Characterization of a massive transformation by microstructural analysis
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I.
INTRODUCTION
SOLID-STATE phase transformations in which identifiable nodules of a daughter phase emerge from the parent phase accompanied by the formation of a distinct interface separating the daughter phase from the parent phase and which then grow by means of interface migration until the reaction is complete are termed "nucleation and growth" or reconstructive reactions.tq The massive transformation in alloys is one such reaction. It has been the goal of phase transformation research to establish the atomistic mechanisms responsible for the various nucleation and growth phenomena in metals and alloys and to develop analytical microstructural representations of the overall reaction processes. While considerable progress has been accomplished over the years, particularly for specific transformations such as the precipitation of proeutectoid ferrite from austenite in iron-base materials,t2,3] much remains to be done. The approach used by investigators to identify and elucidate the atomic mechanisms of nucleation and growth reactions in specific systems consists of experimentally measuring isothermal nucleation rates and interface migration rates and then comparing these with predictions of the existing theories. While obtaining measurements of interface migration rates is relatively straightforward, meaningful experimental nucleation rates, by contrast, are most difficult to achieve. Indeed, to make the nucleation rate measurement correctly in an opaque solid, a number of specimens reacted for different times must be examined. A unit volume of material in each specimen must be sampled by serial sectioning procedures and the number of nodules in that volume counted.t4] The task is made more complicated by the fact that the nodules of the daughter phase can
R.A. VANDERMEER, Branch Consultant, and B.B. RATH, Associate Laboratory Director, are with the Naval Research Laboratory, Washington, DC 20375-5000. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled "Atomistic Mechanisms of Nucleation and Growth in Solids," organized in honor of H.I. Aaronson's 70th Anniversary and given October 3-5, 1994, in Rosemont, Illinois.
METALLURGICAL AND MATERIALS TRANSACTIONS A
be very small and not easy to detect during the early stages of transformation. In this article, an alternative method of investigating nucleation and growth rates in a solid undergoing isothermal phase transformation will be presented. This approach is indirect and relies on the experimental characterization of the evolving microstructure at every stage of transformation by the measurement of the microstructural variables, Vv, the volume fraction transformed, Sv, the parent/daughter interfacial area density, and A~=, the largest intercept-free nodule length of the newly forming daughter phase. These data are then compared to various statistically based geometrical representations or models of the microstmcture, also expressed analytically in terms of these same variables. The models are formulated in ter
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