Nonisothermal Kinetics Study of Phase Evolution of Zn-Fe Intermetallics
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t. Res. Soc. Symp. Proc. Vol. 398 01996 Materials Research Society
powder X-Ray Diffraction (XRD) measurements through which the stages of the amorphous to crystalline phase transition accompanying subsequent heat treatments would be revealed. In addition, Differential Scanning Calorimeter (DSC) measurements serve as a means to unravel the characteristic steps, or stages, associated with the attendant transformation of the non-equilibrium or metastable phase alloy or material, to the equilibrium state. By performing DSC measurements at varying heating rates, the nonisothermal kinetics analyses of the Augis-Bennett type [13, 14] or the modified Kissinger [15] method developed and applied by Mittemeijer et al. [16, 17, 18] can subsequently be applied to determine the activation energy for any stage identified in the DSC measurements. In this study, both DSC and XRD measurements will be used to study the kinetics of phase evolution in Fe-Zn intermetallics, in order to identify the nature/state of these phases from the as-ball milled states, to the final crystalline equilibrium forms after heat treatments. Using ball milling, nonequilibrium alloys are formed which then serve as the precursor to the phase evolution study of these different intermetallics. Since pure alloy phases will be ball milled, the data obtained on their phase evolution characteristics can be used to analyze steel coatings and coating operations, i.e., in either predicting or determining the phases likely to form. EXPERIMENTAL Pure Fe and Zn powders corresponding to 99.9+% and 99.9% purity respectively were mixed in the proportions given in Table I. The powders were carefully weighed out under a controlled atmosphere of argon, sealed in a hardened stainless steel vial fitted with O-rings, and subsequently mounted in a SPEX-8000 mixer/mill. Hardened stainless steel balls were used in the ball milling process. An argon gas atmosphere was maintained throughout the ball milling process to prevent possible oxidation or oxygen adsorption or absorption of the powders. Milling times were varied from 0.5 to 8 hours for all the compositions. To maintain consistency, only the materials milled for 8 hours will be analyzed here. Following the milling, both DSC and XRD measurements were performed. Table I. Composition of the Ball-milled Powder Homogeneous Fe-Zn phases Powder Mixtures Phase/Formula Fe at% Zn at% r (Fe 3 Zn o) 25.4 74.6 F, (FesZn 21 ) 18.05 81.95 8•(FeZn 7 ) 9.11 90.89 C(FeZn 13 ) 5.6 94.4
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Samples for XRD measurements were fine powders obtained from the bulk material through filing. This was necessary to minimize texture effects, associated with the bulk milled material. Aging of the fine powders were carried out under argon gas atmosphere in a horizontal furnace. Rapid cooling to room temperature was ensured with argon gas. A Thermal Science PL-DSC machine was used, and the corresponding purging gas was argon. The heating rates chosen for this study were 5 'C/min., 10 °C/min., 20 °C/min. and 30 °C/min. The X-ray diffraction measurements we
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