FeNbCuSiB based materials were produced in the form of ribbons by rapid solidification techniques. The crystallization, magnetic, mechanical and corrosion behaviour were studied for the prepared materials as

a function of quenching rate from liquid to the solid state. Higher quench rates produced a more amorphous

structure exhibiting superior soft magnetic properties with improved corrosion resistance

Basu, S

Chattoraj, I

Mitra, A

Panda, A K

English

eprints@NML

Bull. Mater.  Sci., Vol.  25, No. 6, November  2002,  pp. 573–575.  © Indian  Academy  of  Sciences.  573 Influence of quench rates on the properties of rapidly solidified FeNbCuSiB alloy A  K  PANDA,  I  CHATTORAJ,  S  BASU*  and  A  MITRA  National Metallurgical Laboratory, Jamshedpur 831 007, India *Materials Science Centre, Indian Institute of Technology, Karagpur 721 302, India  Abstract. FeNbCuSiB based materials were produced in the form of ribbons by rapid solidification techn ques. The crystallization, magnetic, mechanical and corrosion behaviour were studied for the prepared materials as a function of quenching rate from liquid to the solid state. Higher quench rates produced a more amorphous structure exhibiting superior soft magnetic properties with improved corrosion resistance.  Keywords. Amorphous and nanocrystalline; magnetic properties; corrosion; c y tallization.   1. Introduction FeNbCuSiB based amorphous alloys exhibit excellent soft magnetic properties in their nanocrystalline state obtained by optimum heat treatment schedule (Yoshizawaet al 1988). These materials are initially prepared in the amorphous state by melt spinning technique, which in-volves rapid solidification of the alloy melt by impinging it on a rotating quenching wheel. As these materials are in a metastable state, their properties depend on the quen-ching conditions like crucible dimension, quenching rate etc. The quenching rate changes the amount of free volume frozen in an amorphous structure which lead to different extent of topological disorder and subsequently inflences the physical properties of the material (Ghannami et al 1984; Hasiak et al 1984). The present study is an attempt to investigate the effect of quench rate on crystallization, magnetic, mechanical and corrosion behaviour of the as-prepared materials. 2. Experimental The crystallization behaviour has been studi d using differ-ential scanning calorimeter (DSC) and thermal electrical resistivity (TER) apparatus. The magnetic properties were evaluated in an open flux configuration. Electrochemical corosion behaviour was evaluated using potentiodyna-mic polarization study. The details of the experimental  arrangement has been described elsewhere (Chattoraj and Mitra 1998; Mitra et al 2001). In the present study a quartz crucible with conical nozzle and 0×5 mm nozzle to wheel distance was used for the preparation of melt spun ribbons. Hence, wheel velocity represents the quenching rate of the material. 3. Results and discussion DSC study showed that crystallization temperature changed with the wheel velocity. Figure 1a shows the representative DSC plots for two wheel surface velocities, VS. Similar change in crystallization temperature was also observed in resistivity study (figure 1b). The broad exo-thermic peak in DSC study indicated the transformation from amorphous to nanocrystalline phase. The rise in resistivity at the onset of crystallization temperature was du to the scattering of electrons from the nanocrystalline grain boundaries. The onset of crystallization, TX, in-creased with the increase in quenching wheel velocity up to 32 m/s, above which no further c ange in crystalliz tion temperature was observed. The parameters obtain d from the resistivity studies are shown in table 1. The low onset temperature at low wheel velocity (< 32 m/s) was due to the low cooling rate that generated nucleation centres f rthe crystallization in as-cast state. Different quenching wheel velocities also affected the curie temperature, TC, as seen from the thermal variation of a.c. susceptibility plots (figure 2).  Curie temperature, TC, of the ribbons have been found to decrease from 552 K to 528 K with the increase in wheel velocity from 27 m/s to 40 m/s. This indicated that the materials with high quenching rate were having less extent of long range ordering. Similar effect was also observed by others (Mizoguchi et al 1980; Greer 1992). Room temperature resistivity, r300 has been found to increase with increasing quenching wheel velocity. An effect analogous to annealing at lower quenching wheel velocity probably led to some incipient crystallization resulti g in their lower room temperature resistivity values. Temp ra-ture coefficients of resistivity (TCR), a, below crystalliza-tion temperature (TX) were found to increase with quen-ching wheel velocities. The fastr transformation ki etics of Cu clustering at higher quenched samples was the cause of such increase in a value (Zbroszczyk et al 1994).  *Author for correspondence A  K  Panda  et al  574 The results of the magnetic measurements for samples with different quenching rates are shown in table 2. The decrease in coercivity and increase in susceptibility i.e. enhanced soft magnetic properties with increasing wheel velocity indicated that the lesser long range ordering  existed at higher quenching rate. The variation of fracture strain with the quenching rate is shown in table 2. The materials prepared at high quenching rate had high amor-phous volume fraction with lower crystallinity and lesser grain boundary as weak links which led to higher fracture strain i.e. a more ductile ribbon.  The corrosion behaviour of the prepared alloy was studied by electrochemical technique. The potentiodynamic polarization plots for different prepared ribbons are shown in figure 3. Curves (a) and (b) represent the electro-chemical behaviour at two different molar strengths of NaCl solution. In the ribbons prepared at 27 m/s wheel velocity, the profile of the anodic current at 0×01 M halide strength exhibit a transient passivation around + 200 mV (SCE) and a passivation zone extended to about + 800 mV (SCE). However, for ribbons prepared at wheel velocities of 32 m/s and above, the anodic activity revealed a longer passivation zone which extended close to + 1200 mV (SCE). At a still drastic environment of 1 M halide strength, the passivation tendency was observed only when the ribbons were prepared at wheel velocities more than 32 m/s. This portrayal of passivation profiles by a rducedTable 1. Various characteristic temperatures of ribbon samples obtained from DSC and TER.       Parameters from TER        Wheel velocity (m/s) Crystallization onset temperature measured from DSC, TX (K) Room temp. sistivity, r300 (× 10–6 Wm)  Crystallization onset (TX (K)) Temp. coeff. ofresistivity below TX,a (× 10–4) K–1           27 826 1×85 822 0×28 32 830 1×97 830 0×60 36 832 2×11 832 1×12 40 832 2×25 832 1×01            Figure 1. (a) DSC plots and (b) thermal electrical resistivity of ribbons prepared at different wheel velocities.   Figure 2. Thermal variation of a.c. susceptibility plots of ribbons prepared at different wheel velocities.  Influence  of quench  rates  on FeNbCuSiB  alloy  575galvanic activity in ribbons prepared at quenching wheel velocities greater than 32 m/s showed the formation of a more homogeneous amorphous phase at higher quench rates (Naka et al 1974).  4. Conclusion The quenching wheel velocities which determine the quench rate of the prepared alloy in the present study affected the crystallization, magnetic, mechanical and corrosion behaviour of the ribbons. With increase in quench rates there was an increase in crystallization onset of nanophase while the curie temperature registered a decrease. Superior soft magnetic properties, higher ducti-lity and enhanced corrosion resistance were observed in ribbons prepared at higher quenching wheel velocities. This was due to the lack of long range ordering obtained at high quench rates. References Chattoraj I and Mitra A 1998 Scr. Mater. 39 755 Ghannami M E I, Kulik T, Hernando A, Barquin L F, Gomez Sal J C, Gorria P and Barandiaran J M 1994 J . Magn. Magn. Mater. 133 314 Greer A L 1982 J. Mater. Sci. 17 1117 Hasiak M, Zbroszczyk J, Olszewski J, Ciurzynska W H, Wyslocki B and Blachowicz A 1994 J . Magn. Magn. Mater. 215–216 410 Mitra A, Panda A K, Rao V, Singh S R and Ramachandrarao P 2001 J . Appl. Surf. Sci. 182 321 Mizoguchi T, Hatta S, Kato H, Arai H, Maeda K and Akutsu N 1980 IEEE Trans. Magn. MAG16 147 Naka M, Hasimoto K and Masumoto T 1974 J . Jpn Inst. Metals 38 835 Yoshizawa Y, Oguma S and Yamauchi K 1988 J . Appl. Phys. 64 6047 Zbroszczyk J, Varga L K, Olszewski J, Ciurzynska W H,  Wyslocki B, Szymura S, Hasiak M and Haneczak G 1994 J . Magn. Magn. Mater. 215–216 410  Table 2. Effect of quenching wheel velocities on magnetic properties and fracture strain.           Wheel speed (m/s) Ribbons thickness ± 2 (mm) Coercivity (HC) (A/m) Initial suscept., cI (× 103)  Fracture strain, lf (× 10–2)           27 36 3×73 4×82 0×54 32 34 1×75 6×52 0×94 36 33 2×07 7×02 1×11 40 28 1×88 8×59 1×13            Figure 3. Potentiodynamic polarization plots of ribbons prepared at different wheel velocities carried out in 0×01 M (a) and 1×0 M (b) NaCl in buffer media.  

Influence of Quench Rates on the Properties of Rapidly Solidified FeNbCuSiB Alloy

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Influence of Quench Rates on the Properties of Rapidly Solidified FeNbCuSiB Alloy

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