Kinetics of phase evolution of Zn-Fe intermetallics
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I.
INTRODUCTION
THE Fe-Zn intermetallics play a vital role in the Zn protection of steels as they form in varied proportionsV 51 under different coating operations. E~,3,61The suitability of a coating for a given application is determined by the phases formed and their morphologies. [7,8,9J Most coating applications occur through nonequilibrium processes (which involve rapid solidification of the molten alloy on the object being coated in the case of galvanization, or electrodiffusion during electrogalvanization), except the old sherardizing situation in which slow diffusion of Zn into the steel or iron matrix takes place over a period of time in the solid state. Thus, in a coated steel, one observes the formed phases in varied proportions depending on the time and temperature of treatments involved. Under equilibrium conditions, three of the Fe-Zn intermetallics (F, 6, and ~) form peritectically, while F~ forms peritectoidly. As a result of this fact, the microstructure of the coated steel will be a function of the nonequilibrium transformation characteristics of the Fe-Zn alloy system. This explains the reason for studying the nonequilibrium transformation process of each of the Fe-Zn intermetallic phases. Through ball milling, phases can be alloyed mechanically.VO..,~21 The state of the mechanically alloyed phases of as-ball-milled powders could be amorphous[HI or crystalline, depending on the system involved.[~21 In both cases, the as-ball-milled powders exist initially in a metastable state that is subject to evolution with heat treatment. The crystallization kinetics can be studied using 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 nonequilibrium or metastable phase alloy or material to the Z.T. LIU, Graduate Student, and O.N.C. UWAKWEH, Assistant Professor, are with the Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012. M. BOISSON, Student, is with the Foundation Ecole Polytechnique Feminine, 92330 Sceaux, France. Manuscript submitted February 6, 1995. 2904~VOLUME 27A, OCTOBER 1996
equilibrium state. By performing DSC measurements at varying heating rates, the nonisothermal kinetics analyses of the A u g i s - B e n n e t t type03,14jor the modified Kissinged ~51method developed and applied by Mittemeijer and co-workersll6,17,1s] can subsequently be applied to determine the activation energy for any stage identified in the DSC measurements. The interpretation of the activation energies determined can be used to control industrial processes during coating operations, such as in the hot-dip galvanization process. Similarly, one can better predict the nature of a coated steel in terms of the phases present, or in analyzing the sequence of the inter
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