Anomalously fast diffusion in the Fe-Zn System
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I.
INTRODUCTION
W H E N a diffusion couple is made by placing two pure elements in contact the driving force for interdiffusion is extremely large. With higher temperatures, longer times, and thicker diffusion zones, the phases formed by diffusion are those found in the equilibrium diagram and the phases thicken parabolicly with time. Many authors have treated diffusion in such binary systems, and the equations that need to be solved are well understood, though in multiphase systems the analysis can be complicated. 1,2,3 With lower annealing temperatures and thinner diffusion zones the system may go through a variety of metastable states which are as yet poorly understood. If the temperature is quite low and the interface sharp, the diffusion zone may initially form as an amorphous layer. 4'5'6 When the interdiffusion zone is small, as is always the case if only a thin film of one element is applied, only one of several equilibrium intermetallic phases will form. If the transfer of atoms across the interface is rate limiting rather than diffusion, then the intermetallic phase will thicken linearly with time; in other cases the growth is diffusion controlled and increases parabolicly with time. 7'8'9 The reasons for the growth of only one intermetallic at a time is poorly understood, but kinetics seems to allow only one to be present. If a second phase is artificially inserted into the couple it will dissolve, and the layer revert to only one phase as diffusion in the thin layer proceeds. 10 Herein we report on the interdiffusion of Fe-Zn couples. Several authors have studied the formation of intermetallic phases in this system, 1l'12but relatively little work has been reported on the kinetics of diffusion of zinc into ferrite. The diffusion data of Richter and Feller-KniepmeierI3 indicate the penetration by lattice diffusion will be a few tenths micron in 12 hours at 550 ~ However, the Fe-Zn system is one in which several authors have studied diffusion induced grain-boundary migration (DIGM) in the 400 to 600 ~ temperature range. 14-17All of the DIGM studies have been made with Fe/Zn-vapor diffusion couples, and all have dealt with near surface effects. There has been no systematic study of either the depth to which DIGM would carry zinc into a thick piece of iron, or the time dependence of such PAUL SHEWMON and GLYN MEYRICK are Professors of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210. MAHMOUD ABBAS is on the faculty of Suez Canal University, Department of Metallurgical Engineering, Suez, Egypt. He was a Visiting Scholar at OSU at the time this work was done. Manuscript submitted July 15, 1985.
METALLURGICALTRANSACTIONS A
penetration. In fact, there have been no studies of the kinetics or depth of penetration of solute into bulk samples by DIGM for any system. This work was undertaken in an effort to determine the depth, and kinetics of penetration of zinc into iron by DIGM.
II.
EXPERIMENTAL PROCEDURE
Samples of commercial 25 mil (0.6 mm) thick, hot dipped galvanized steel were
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