Obtaining ferromagnetic bulk nanocrystalline alloys with high mechanical strength.

Buzdugan, Dragos ; Codrean, Cosmin ; Serban, Viorel Aurel 等

1. INTRODUCTION

Year 1988 is considered by most experts as the year of the first discovery of ferromagnetic nanocrystalline alloys. Yoshizawa and his colleagues showed on Fe-Cu-Nb-Si-B and later on Fe-Cu-M-B-Si family (where M = Nb, V, Mo, Zr) nanocrystallized alloys, exceptional magnetic characteristics: saturation induction of 1.2 T, low coercivity, magnetic permeability at high frequencies above [10.sup.3] (Codrean et al., 2007). These alloys named "Finemet" are the first family of ferromagnetic nanomaterials with significant economic impact.

They are followed by nanoalloys called "Nanoperm" (Fe-M-B families, with M = Zr, Nb, Hf) recommended for special applications, mostly in manufacture of power transformers (Codrean et al., 2007).

Meanwhile, the strength of nanocrystalline alloys is higher than crystalline and even amorphous alloys. It is well known that grain size is affecting the limit of elasticity, resistance properties and the deformability of materials. Thus, for [Al.sub.85][Ni.sub.5][Y.sub.10] alloy nanocrystallized by controlled devitrification have been noticed spectacular increases in hardness compared to the amorphous state (with 20-40%) and compared with best conventionally cast crystalline aluminium alloy and heat-treated later (200-300%) (Inoue et al., 2000).

Therefore, the purpose of this work is to determine the mechanical characteristics of ferromagnetic nanocrystalline alloys and how are influenced by their chemical composition.

2. ELABORATION OF THE NANOCRYSTALLINE ALLOYS AS RODS

One of the obtaining methods of ferromagnetic alloys with nanocrystalline structure is based on ultrarapid melt cooling.

The first step to get nanocrystalline alloys was to obtain a master alloy in form of bars, with an optimal chemical composition.

In our previous researches, the used chemical compositions in order to obtain the nanocrystalline alloys were: [Fe.sub.75-x][Cr.sub.x][Ga.sub.4][P.sub.13][Si.sub.5][C.sub.3], where x = 2, 3, 4, 5 [Serban et al., 2009]. The elaboration of the master alloy was done by melting induction in argon atmosphere of the raw materials: iron based alloys Fe-P, Fe-C, Fe-Si and high purity powders of Cr and Ga. Microscopic structure of the master alloy revealed the presence of a fine eutectic and some dendritic crystals which belong to a solid solution based on iron (Fig. 1).According to previous experiments, the elaboration of bulk nanocrystalline alloys as rods of 2 mm diameter was achieved by copper mold casting (Fig. 2). The technology of elaboration involves the following steps:

--cutting the master alloy to obtain sections of 5 grams;

--inserting the master alloy into the quartz crucible which has at the bottom an evacuate nozzle of the melt with 1 mm in diameter. In this crucible the master alloy is remelted by induction in argon atmosphere;

--melt ejection into the cavity of the mold.

The main used parameters of the process were:

--process temperature is 100 [degrees]C higher than fusion temperature of the alloy

--overpressure applied on melt: 0,25 atm

Following these steps rods with 2 mm in diameter and 20-25 mm length were obtained (Fig. 3), marked according to Table 1.

The rods thus obtained were structurally analyzed by X-ray diffraction, performed on a DRON 3 diffractometer, using Mo radiation and wave length [alpha] = 0,71 [Angstrom]. The diffraction pattern obtained (Fig. 4) indicates a nano crystalline structure, reduced intensity peaks belong to a supersaturated solid solution based on iron.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

It can be noticed also that an increased content of chromium leads, in the same cooling conditions, to a light increase of crystals, evidenced by enlargement of intensity of top interferences.

4. MAGNETIC AND MECHANICAL PROPERTIES OF THE OBTAINED ALLOYS

Nanocrystalline alloys proved to have excellent soft magnetic properties, better than crystalline and even amorphous alloys because of the crystalline grains dimension.

The coercivity of crystalline alloys depends strongly of their size because the magnetic domain walls interact with the grain boundaries. As the grain size decreases, Hc growths and reaches to a maximum value for grain sizes of approximately 100 nm. Further grain size reduction leads to a rapid decreasing of coercivity which varies by D6 law (Scwarz et al., 2004).

Magnetic characterization of the elaborated alloys was performed with a VSM 880 vibrating sample magnetometer. Figure 5 shows the hysteresis loop for the obtained alloys.

It can be noticed that these alloys present good soft magnetic properties (coercive field beetwen 1.24 and 1.91 kA/m) and the higher Cr content alloy has both high saturation magnetization and high coercive field.

Mechanical properties of the elaborated alloys were determined by micro-hardness measurements and compression tests. Compression tests were performed on an Instron machine. The speed of load implementation was 5 x [10.sup.-4] [s.sup.-1]. Microhardness tests were performed on a digitally Volpert Micro Vickers Hardness Tester using a load of 500 grams.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Compression curves obtained (Fig. 6) shows a brittle behaviour of alloys. However, for alloys with less chromium is remarkable a little plastic deformation before breaking. Values obtained from compression and microhardness tests are shown in Table 2.

5. CONCLUSIONS

The nanocrystalline structures can be obtained by ultrarapid melt cooling. The researches performed showed the possibility of obtaining ferromagnetic bulk nanocrystalline alloys of Fe-Cr-Ga-P-Si-C as rod form by copper mold casting. The elaborated alloys have high mechanical strength, presenting a fragile behaviour during compression test. It was found that with increasing the content of chromium increases the mechanical strength (hardness and compression strength). The further work will be focussed on the obtaining of bulk nanocrystalline ferromagnetic alloy in shape of socket used for magnetic shielding.

6. ACKNOWLEDGEMENTS

The studies were performed in frame of the researh project ID_18, financed by UEFISCSU, research contract no.66/2007.

7. REFERENCES

Codrean, C. Serban, V.A. (2007) Amorphous and nanocrystalline alloys, (Ed. Politehnica, Timisoara,), pp. 94-96

Inoue, A. Koshiba, H Itoi, T. (2000) Ferromagnetic Bulk Glassy Alloys with Useful Engineering properties, Materials Science forum, Vols.343-346, pp. 81-89

Hu, K Liu, L Chan, K.C. Pan, M. Wang, W. (2006) The effect of crystallization on microstructure and magnetic properties of [Fe.sub.61][Co.sub.7][Zr.sub.9]5[Mo.sub.5][W.sub.2][B.sub.15.5] bulk metallic glass, Materials Letters 60, pp.1080-1084

Serban, V.A. Codrean, C. Utu, D. (2009) Bulk amorphous soft magnetic iron based alloy with mechanical strength and corrosion resistance, Key Engineering Materials Vol. 399 pp. 37-42

Schwarz, R.B. Shen, T.D. Harms, U. Lillo, T. (2004) Soft ferromagnetism in amorphous and nanocrystalline alloys, Journal of Magnetism and Magnetic Materials 283 pp.223-230

Tab. 1. The mark of the obtained rods

Chemical compositions, % at. Mark

[Fe.sub.73] [Cr.sub.2] [Ga.sub.4] NB2
 [P.sub.13] [Si.sub.5] [C.sub.3]
[Fe.sub.72] [Cr.sub.3] [Ga.sub.4] NB3
 [P.sub.13] [Si.sub.5] [C.sub.3]
[Fe.sub.71] [Cr.sub.4] [Ga.sub.4] NB4
 [P.sub.13] [Si.sub.5] [C.sub.3]
[Fe.sub.70] [Cr.sub.5] [Ga.sub.4] NB5
 [P.sub.13] [Si.sub.5] [C.sub.3]

Tab. 2. The values of mechanical properties

 Compresive
 strength, Fracture
 [[sigma].sub.f] [strain.sub.[epsilon]f],
Alloy [Gpa] [%]

NB2 1,80 1,90
NB3 1,97 1,82
NB4 2,14 1,77
NB5 2,38 1,68

 Yield Yield
 strength strain
 [[sigma].sub.f] [strain.sub.[epsilon]f],
Alloy [Gpa] [%]

NB2 1,72 1,81
NB3 1,93 1,75
NB4 2,1 1,70
NB5 2,38 1,68

 Vickers
 Hardness
Alloy [HV.sub.0,5]

NB2 690
NB3 770
NB4 820
NB5 890