Permanent magnet and method of making it

A rare earth permanent magnet comprising an alloy consisting essentially of: EQU RE.sub.2 (CO.sub.1-x-y Fe.sub.x TM.sub.y).sub.17+z Wherein: PA1 Re is at least one rare earth element; PA1 Tm is at least one transition element selected from the group consisting of chromium, manganese, titanium, tungsten and molybdenum; PA1 -2 .ltoreq. z .ltoreq. 1; PA1 0.5 < (1-x-y) < 1 PA1 0.05 .ltoreq. x .ltoreq. 0.4 PA1 0.01 .ltoreq. y .ltoreq. 0.2 Wherein said rare earth permanent magnet is further characterized by possessing high values of coercive field strength, an ideal demagnetization curve and a remanence of more than 9KG and wherein said rare earth permanent magnet is prepared by the process which comprises mixing together a starting alloy of the composition RE.sub.2 (Co.sub.1-x-y Fe.sub.x TM.sub.y).sub.17+z and 8 to 14 wt. % of a samarium-rich sinter additive compound composed of 50-60 wt.% samarium and 40-50 wt.% of an alloy Co.sub.1-x-y Fe.sub.x TM.sub.y wherein both said starting alloy and said sinter additive are each in powder form of average grain size 2.0 to 10.mu.m; magnetically aligning the mix; compressing it to a greenling; sintering it to form a magnet; and subjecting said magnet to a heat treatment to 400.degree. C - 600.degree. C.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a permanent magnet composed of at least 
one rare earth element and other elements, including cobalt, as well as a 
method of making it. 
2. Description of the Prior Art 
Permanent magnets of the above-mentioned type which are based on SmCo.sub.5 
and CeMMCo.sub.5 are known. High coercive fields are attainable with 
these. However, their magnetic remanence is below 10KG in all cases. 
For many uses, a lower coercive field and a higher magnetic remanence with, 
at the same time, an ideal demagnetization curve are required. 
Consequently, it is most desirable to improve rare earth-cobalt magnets so 
as to obtain, along with a high coercive field, a magnetic remanence of 
more than 9KG. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide a rare 
earth-cobalt magnet which simultaneously possesses high values of coercive 
field strength and remanence as well as an ideal demagnetization curve. 
Briefly, this and other objects of this invention as will hereinafter 
become clear, have been attained by including along with at least one rare 
earth element and cobalt, the elements iron and at least one of the 
transition metals (TM) selected from the group consisting of chromium, 
manganese, titanium, tungsten and molybdenum wherein approximately 17 
moles of all elements other than the rare earths are present for every 2 
moles of the rare earths (RE). 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To make the permanent magnets of this invention, a powder, with a mean 
grain size from 2.0 to 10 .mu.m, of a starting alloy of composition 
RE.sub.2 (Co.sub.1-x-y Fe.sub.x TM.sub.y).sub.17+z is mixed with from 8 to 
14 wt.% of a samarium-rich sinter additive (composed, for example, of 
50-60 wt.% of samarium and 40-50 wt.% of the alloy Co.sub.1-x-y Fe.sub.x 
TM.sub.y) wherein -2 .ltoreq. z .ltoreq. 1; 0.5 &lt; (1-x-y) &lt; 1; 0 &lt; x 
.ltoreq. 0.4; 0 &lt; y &lt; 0.2, preferably 0.05 .ltoreq. x .ltoreq. 0.4 and 
0.01 .ltoreq. y .ltoreq. 0.2. The mixture is magnetically aligned, 
compressed to a greenling and sintered to form a magnet. The magnet is 
subsequently subjected to a heat treatment above 400.degree. C. 
The permanent magnets of this invention, in contrast to known magnets, 
e.g., Alnico, exhibit a much higher coercive field and yet have a 
comparable remanence and an ideal demagnetization curve. 
Preferred rare earths are (1) samarium and (2) a mixture of samarium and a 
light rare earth element from atomic elements 57-62, misch metal or 
mixtures thereof. 
In the making of the permanent magnets of this invention, the following 
basic procedure is advantageous. A quantity of the desired RE.sub.2 
(Co.sub.1-x-y Fe.sub.x TM.sub.y).sub.17+z starting alloy, i.e., from 92-86 
wt.%, on the one hand, and from 8-14 wt.% of a samarium-rich sinter 
additive Sm/(Co,Fe,TM) on the other, are each melted together from their 
individual alloy components. The sinter additive should contain 50 to 60 
wt.% of samarium. The proportion of Co:Fe:TM in the sinter additive is 
preferably the same as that of the starting alloy. The sinter additive 
creates, in a known way, particularly favorable sintering conditions. It 
does not figure quantitatively in the magnetic end-alloy, since, by 
appropriate selection, it only compensates the oxide losses occurring 
during the production process. 
The fused starting alloy is subjected to a stabilizing annealing treatment 
at about 1150.degree. C. for about 6 hours, i.e., at a temperature below 
the liquidus temperature. The starting alloy, thus annealed, and the fused 
sinter additive are crushed to a grain size of .ltoreq. 1mm. The crushed 
starting alloy is then mixed with 8 to 14 wt.% of the crushed sinter 
additive and the mixture reduced to a powder of average grain size from 
2.0 to 10 .mu.m, desirably from 2.0-5.0 .mu.m, preferably less than 3 
.mu.m, in a counter-jet mill. There can also be used, in place of the 
counter-jet mill, an attritor or a ball mill. The two alloys can also be 
ground separately and the powders subsequently mixed in the correct ratio. 
The powder is next magnetically aligned in a pressing die and compressed 
isostatically or uniaxially to a greenling with pressures up to 8000 atm. 
The greenling is then sintered at temperatures between 1110.degree. C. and 
1180.degree. C. in a protective gas atmosphere. After the sintering, its 
density should be at least 92% of the theoretical density. 
Next the magnet is advantageously subjected to homogenization annealing at 
temperatures between 900.degree. C. and 1100.degree. C., preferably 
1000.degree.-1100.degree. C., and cooled to room temperature. After 
cooling, it is tempered at 400.degree. C. to 600.degree. C. and finally 
magnetized. The tempering is particularly important. The heating and 
cooling rates used during tempering are relatively irrelevant to the 
magnetic properties of this type of alloy unless exaggerated values lead 
to mechanical destruction of the magnet by thermal stresses. Regarding the 
heating time, values of 1 hour up to a maximum of 300 hours are suitable, 
the range of 80 to 100 hours being preferred. The resultant products 
typically have a predominantly single-phase structure.