Effect of heat treatment on magnetic property of high-coercivity FeCrCo magnet

FeCrCo based alloy is a type of permanent magnet that can be used for special purpose, its most impressive performance is about its machinability, it can be easily drawn, pulled, rolled, forged, bent, making it far outshine any other permanent magnetic materials, besides, it also boast for  the characteristic of outstanding hydrogen-resistance. For above advantages, FeCrCo based permanent alloy has attracted a high attention from people of different lines.

However, its magnetic property is relatively low among other types of permanent magnetic materials, following is the property for typical FeCrCo alloy: Br=1~1.3T, Hc=40~48kA.m-1, (BH)max=32~40kJ.m-1. It shows that, its Br is not low, while its Hc and (BH)max is far lower than that for High-coercivity permanent magnetic materials:NdFeB and SmCo, thus to improve the Hc is of high importance to the development of FeCrCo alloy.

1 .Production and experiment method of the alloy

The alloy is designed to have 30%-35%Cr, 20%-25%Co, some Fe content and a bit of Mo and Zr, smelted in Vacuum induction furnace and then be cast into shapes. Heat treatment for FeCrCo alloy often goes as follows:

High-temp solid solution treatment→quenching→thermomagnetic treatment →step-tempering, as shown at the right figure 1

During experiment, keep the alloy at a temperature of 1100~1250℃ for 20 min, then put them into ice salt solution for quenching to form over-saturated solid solution. Temperature for the magnetic treatment should be controlled within the range of 630~660℃, while the magnetic intensity is 240, 400, 560, 720kA.m-1 and the treatment time is 0.5~2h. For step-tempering process the temperature should be at 620℃, 0.5h+600℃, 1h+580℃,2h+560℃, 2h+540℃, 4h. The sample property can be measured by X-Y function recorder, with sample size of F8mm x 8mm.

2 . Conclusion and discussion

Solid solution treatment under high temperature

During the beginning of its development, FeCrCo alloy has a solid solution at around 1300℃; then as study on it goes further, it is found that addition of some element such as Mo and Ti will reduce the temperature of solid solution to 1100℃. Since there will be easy creation of non-magnetic phase in the process of slow cooling, which will debase the magnetic property, thus after solid solution step the alloy needs to be cooled down at a very quick speed, with ice salt solution quenching as the typical method. Zr is the most effective element for counteractingγphase, it can minimize the range forγphase. In view of this aspect, a small amount of Mo and Zr is added to the alloy. To explain the effect of cooling speed on the property, the alloy is solid melted at a temperature of 1150℃ for 20 min, after that cool it down at different speeds, i.e. submit the alloy to ice salt solution quenching, room temperature solution quenching, furnace pipe quenching, air quenching, then step into thermomagnetic treatment and step-tempering, the magnetic property as shown in the table 1.

Influence quenching speed on magnetic properties ally

Parameter Salt solution quenching Room temperature water quenching Quenching within furnace pipe Air quenching
Br / T 0.89 0.92 0.75 0.26
Hc / (kA · m-1) 82 83 86 82

One obvious conclusion for above is that: Br for alloy cooled down by air is far low than those by other cooling methods, which means its cooling speed is not quick enough and non-magnetic phases would form during cooling process; Br for alloy cooled down within furnace pipe also is kind of low, which means only by graduate temperature decreasing in furnace pipe and without direct contact with water, the cooling speed is also not quick enough; while for the ice salt solution quenching and room temperature solution quenching, both magnetic properties produced are good enough, this means addition of Zr element will counteract theγphase successfully to a great extent, and the transformation becomes slowly, then good effect will be obtained by room temperature solution quenching. The fig 2 shows property variation of the alloy that were solid melted under different temperatures, you can see, except for the relative low Br at a temperature of 1100℃, the Br obtained at any temperature within1150-1250℃will be constant, which results from the addition of Mo.

Thermomagnetic treatment

Thermomagnetic treatment is an important step in the process of FeCrCo alloy treatment. It means to submit the alloy to isothermal treatment within magnetic field to realize Spinodal discomposition, and to form amplitude modulation structure for the two phases ofα1+α2. Under the influence of exterior magnetic field, FeCo-concentratedα1 phase granules will separate out along direction of exterior field and array in order, while the Curie Temperature for Cr-concentrated ferromagneticα2 phase is higher than room temperature. There will be an optimal treatment temperature for each FeCrCo alloy, if temperature is within range of 630~660℃, then the property obtained as shown in fig.3, you can see, optimal property for the alloy will be achieved at temperature of 640~650℃, and when it is at 630℃ or 660℃, the Hc has been reduced remarkable, it shows the sensibility of alloy against the temperature. During heat treatment, the magnetic intensity won’t play an obvious role to the alloy property, the magnetic property will undergo evident change within range of 240~720kA•m-1, which shows that the alloy property will not improve any more as long as the magnetic intensity increase to a certain value. Besides, influence of the duration of thermomagnetic treatment on the property is also not remarkable, the result will be basically same for duration of 20min to 2h. To sum up, the key for thermomagnetic treatment is the selection of treatment temperature.

Step-tempering

After thermomagnetic treatment on the alloy, better magnetic property will not be obtained without being tempered, and the coercivity will become low too, for after thermomagnetic treatment at 640~650℃,the distance between the two phases is not big enough. To get high property, aging must be undertaken after thermomagnetic treatment, and the effect resulted from the single-step aging is not as good as that for the multi-step aging. During treatment, difference of temperatures for neighboring steps aging should not be wide, and in general should not exceed 20℃. By atomic negative dispersing, the two phase structure ofα1+α2 will be further improved, so that the curie temperature for weak magneticα2 phase will be reduced to below room temperature, and full play will be given to the single domain property of strong magneticα1 phase so as to achieve a better permanent magnetic performance, just as shown in fig.4.

Conclusion

By experiment above, conclusion can be reached that the coercivity for this alloy can be made to 70-84kA•m-1 if under suitable processing technique, far higher than 40~48 for ordinary FeCrCo alloy, which is beneficial for expanding its application range. Besides, only a little reduction happens to Br value, while the (BH) max is basically maintained around 32 kJ•m-1. However, the quenching after solid solution treatment and the temperature sensitivity during thermomagnetic treatment is difficult to controlled, and they are the steps that have decisive influence on final property of the alloy.

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