JOURNAL OF MATERIALS SCIENCE 27 (1992) 5863-5868 Crystallization study of amorphous FeioNiioB2o by electrical resistivity measurement A. MITRA, V. RAO, S. PRAMANIK, O. N. MOHANTY National Metallurgical Laboratory, Magnetic Materials Group, MTP division, Jamshedpur 831 007, India The crystallization in amorphous Fe4oNi4oB2o alloy has been studied by electrical resistivity measurement. It shows a three stage compared to the more conventional two-stage crystallization behaviour in many metallic glasses. The Johnson-MehI-Avrami equation was found to be operative only for 40% transformed crystallized volume for the highest measured isotherm (740 K). The Avrami exponent and activation energy were found to be 1.75 and 53 kcal mo1-1, respectively. The low activation energy in the amorphous alloy has been explained by the structural relaxation model. 1. Introduction Metallic glasses based on transition metals are soft magnetic materials and are extensively used in the electronic and power industries [1]. The metastable nature of the different phases, however, makes them unsuitable for use in the industry in its as-quenched state. Consequently, the manner in which these phases proceed towards equilibrium is not only of funda- mental interest but of technical importance as well. Amongst the transition metal-based metallic glasses, Fe Ni-M (where M is the metalloid) mater- ials are the most extensively studied system because of their interesting magnetic properties coupled with high strength and good corrosion resistance [2-4]. Fe4oNi4oB2o is a simple ternary system whose thermal behaviour has been reported by several researchers employing techniques such as calorimetry, dilato- metry, magnetometry, resistometry [5 9]. Differential scanning calorimetry (DSC), however, is used widely for studying crystallization behaviour. In the present work the thermal behaviour of amorphous Fe40Ni4oB2o alloy has been studied using electrical resistivity measurements. The crystallization and Curie temperatures have also been determined by DSC and magnetization studies. 2. Experimental procedure The metallic glass studied here, prepared by the melt spinning technique, had a nominal composition Fe4oNi4oB2o. The electrical resistivity measurements were made by a d.c. four-probe method using an infrared gold image furnace (TER-2000, Ulvac/ Sinku-Rico, Japan) where the temperature could be varied from 300-1400 K. A 1 mA current was passed through the sample during the measurement. Typical dimensions of the sample were 3 cmx 0.2 cmx 30 I~m. During isothermal measurements, the sample was kept at the desired temperature for sufficient time for the resistivity to become constant. The calorimetric study was carried out using a Perkin-Elmer DSC-7 calorimeter. Magnetization was measured using a vibrating sample magnetometer at a residual magnetic field strength of 3.5 kA m- i. The X-ray diffractograms of the alloys heat-treated at different temperatures for 30 min were taken using a Siemens-D500 diffractometer using Cr K= radiation. The microstructure of the sample annealed at 630 K for 20 min was studied by electron microscopy (Philips EM-400). A circular spe- cimen, 3 mm diameter, was punched out andelectroly- tically thinned and polished in a mixture of 90% acetic acid and 10% percholoric acid using a twin-jet Struers tenupole - 2. 3. Results and discussion The temperature variation of normalized resistivity (QT/Q290) measured at a heating rate of 5 K rain-1, is shown in Fig. 1. The resistivity increases with temper- ature up to 470 K with the temperature coefficient of resistivity (TCR = (1/O29o~dp/dT)) 1.934 x 10 -4 K- 1 From 470 K onwards, the value of TCR begins to change. A small decrease in resistivity is observed at 600 K which is shown much more prominently in Fig. lb. Resistivity drops sharply at 730 K showing the onset of primary crystallization. Another drop in resistivity is observed at 870 K which corresponds to the crystallization of secondary phases. The DSC study of the same sample showed similar different transitions (Fig. 2). Two sharp exothermic peaks at 766 K and 845 K and a broad exothermic maximum in the temperature range 500-660 K were observed. This broad maximum may be attributed to the super- position of two transitions which are observed at 470 and 600 K in the resistivity measurement. Fe4oNi4oB2o is a ferromagnetic material at room temperature. The Curie temperature of this alloy is 0022-2461 9 1992 Chapman & Hall 5863