Рік видавництва: 2019
Рік видавництва: 2019
?MAGNETIC PROPERTIES OF POWDER Fe-30Cr- (8-16) Co ALLOYS OBTAINED BY LOW-TEMPERATURE SINTERING
A. S. Ustyukhin ", V. A. Zelenskii", I. M. Milyaev ", M. I. Alymovfi, and S. F. Zabelinc
aBaikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences,
Moscow, 119991 Russia
bMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of
Sciences, Chernogolovka, 142432 Russia cTransbaikal State University, Chita, 672039 Russia
DOI: 10.24411 / 9999-0014A-2019-10181
In conventional production of magnetic alloys in the Fe-Cr-Co system by powder metallurgy methods, the sintering temperature of compacts is high (about 1400 ° C) [1-3]. It is well known that magnets of the Fe-Cr-Co system lose strongly in magnetic hysteresis properties if the nonmagnetic y-phase is present in their structure. According to Fe-Cr-Co phase diagrams the presence of the y-phase in material depends on Cr and Co content and also sintering temperature. Sintered at a conventional temperature of the order of 1400 ° C, the powder low-cobalt and high-cobalt magnets do not have the y-phase in their composition. In the previous work , it was shown that for magnetic materials with a low and moderate cobalt content (8-16 wt%), the use of a lower sintering temperature (down to 1150 ° C) is possible, producing dense compacts with practically no nonmagnetic y-phase precipitation after quenching.
In present work, sintering of Fe-30Cr-8Co, Fe-30Cr-12Co, and Fe-30Cr-16Co powder alloys was carried out at 1100 ° C. Magnetic hysteresis properties of materials obtained in this way are given. The green compacts had a relative density of ~ 75-80%, while with increasing cobalt content, the density of the compacts decreased somewhat, probably because of the low bulk density of the cobalt powder. The results of measurements of the density of sintered specimens are shown in Table 1. It is seen that all 3 alloys have a good density for this low-temperature sintering conditions. Specimens of the Fe-30Cr-8Co, Fe-30Cr-12Co, and Fe-30Cr-16Co alloys were heat-treated under several conditions and their magnetic properties were measured. A magnetic field in the course of thermomagnetic treatment was applied along the easy magnetization axis coinciding with the axis of a cylindrical specimen. The results of measurements of the magnetic hysteresis properties of sintered specimens are shown in Table 2.
Table 1. Density of sintered Fe-Cr-Co samples.
Alloy Sintering temperature, ° C Relative density,%
Fe-30Cr-12Co 1100 93.5
Table 2. Magnetic hysteresis properties of sintered Fe-Cr-Co samples.
Alloy Sintering temperature, ° C Br, T HCB, kA / m (BH) max, kJ / m3
Fe-30Cr-8Co 1.18-1.19 40.7-40.8 27.6-27.8
Fe-30Cr-12Co 1100 0.86-0.88 22.7-23.8 7.4-8.0
Fe-30Cr-16Co 0.06-0.12 6.7-7.7 0.1-0.3
_XV International Symposium on Self-Propagating High-Temperature Synthesis_
It is seen that the magnetic properties strongly decreased with the increase of Co content and tends to zero for Fe-30Cr-16Co alloy. Magnetic properties of Fe-30Cr-8Co alloy are good and meet the standards for low-cobalt Fe-Cr-Co cast alloys.
The X-ray powder diffraction analysis of the samples has shown differences for each alloy after thermomagnetic treatment (Fig. 1).
Fig. 1. X-ray powder diffraction patterns of Fe-30Cr-8Co (a), Fe-30Cr-12Co (b), and Fe-30Cr-16Co (c) samples after thermomagnetic treatment.
The X-ray diffraction pattern of the Fe-30Cr-8Co alloy (Fig. 1a) sintered at 1100 ° C shows the presence of the a-phase only. The X-ray diffraction pattern of the Fe-30Cr-12Co alloy (Fig. 1b) sintered at 1 100 ° C shows traces of the presence of the y-phase, what explains the decrease of magnetic properties. The X-ray diffraction pattern of the Fe-30Cr-16Co alloy (Fig. 1c) sintered at 1 100 ° C is completely different. It shows the presence of the a-phase and tetragonal intermetallic phase FeCr (a-phase), which is also undesirable for obtaining good magnetic properties. Also, in Fe-30Cr-16Co sample weak traces of some metal oxide or metal carbide phase were discovered, however, accurately identify the phase is not possible due to the small number of diffraction maximums and their low intensity.
Summarizing, Fe-30Cr-12Co and Fe-30Cr-16Co powder alloys do not have good magnetic properties despite the fact that they have good density after low-temperature sintering and almost no traces of the y-phase presence after quenching. It is known that alloying additives could significantly increase magnetic hysteresis properties of Fe-Cr-Co alloys, but in this case phase composition after thermomagnetic treatment may be determined by inhomogeneity of the a-phase itself due to low diffusion coefficient during sintering at 1100 ° C. As for Fe-30Cr-8Co alloy good magnetic properties can be obtained after low-temperature sintering even without alloying additives.
The research was supported by the Russian Foundation for Basic Research (project no. 18-03-00666-a).
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