Rare earth-cobalt system permanent magnetic alloys and method of preparing same

This invention relates to permanent magnetic alloys and, in particular, to rare earth-cobalt system permanent magnetic alloys.

Among the intermediate substances of RCo 5 and R 2 Co 17 intermetallic compounds, those which are composed of R(CoFeCu)z (z=5-85) where the Co or Co and Fe components have been partially substituted with Cu are known to be excellent material for permanent magnets, see, for example, U.S. Pat. No. 3,560,200. In recent years, rather than the high coercive force ( 1 Hc) of rare earth-cobalt magnets, high residual magnetic flux density (Br) is in demand from applied fields. Thus, the main stream of rare earth magnets is shifting from the 1 Hc-dominant 1-5 system sintered magnets to Br-dominant 2-17 system magnets.

It is disclosed in Japanese Patent Application 52-154207, which is incorporated herein by reference, that alloys of (Sm, Y) (Co, Fe, Cu)z composition in which the R component consists of Sm and Y result in a permanent magnet with a high Br value of approximately 11KG while maintaining the coercive force of above 3KOe. However, in the case of rare earth magnets, due to the low value (3-6KOe) of coercive force, their application is limited by the fact that the maximum efficiency is obtained when used on the side of relatively high permeance coefficient (B/H=2-5) with respect to the magnet circuit.

It is thus a primary object of this invention to provide 1 HC upgrading lowering the Br of the magnetic alloy.

Accordingly, it is an object of this invention to provide for the addition of a trace amount of hafnium (Hf) to the permanent magnetic alloys described in the above-mentioned Japanese application so that the permanent magnetic alloys of this invention are characterized by the fact that the main components are R 2 T 17 intermetallic compounds composed of rare earth metals (R=Sm, Y) and 3d transition metals (T=Co, Fe, Cu), to which a trace amount of Hf element is added.

It is a further object to provide an improved method for making the above permanent magnet alloys.

Other objects and advantages of this invention will be apparent from a reading of the following specification and claims taken with the drawing.

FIG. 1 shows the dependency of the magnetic characteristics on Hf quantity of an illustrative permanent magnetic alloy in accordance with the invention having a composition formula of Sm 0 .9 Y 0 .1 (Hf x Co 0 .72-x Fe 0 .1-8 Cu 0 .10) 7 .2.

FIG. 2 shows the dependency of 1 Hc on cooling velocity.

FIG. 3 shows the dependency of 1 Hc on annealing temperature and annealing time.

The permanent magnetic alloy of this invention is generally manufactured in the following sequence after weighing the raw material: melting, pulverizing, magnetic field orientation, compressive forming, sintering, and annealing. The melting and the pulverizing processes may also be carried out by the direct reduction method of oxides to manufacture the powder. In cooling to room temperature after sintering, quenching to 900° C. or lower from the sintering temperature was found effective. Melting is effective when it is carried out in an inert atmosphere with a high frequency induction furnace, electric arc furnace, etc. Pulverizing into fine powder should be done in an inert atmosphere or organic solution. There is no great difference in the performance of various pulverizers. The grain size of the powder is not as sensitive as in the case of 1-5 system magnets, and fairly constant values of coercive force are maintained in the range of 1-50 μm. However, considering the aspect of grain orientation level, the grain size of 1-5 μm is desirable. The sintering process is carried out most effectively in an inert atmosphere or in vacuum at the temperature range of 1160°-1220° C., and the sintering time of 1-10 hours is favorable in the industrial sense. The range of sintering and sintering temperature depends on the permissible composition range of the magnetic alloys of this invention and the grain size of the powder. The rapid cooling treatment after sintering is one of the processes required to obtain the desired magnetic characteristics of this invention. The cooling velocity should be at least 1° C. per second until the sintering temperature is lowered to below 900° C. This process is believed to have a strong influence on the coercive force increment during the next process of annealing carried out at 750°-900° C.

The preferred ranges for components of the permanent magnetic alloys of this invention may be limited to 11.5-12.5% in atomic ratios for rare earth components (Sm and Y), 0.2-2.5% for Hf, 10.5-26.5% for Fe, 7-10.5% for Cu, and 52-70.8% for Co. The 11.5-12.5% rare earth components should be 0.5-6.2% Y and 6.3-12% Sm. These ranges are related to the magnetic characteristics. The coercive force increment action of the Hf component is apparently influenced more strongly by the mixed state of Sm and Y rather than Sm alone as the rare earth component. However, the coercive force increment is not marked at a Hf quantity of less than 0.2%. Further, at above 2.5% Hf, although a magnetic force of 7-8KOe is obtained, the saturation magnetization is lowered. Thus, the Hf range should be between 0.2-2.5% with a Sm and Y mixture. Regarding the rare earth components, when Y is below 0.5%, both Br and Hc decrease, and the 1 Hc increase resulting from the Hf addition is also reduced. At above 6.2%, virtually no change occurs in the saturation magnetization compared to the case of Sm because of the increase in the magnetic alloy of the Y 2 (CoFe) 17 compound with low anisotropism to begin with, but both Br and 1 Hc are reduced. Thus, the range can be limited to 0.5-6.2%, and the remaining rare earth component is provided by 6.3-12% Sm. The Fe component contributes the most to increase the Br value of the alloy as a whole. At below 10.5%, although the coercive force increases by about 0.5-1 KOe, high Br value, which is an object of this invention, cannot be obtained and thus the Fe component should be at least 10.5%. At 26.5% or above, extreme deterioration of coercive force is caused. Thus, the effective range of the Fe should be limited to 10.5-26.5%. The Cu component becomes the generating element for the precipitating action during annealing and plays an important role in the mechanism to generate the coercive force. However, sufficient precipitating action cannot be obtained at below 7%. At above 10.5%, the Cu component being a nonmagnetic element, causes lowered saturation magnetization. Thus, the Cu range should be limited to 7-10.5%. The remainder is the Co component of 52-70.8%.

This invention is described in further detail using the practical, illustrative examples below.