The Growth Kinetics and Shape Evolution of Precipitates Growing by the Ledge Mechanism
- PDF / 737,414 Bytes
- 12 Pages / 420.48 x 639 pts Page_size
- 26 Downloads / 184 Views
THE GROWTH KINETICS AND SHAPE EVOLUTION OF PRECIPITATES GROWING BY THE LEDGE MECHANISM
MASATO ENOMOTO*, GEORGE SPANOS** AND ROBERT A. MASUMURA** *National Research Institute for Metals, 2-3-12, Nakameguro, Meguroku, Tokyo 153, Japan **Physical Metallurgy Branch, Naval Research Laboratory, Washington, D.C. 20375-5000 ABSTRACT The characteristics of ledge growth of precipitates are investigated by a previously developed finite-difference computer model. The plate lengthening simulation results which incorporate measured ledge heights and spacings are in reasonable agreement with the observed lengthening behavior of plates in Fe-C alloys. The simulation which takes into account the diffusion field intershows that a variety of action among multiple precipitates precipitate morphologies actually observed can possibly be formed solely by the change in the extent of diffusion field overlap among ledges and/or neighboring precipitates. This overlap is in turn caused by changes in the solute supersaturation, distribution of ledge nucleation sites and ledge nucleation rates. A brief discussion is made of the relative growth kinetics of ledged and disordered interfaces. INTRODUCTION It has been observed in many alloy systems that the growth of precipitates occurs by the lateral migration of ledges along their interphase boundaries. Except at very large undercoolings, the partition of solute atoms occurs simultaneously with the advancement of the interface, presumably at the risers of mobile growth ledges. Hence, it is likely that the diffusion field builds up around the riser and the diffusion flux of solute to or from the ledge riser controls the migration velocity of ledges. Thus, the diffusion field associated with each riser may interact in a complex manner and affect the motion of individual ledges moving in the train. Accordingly, it is expected that the ledge density and growth rate and solute supersaturation affect the overall precipitate morphology as well. Several authors have investigated the effects of diffusion field interaction on ledge motion by analytical or numerical methods using boundary conditions which may be relevant to the growth of precipitates in solids [1-6]. The earlier efforts [1,2] assumed the steady state motion of ledges and thus can be applied stages. Recently, late growth at relatively to precipitate analytical solutions which describe the transient motion of a multistep train have been presented [3]. A computer model utilizing the finite difference method can analyze the motion of a train of steps under various growth conditions in a simple and unified manner and was used to simulate precipitate growth [6] . It was shown that by including ledge nucleation the model can predict a major change of precipitate morphology with increasing solute supersaturation and growth kinetics which are consistent with actual observations. In this report, simulations are conducted further to study Mat. Res. Soc. Symp. Proc. Vol. 237. 01992 Materials Research Society
120
several aspects of precipitate growth
Data Loading...
