Lamellar Phase Transformations in Fe-C-X Alloys

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9628936 Shiflet This is a fundamental examination of pearlite formation in iron-carbon steels. Pearlite is a lamellar product of eutectoid decomposition and is one of the most familiar of all metallographic structures. It can form in steels and also in a range of non-ferrous alloys during transformation under isothermal, continuous cooling and forced-velocity growth conditions. The aim of this project is to extend crystallographic concepts concerning the growth of pearlite and examine the role of ternary substitutional alloying elements in pearlite development. Key questions addressed: Is the alloying element atomic diffusion path at the pearlitic cementite/austenite or pearlitic ferrite/austenite interphase boundaries. Are these boundaries really the rapid diffusion paths they are perceived to be? How does the partitioning of substitutional elements affect pearlite development? Extensive thermodynamic calculations are performed and correlated with experimental observations as to element partitioning and lamellae development. The primary experimental approach includes making all ternary steel alloys in-house and performing heat treatments isothermally in lead baths. Examination of the structure is by high-resolution transmission electron microscopy. Chemical partitioning studies are performed in a Field Emission Gun transmission electron microscope that is capable of collecting chemical data from areas less than 1 nanometer in diameter. %%% Substitutional alloying elements are important in steel production because they can greatly increase the hardenability of the alloy, in part, by delaying the pearlite reaction which leads to a greater ability to produce martensite. Also, small additions of elements that are strong carbide formers, such as vanadium and titanium, can lead to ultra-high strength pearlite structures in medium carbon steel alloys through precipitation hardening the ferrite lamellae with parallel sheets of carbides roughly 10 nanome ters apart. ***