordered L10 phase The blocking temperature as a function of the annealing temperature was investigated.

Abstract: Magnetic stability of a Fe-Cr-Co permanent magnet material at high temperatures has been investigated. Changes of microstructures and hyperfine field distributions after high temperature holding were analyzed by transimission electron microscopy (TEM) and Mössbauer spectroscopy. The results of experiments indicate that the changes of magnetic properties of the Fe-Cr-Co alloy tend to decrease, and then gradually keeps a certain level with increasing the holding time at high temperature. TEM observations show that microstructures of the alloy do not change distinctly after high temperature holding. Hyperfine interaction study through Mössbauer spectroscopy finds that the composition difference between α1 and α2 phases increases, and the composition of the two phases fluctuates. This study attributes the change of magnetic properties to element redistribution of Cr atom rather than the change of microstructure.

Abstract: The local environment issued from the phase decomposition in Fe-25Cr-12Co-1Si alloy during step ageing has been studied by 57Fe Mössbauer spectrometry. The phase decomposition is influenced by both ageing time and temperature. It processes rapidly when ageing temperature is high. The formation of different kinds of (Fe, Co)-rich, Fe-rich and Cr-rich domains with independent hyperfine parameters during phase decomposition was characterized by hyperfine field distribution. The magnetic hardening of the alloy was discussed basing on single domain particle model.

Abstract: Manufacture of 2:17 Sm-Co magnets by powder injection molding was investigated. The binder of thermoplastic polymer was selected as the wax-based system including paraffin wax, stearic acid and high density polyethylene. Before mixing with paraffin wax and high density polyethylene, the powder of 5-8 µm was coated by stearic acid. The molding compacts were obtained under 200°C without deficits. Solvent debinding and thermal debinding were combined to remove the binder. The basic magnetic characteristics of the specimen were of the same level as those by powder metallurgy technique, which indicated that to fabricate 2:17 Sm-Co magnets by powder injection molding was feasible.

Abstract: The microstructure and the coercivity of nanocomposite Nd4.5Fe76.5-xGaxCo1.0B18 (x=0~0.6at%)bonded magnets were studied. The results showed that addition of small amount of Ga in Nd4.5Fe76.5-xGaxCo1.0B18 made the grains finer and more uniform. A notable peak of coercivity was observed in the variational curve of coercivity with annealing time when Ga content was low (x=0.2). This phenomenon could be explained by the microstructure and the exchange coupling interaction of two-phase nanocomposite.

Abstract: Electrochemical impedance spectroscopy (EIS) tests of anisotropic Nd-Fe-B magnets prepared by spark plasma sintering (SPS) technique were carried out to investigate their corrosion kinetics in different solutions. According to the Nyquist and Bode diagrams, convenient equivalent circuit was modeled and the values of the proposed equivalent circuit components were calculated. It was found from the EIS plot formats that the electrochemical reaction of SPS Nd-Fe-B magnets was controlled by the step of surface charge transfer i.e. activation polarization in alkaline solution, while controlled by step of active substances adsorption on the magnet surface in salt and acidic solutions. Compared with conventional sintered Nd-Fe-B magnets, the SPS Nd-Fe-B magnets possess better chemical stability in saline and weak acidic environment.