Mechanical instability of heat-treated ribbons Fe(Co)NbB.

Fabian, Stanislav ; Krenicky, Tibor

Abstract: In this study, the influence of the formation of a nanocrystalline structure on the mechanical properties in [([Fe.sub.1-x][Co.sub.x]).sub.81][Nb.sub.7][B.sub.12] (x=0, 0.5) alloys is investigated using simple bending tests. The tests have revealed that the transition from flexible to brittle behaviour develops predominantly just before crystallization when material is still amorphous. The subsequent crystallization causes only slight changes in the embrittlement level. We have found that the Co-containing alloy exhibits a higher tendency towards the embrittlement in the amorphous phase as compared to the ternary FeNbB. On the other hand, after advanced crystallization (irrespective of the Co-content), the embrittlement level of the samples is comparable.

Key words: soft magnetic materials, mechanical properties

1. INTRODUCTION

Nanocrystalline soft magnetic alloys prepared by devitrification of melt-spun amorphous ribbons are interesting because of their magnetic properties superior to previously used materials, offering large inductions and good temperature stability. There are three major classes of these alloys--FINEMET, NANOPERM and HITPERM, suitable for applications such as rotors of aircraft generators, transformers or magnetic bearings. Magnetic properties of the nanocrystalline alloys are governed by structural arrangement of two constituent phases--crystallites with diameter ~ 10 nm, embedded in amorphous residual phase.

In this case, two-phase structure of the nanocrystalline ribbons is closely connected with mechanical instability that causes serious problems for their applicability. Amorphous precursor in the form of ribbon is a multi-component alloy consisting of the chemical clusters and free volumes. The free volumes originate during the preparation of the ribbon when melted alloy is rapidly quenched on the copper wheel (Gerling et al., 1990). Previous studies performed on this class of materials produced by isothermal annealing revealed that annealing-induced changes of the mechanical properties are closely connected with the free-volumes reduction (Allia et al., 1993; Skorvanek & Gerling, 1992; Skorvanek et al., 1994). This process results in enhancement of the material density. As was reported, the process of embrittlement prevails in the intergrain area and is enhanced when the diameter of the grains is decreasing (Gan & Zhou, 2001). Alternative method used for development of the nanocrystalline structure, when electrical current is used for the the sample heating (Joule heating), results in partial improvement of mechanical properties as reported for the FINEMET (Allia et al., 1993). This improvement is assumed to be the consequence of the shorter annealing-time used to develop nanocrystalline structure (few seconds, in comparison with a hour using isothermal annealing) which is not sufficient for an atomic re-arrangement in the amorphous phase.

In this paper we focused on the identification of the temperature area in which deterioration of the mechanical properties in the NANOPERM-type and HITPERM-type samples develops and to quantify the extent of this deterioration. Most papers concerning mechanical properies of the nanocrystalline soft magnetic alloys are dedicated to FINEMET and there is a lack of informations quantifying mechanical stability of mostly magnetically characterized nanocrystalline alloys NANOPERM and HITPERM. As mechanical instability is a serious limiting factor for applicability of this class of materials, there is real need to continue in its exploration.

2. EXPERIMENTAL

Amorphous ribbons (6 mm wide and about 20 [micro]m thick) of the nominal composition of [Fe.sub.81][Nb.sub.7][B.sub.12] (NANOPERM) and [(FeCo).sub.81][Nb7.sub.][B.sub.12] (HITPERM) were prepared by the method of planar flow casting at the Institute of Physics, SAS in Bratislava. Pieces of the length 10 mm and 3 mm wide were treated by a controlled isothermal annealing for one hour in vacuum at the Institute of Experimental Physics, SAS in Kosice. Annealing temperatures were applied which covered the values where heat-treatment does not affect amorphous state up to the temperatures where the second step of crystallization takes place, deteriorating soft magnetic character of the materials (Krenicky et al., 2004). So the series of amorphous and nanocrystalline samples with varying amounts of bcc-Fe grains were produced.

Mechanical properties were examined using simple bending test. To diminish the values scattering, sets consisting of six samples for every annealing temperature were examined.

Following the schematic representation of the test (Fig.1), the sample was bended by approaching of two parallel plates and the distance D at which sample broke was measured. The distance D of the plates is conneced with the relative strain in the sample [[epsilon].sub.f] by the equation

[[epsilon].sub.f] = d/D - d (if D[greater than or equal to]2r)(1)

[[epsilon].sub.f] = d/[D.sup.2] + 3[l.sup.2]/4. [square root of (3[l.sup.2] - 3[d.sup.2])] (if D < 2r)(2)

d is sample thickness, l is length of the sample, r is critical radius of the sample bending (Skorvanek et al., 2002). So, the value [[epsilon].sub.f] = 1 means total bending without sample breaking and small values of [[epsilon].sub.f] belong to the samples that broke at large distance D.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Results of the simple bending tests performed on the samples annealed at various values of temperature are shown in Fig. 2. To compare annealing temperature with the onset of the first step of crystallization which was already stated (Krenicky et al., 2004), arrows are used. All the samples FeCoNbB (FeNbB) annealed at the temperature value up to 525 K (625 K) respectively, show full bendability i.e. can be bended without breaking ([[epsilon].sub.f] = 1).

The process of embrittlement starts obviously in still amorphous state, just before onset of crystallization. That holds for both alloys independently of the Co-content. However, in the samples with equiatomic portions of Fe and Co the embrittlement develops at lower value of temperature in comparison with value for the ternary alloy without Co, following temperature shift in the crystallization onset.

After onset of the embrittlement process, flexibility of the samples deteriorates rapidly and all samples with nanocrystalline portion show markedly low mechanical stability with respect to bending. So, embrittlement of the samples at this stage of heat treatment shows just slight changes dependent on Co-content and portion of crystalline phase.

Such a behaviour is in good agreement with studies performed on FINEMET (Skorvanek & Gerling, 1992; Skorvanek et al., 1994) and NANOPERM with composition of [Fe.sub.80.5][Nb.sub.7][B.sub.12.5] (Skorvanek et al., 2002).

4. CONCLUSION

Remarkable deterioration of mechanical stability with respect to bending strain was observed for both (NANOPERM and HITPERM) types of the samples after heat-treatment leading to nanocrystallization. Embrittlement started and developed in still amorphous state just before creation of the crystalline phase. On the other hand, sufficient portion of the crystalline phase is necessary for developing of extremal magnetic propeties in this type of materials.

Brittleness appears to be common property for each three main types of the nanocrystalline soft magnetic alloys prepared by a controlled isothermal annealing. However, the heat-treated amorphous NANOPERM alloys show better resistance against the embrittlement as compared with FINEMET or HITPERM. So the task to produce magnetically excellent nanocrystalline ribbon that will be mechanically strong persists to be solved. Possible way to accomplish this task is to find new composition of an amorphous alloy with positive effect on the processing. The limiting factors on the chemical composition are closely connected with technology of amorphous precursor preparation and required magnetic properties of the final material. Other alternative could be modification of the heat-treatment process, especially leading to the shortening of heating time, as the annealing out of excess free volume is believed to be a reason for the observed embrittlement. In this respect a heat-treatment, which allows us to preserve more free volume in the residual amorphous phase, should be beneficial. But this solution should be counterproductive in the case of magnetic annealing with requirement of time long enough for atomic re-arrangement. At present, usual solution of prevention materials for application from damage is to anneal material in its final shape. Products also can be covered and protected by proper kind of isolation.

To conclude, mechanical stability of the nanocrystalline ribbons is extremely sensitive to bending. However, mechanical properties being so complex cannot be fully characterized by this kind of strain. For better understanding of the embrittlement it is necessary to continue investigations using other methods and kinds of stress, particularly as there are still not quantified claims about existence of nanocrystalline soft magnetic ribbons that are not brittle.

5. REFERENCES

Allia, P.; Barico, M.; Knobel, M.; Tiberto, P. & Vinai, F. (1993). Nanocrystalline [Fe.sub.73.5][Cu.sub.1][Nb.sub.3][Si.sub.13.5][B.sub.9] obtained by direct-current Joule heating: Magnetic and mechanical properties. Philosophical Magazine B--Physics of Condensed Matter Structural, Vol. 68 (1993) pp. 853-860, ISSN 1364-2812 (Print); 1463-6417 (Online)

Gan, Y. & Zhou, B. (2001). Effect of grain size on the fracture toughness of nanocrystalline FeMoSiB. Scripta Materialia, Vol. 45, No. 6 (2001) pp. 625-630, ISSN 1359-6462

Gerling, R.; Schimansky, F.P. & Wagner, R. (1990). The thermal stability of irradiation-induced excess volume in amorphous Fe, Ni, P, and its influence on ductile behaviour. International Journal of Rapid Solidification, Vol.5 (1990) pp. 137-147, ISSN 0265-0916

Krenicky, T.; Marcin, J.; Skorvanek, I. & Svec, P. (2004). Magnetic properties of FeCoNbB nanocrystalline alloys heat treated under longitudinal magnetic field. Czechoslovak Journal of Physics, Vol. 54, Supplement D (2004) pp. 185-188, ISSN 0011-4626 (Print); 1572-9486 (Online), http://www.springerlink.com/content/w60mt07936672412/ ?p=4e7d42de41cc4b29a0c3fd1b94a49592&pi=41

Skorvanek, I. & Gerling, R. (1992). The influence of neutron-irradiation on the soft magnetic and mechanical properties of amorphous and nanocrystalline [Fe.sub.73.5][Cu.sub.1][Nb.sub.3][Si.sub.13.5][B.sub.9] alloys. Journal of Applied Physics, Vol. 72, No. 8 (1992) pp. 3417-3422, ISSN 0021-8979

Skorvanek, I.; Gerling, R.; Graf, T.; Fricke, M. & Hesse, J. (1994). Neutron Irradiation Effects on the Structural, Magnetic and Mechanical Properties of Amorphous and Nanocrystalline FeCuNbSiB. IEEE Transactions on Magnetics, Vol. 30, No. 2 (1994) pp. 548-551, ISSN 0018-9464

Skorvanek, I.; Svec, P.; Greneche, J.M.; Kovac, J.; Marcin, J. & Gerling, R. (2002). Influence of microstructure on the magnetic and mechanical behaviour of amorphous and nanocrystalline FeNbB alloy. Journal of Physics: Condensed Matter, Vol. 14, No. 18 (2002) pp. 4717-4736, ISSN 0953-8984 (Print); 1361-648X (Online)