The role of ledges in the proeutectoid ferrite and proeutectoid cementite reactions in steel
- PDF / 1,950,637 Bytes
- 14 Pages / 597 x 774 pts Page_size
- 0 Downloads / 233 Views
I.
INTRODUCTION
SINCE the seminal work of Gibbs [11 in 1878, ledges at interphase boundaries have been known to play an integral role in the growth of solids from both the vapor and liquid phases, tt-a[ The importance of ledges in solidsolid phase transformations, however, was not recognized until much later. In a general theory of precipitate morphology, Aaronson [9] first proposed that the shape of a precipitate is determined by the orientation-dependent mobility of the precipitate:matrix boundary; the corresponding "kinetically determined" shape can differ significantly from the predicted equilibrium shape. An anisotropic interphase boundary mobility was taken to arise from the presence of partially coherent interfaces at special boundary orientations, tg] It was suggested that boundaries with a partially coherent structure migrated with reduced kinetics during growth through the agency of a ledge mechanism. These conclusions were reached largely through optical microscopy observations of proeutectoid ferrite and cementite precipitates in steel.J9] Subsequently, a number of studies employed electron microscopy to confirm the essential features of these predictions, albeit with some modification of the details. This paper seeks to summarize these more recent results on the effect of ledges on the growth kinetics and morphology of proeutectoid ferrite and proeutectoid cementite precipitates and to correlate them to earlier growth G. SPANOS, Metallurgist, and R.A. VANDERMEER, Branch Consultant, are with the Physical Metallurgy Branch, The Naval Research Laboratory, Washington, DC 20375-5000. W.T. REYNOLDS, Jr., Assistant Professor, is with the Department of Materials Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0237. This paper is based on a presentation made in the symposium "The Role of Ledges in Phase Transformations" presented as part of the 1989 Fall Meeting of TMS-MSD, October 1-5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL. METALLURGICAL TRANSACTIONS A
studies. A brief description of models for precipitate growth by the ledge mechanism will be given first so as to provide a framework for the interpretation of experimental results. Data on the effects of ledges will then be reviewed separately for the ferrite and cementite reactions. Finally, it should be noted that a sizeable amount of literature available on the effects of ledges on the formation of ferritic structures containing interphase boundary carbides has not been considered here, since these microstructures constitute bainite (on the microstructural definition of bainite u~ rather than proeutectoid ferrite; the reader is instead referred to two excellent review articles by Honeycombe on this topic. [1z,'3[
II. THEORIES OF SOLID-SOLID GROWTH BY THE LEDGE MECHANISM The overall growth rate of a planar interphase boundary advancing by the lateral migration of ledges is given by 18A41 h ~L = ~ v~
ill
where h and A
Data Loading...
