Unified Model for Plate and Lath Martensite with Athermal Kinetics
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NTRODUCTION
MODERN low alloy structural steels sport engineering properties developed by thermal and thermal mechanical treatments aiming at a fine-grained austenite and its controlled phase transformation. This phase transformation frequently includes a small fraction of martensite to achieve a broader range of mechanical properties. In the other extreme of the composition range, the higher alloyed tool or engineering steels are used in the quench-hardened and tempered condition. In previous articles,[1–3] we have described the factoring of the martensite reaction rate into an athermal factor (driving force dependent, inferred from small FeNi particles transformed by quenching) and a thermally activated factor expressed by the Boltzmann factor. By incorporating a modified grain partitioning concept[2] and associating autocatalysis to the martensite-austenite interfaces, it was possible to work out a unified description of martensite transformation kinetics, including athermal, isothermal, isothermal induced by depressurization, magnetic field, and applied strain in iron alloys, and to obtain activation energies within an order of magnitude of one another and compatible with results found in the literature.[3] Martensite is observed in iron base alloys exhibiting two distinct morphologies: lath/packet and plate martensite.[4] The former is predominant when the carbon and alloy content of the martensite is low, whereas the latter is J.R.C. GUIMARA˜ES is Researcher at large with the Universidade Federal Fluminense, Escola de Engenharia Industrial Metalu´rgica de Volta Redonda, Volta Redonda, RJ, 27255-125, Brasil and Mal. Moura 338H/22C, Sa˜o Paulo, SP, 05641-000, Brasil. P.R. RIOS, Professor, is with Universidade Federal Fluminense, Escola de Engenharia Industrial Metalu´rgica de Volta Redonda, and RWTH Aachen University, Institut fu¨r Metallkunde und Metallphysik, D-52056 Aachen, Germany. Contact e-mail: [email protected]. uff.br Manuscript submitted January 10, 2010. Article published online May 22, 2010 1928—VOLUME 41A, AUGUST 2010
normally found in higher alloyed high-C steels. It is evident that both lath and plate martensites are of considerable practical interest. In either case, martensite obtained by cooling is generally considered to be athermal. Formally, we will follow this operational view in this article, although, as described previously, a thermal activated barrier may exist along the reaction path. The displacive nature of martensite is crucial regarding the microstructure and the entailed properties. Martensite transformation is accomplished by coordinated displacements of atoms over distances shorter than an atomic distance. The resultant shape change allows interaction with internal and external stress fields, but the size of a martensite unit is limited by obstacles opaque to the mechanism of the reaction. The martensite units cannot cross grain boundaries; consequently, the austenite grain size has a notable effect[5,6] on the resultant microstructure. Additionally, the thickening of the martensite
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