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Doctor of Philosophy (PhD)


The kinetic morphologies of proeutectoid ferrite developed in a coarse-grained 0.29% C, 0.76% Mn steel during isothermal reaction at temperatures from 775°C through 550 C have been catalogued and their development as a function of transformation time has been studied by means of intensive optical metallographic examination. Supplementary electromicrographic studies were made on specimens of special interest. On the bases of these results and of published theoretical concepts of homophase boundary movement processes, a qualitative general theory of crystal growth in the solid state was constructed in terms of the effects of the ‘Structural Variables" of nucleation site, interphase interface migrations! characteristics and strain energy minimization. These considerations were applied in attempts to explain both previously established and newly discovered morphological phenomena# Material in the latter category included: Widmanstatten sideplates growing directly from small angle austenite grain boundaries; evidence for a layer-wise thickening mechanism of sideplates degeneration of the sideplate structure in the temperature range 675°C through 575°C in several rather specific fashions, with normal forms being recovered at 550°C sawtooth-shaped crystals of Widmanstatten character growing from grain boundary ferrite, or occasionally, from the austenite grain: boundary itself; nucleation of ferrite at pre-existing austenite:ferrite interfaces; oriented interfaces of even very early reaction time grain boundary ferrite composed of both linear and curved segments, suggesting a discontinuous thickening mechanism; effects on the morphology of structures formed on grain boundary ferrite bases which appear to be ascribable to the detailed shape of the oriented austenite:grain boundary ferrite interfaces; frequent raggedness of grain boundary ferrite formed at 775°C, tending to disappear rapidly as the transformation temperature is reduced; tiny late reaction time sideplatelets developing largely from 775°C grain boundary ferrite; later reaction time structures of grain boundary ferrite forced at 750°C and below noticeably different from those fomed at 775°C, becoming quite blocky in character; considerable irregularity of interface shape as a general feature of intragranular idiomorph; conversion of idiomorphs to thick plates at late reaction times; nucleation of many of the first formed intragranular plates by inclusions and nucleation of virtually all of the remainder at the interphase interfaces of previously formed plates; choking of the interiors of large austenite grains with great numbers of intragranular plates, to the exclusion of the pearlite reaction, at lower reaction temperatures.

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