Crystallization Kinetics in a Ni 24 Zr 76 Amorphous Alloy
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investigation we are interested in studying the isothermal crystallization kinetics and the microstructural evolution of an Ni24Zr 76 amorphous alloy, in order to identify the reasons for the departure of the isothermal kinetics from KJMA formalism. Apart from studying the crystallization kinetics by conventional techniques, such as DSC and transmission electron microscopy (TEM), high-resolution electron microscopy (HREM) has also been applied in this investigation in order to monitor the atomic processes involved during the nucleation and growth of the crystalline phases from the amorphous matrix. EXPERIMENTAL Amorphous ribbons of 2-3 mm wide and 25-30 laim thick were produced by "chill-block meltspinning" technique. Dynamic and isothermal crystallization kinetics were studied in a Du-Pont DSC 910 cell coupled with computer controlled thermal analyzer (TA9900). During isothermal tests high purity Ar was continuously purged (3 liters/hr) into the DSC cell and the temperature was maintained within ±0.150 C. Isothermal annealing of the amorphous alloys was also carried out in salt-bath after wrapping the specimens with Zr-foils and encapsulating them in quartz tubes partially filled with Ar. The temperature of the salt bath was controlled within ±1.50C. The amorphous nature of the melt-spun ribbons and the phases formed after crystallization were characterized by X-ray Diffraction as well as by TEM. The thin foils, prepared by dual jet electropolishing using an electrolyte of methanol (80%) and perchloric acid (20%) at 223K, were examined in JEOL200CX TEM at 200kV or HITACHI9000 HREM at 300kV. 289 Mat. Res. Soc. Symp. Proc. Vol. 321. 01994 Materials Research Society
RESULTS The isothermal crystallization kinetic curves were obtained by integrating the DSC thermograms at different test temperatures, and as shown in Fig. 1 they are typically sigmoidal in nature. The presence of a significant incubation period, r, can also be noticed. The incubation period was evaluated by measuring the time from the instant the DSC cell had reached the required temperature until the onset of the crystallization. This satisfies the operational definition of t given by Christian [10]. The conventional KJMA plots, i.e., ln[-ln(1-X)] vs ln(t-t), over the full range of volume fraction transformed are shown in Fig. 2. It is obvious that instead of being straight lines, such plots exhibit significant departure from linearity from the beginning of crystallization. 1.0
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Fig. 1 Isothermal transformation curves of Ni24Zr 76 amorphous alloy at different test temperatures.
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Fig. 2 KJMA plot of Ni24Zr 76 amorphous alloy at different isothermal test temperatures.
The activation energy for crystallization, Ec, was evaluated from the data of isothermal crystallization and also from dynamic crystallization experiment. Figure 3 shows the Kissinger's plot i.e. ln[T.TI2] (where 17 is the heating rate and Tp is the peak temperature) vs
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