Materials Science Forum Vols. 534-536

Abstract: This paper describes the preparation of tungsten powder and the various types of tungsten alloys and their application.

Abstract: The oxidation of (W,Mo)Si2 powders has been examined at 400, 500 and 600°C for 12.0 hours in air. It was shown that the low temperature oxidation resistance of (W,Mo)Si2 was worse than that of MoSi2, and they showed great changes in mass, volume and colour. At 500°C, the amount of volume expansion of (W,Mo)Si2 was as high as about 7~8 times and color changed from black to yellow after 4.0h with MoO3, WO3, (W,Mo)O3 and amorphous SiO2 as main reaction products. It took about 8.0h to obtain the same results for MoSi2. The mass gain and oxidation rate were relatively slower at 400°C and 600°C than that at 500°C. These were probably due to the lower oxidation rate at 400°C and the protective silica glass on surface of powders formed from the volatilization of MoO3, WO3 and (W,Mo)O3 at 600°C, which would restrain the diffusion of molybdenum from matrix to exterior and oxygen from outside to inside and the further accelerated oxidation.

Abstract: The effects of the content of molybdenum on the dynamic properties of tungsten heavy alloys were investigated.Increase in mechanical properties was observed in molybdenum-added tungsten heavy alloys, due to the refined microstructure. On the other hand, decrease in mechanical properties was also observed in the alloys with high molybdenum concentrations, due to the decreased strength of the matrix phase and the precipitation of an intermetallic phase. Hopkinson bar dynamic test under strain rates ranging from 2000 s-1 to 8000 s-1 at room temperature revealed that the flow stress of tungsten heavy alloys depended strongly on the strain, strain rate, and the content of molybdenum. The variation of flow stresses was caused by the competition between work hardening and heat softening in the materials at different strain rates. The high temperature strength of the matrix phase was increased by the addition of molybdenum, which enhanced the strength of the tungsten heavy alloys in high strain rate test.

Abstract: Microstructure and soft magnetic properties of bulk amorphous and/or nanocrystalline Fe73.5Cu1Nb3Si13.5B9(Finemet) alloys prepared by consolidation at a ultra-high pressure (5.5GPa) were investigated in this paper. The relative density of the bulk sample 1 (from amorphous powders) was 98.5% and the grain sizes were about 10.6nm. While the relative density and grain sizes of bulk sample 2 (from nanocrystalline powders) are 98% and 20.1nm, respectively. Particularly, the bulk samples exhibited a good combined magnetic property: for Sample1, Ms=125emu/g and Hc=1.5Oe; for Sample2, Ms=129emu/g and Hc=3.3Oe. The success of synthesizing the nanocrystalline Fe-based bulk alloys will be encouraging for the future development of bulk nanocrystalline soft magnetic alloys.

Abstract: Titinium carbide (TiCx) was produced by self-propagating high temperature synthesis (SHS) method. The morphology and non-stoichiometric number of the SHS product were observed by scanning electron microscopy and neutron diffractometry, respectively. Tubular titanium carbide with hole inside was formed with different non-stoichiometric number (x), which value increased with combustion temperature.

Abstract: Experiment was carried out to investigate the effect of Ba Stearate as a reducing agent on the magnetic and physical properties of anisotropic BaFe2-W type ferrite magnets. It was found that the magnetic properties of BaO･8.5Fe2O3 were improved by adding 0.3 wt% of Ba Stearate, 0.5 wt% of SiO2, and 0.5 wt% of CaO together. The optimum conditions for making magnets were as follows; chemical composition: Ba1.029Ca0.127Si0.097C0.053Fe2+ 2.456Fe3+ 15.392O27, semisintering condition: 1350 °C×4.0 h in nitrogen gas atmosphere, drying condition: 180 °C×2.0 h in air, sintering condition: 1160 °C×1.5 h in nitrogen gas atmosphere. Magnetic and physical properties of a typical sample were Jm = 0.46 T, Jr = 0.43 T, HcJ = 182.3 kA/m, HcB = 177.2 kA/m, (BH)max = 33.8 kJ/m3, TC = 495 °C and KA = 2.65×105 J/m3 and HA = 1332 kA/m. The lattice constants of this compound were a = 5.883×10-10 m, c = 32.92×10-10 m, and c/a = 5.596.

Abstract: In the paper, the influence of different particle size D: D>125m, D<50m and between on magnetic properties of a standardized dielectromagnetic is presented. The tests were taken at frequencies of between 50Hz, and 500Hz. Presented in the paper results provide important materials property data to allow the selection of the most appropriate dielectromagnetic particle size for different applications.

Abstract: Core loss of soft magnetic powder cores have been focused on to achieve high efficiency of power supplies. In this study the effects of crystal grain size on core loss were investigated by changing heat treatment conditions. It was found that core loss is influenced by crystal grain size because eddy current loss decreased and hysteresis loss increased by making crystal grain size smaller, and it is also influenced by the frequency and particle size.

Abstract: A high-speed motor and a DC brush-less motor for factory automation (FA) were made to investigate the applicability of the powder magnetic core to motor application. The performances of these motors were compared with the similar motors having conventional electromagnetic steel core. Although the permeability and saturated magnetization of powder magnetic core were less than those of electromagnetic steel core, the output performances of each core motor were almost the same. The FA motor with powder magnetic core using three-dimensional magnetic circuit showed higher torque than the electromagnetic steel core motor with the same volume.

Abstract: Eventhough Fe-6.5 wt.% Si alloy shows excellent magnetic properties, magnetic components made of the alloy with the composition are not totally commercialized because of its extremely low ductility. In order to overcome this demerit of alloy, 6.7 wt.% Si alloy powders were produced by gas atomization and then post-processed to form magnetic cores. By doing so, the total core loss could be minimized by reducing both hysteresis and eddy current loss, which were attributed to both grain size adjustment and particle size control. From our experiments, we were able to achive a core loss of 390 mW/cm3 at an induction of 0.1 T and 50 kHz through proper processes and a permeability μeff of 68 at low frequency was kept up to 700 kHz. These properties are compatable with the properties of well-known soft magnetic materials such as Fe-Si-Al and Ni- Fe alloys. From the above results, it can be concluded that Fe-Si alloy powders with high Si content have very high potential for the commercialization and application of the core.