Mechanisms of Pearlite Spheroidization
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I.
INTRODUCTION
A two-phase lamellar structure is commonly observed in metallic systems, e.g., eutectoid pearlite and eutectic alloys. The high thermal stability of some lamellar structures makes them potentially useful in high temperature service. But the mechanism and kinetics of spheroidization must be thoroughly understood to achieve this potential in practical applications. Many models for the mechanism of spheroidization of lamellar structures have been proposed, but the three major ones are: 1,2 1. Rayleigh's capillarity induced perturbation theory.3-9 2. Grain boundary thermal groove theory.l~ 3. Fault migration theory. 17,18.19 Although these theories are useful in explaining certain aspects of the spheroidization process, there are still many details of the phenomena which can not be explained by the theories, and some conflicts exist among them. For instance, none of these models takes into account the factor of anisotropy of the crystallography and interfacial energy or imperfections in microstructures. Moreover, the activation energy estimated from the experiments which were used to support these theories varies over such a wide range (from 30 kcal/mole to 60 kcal/mole) that the corresponding suggested rate-controlling diffusion mechanisms are entirely different from one anotherfl~ In addition, the study of the kinetics of pearlite spheroidization has been given less than adequate attention. Only a few studies have been reported. 2~ Most of them work only qualitatively but fail quantitatively. Hence we can say that a satisfactory explanation of pearlite spheroidization has not yet been achieved. The objective of this research is to clarify the mechanisms and kinetics of pearlite spheroidization during static annealing processes. This paper, as the first part of our research report, focuses on the mechanisms, and the second part reports on the kinetics. 25
II. T H R E E M O D E L S F O R THE M O R P H O L O G I C A L INSTABILITY OF L A M E L L A R S T R U C T U R E S
A. Rayleigh's Perturbation Theory Figure 1 schematically illustrates Rayleigh's perturbation model which deals with the instability of a cylindrical shape caused by capillarity-induced perturbation. It has been demonstrated, both theoretically and experimentally, 3-7 that a long rod shape is intrinsically unstable with respect to a sinusoidal perturbation provided the perturbation wavelength is greater than a critical wavelength, hc. The rod would spontaneously break into a row of spherical particles with intervals of maximum perturbation wavelength, h .... The values of hc and Amax depend on the mass transport mechanisms. For instance, if interface diffusion is the rate controlling mechanism, A, = 2~-r and hmax = 2zrk/~r for an infinite length rod 4 where r is the radius of the r o d ) In contrast, a platelet shape is stable against any capillarity-induced perturbation. Theoretical analysis given by Mullins indicates that any perturbation on a flat interface will decay with time, which means Rayleigh's perturbation theory can not be di
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