Driving Force and Thermal Activation in Martensite Kinetics
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THE properties of engineering steels during production, manufacturing, and use depend upon our ability to exert microstructure control. Among other means, martensite transformation has been proven especially useful. However, martensite is not as trivial as the previous sentence may convey. Although vintage abaci and compilations have been replaced by modern data bases, so did the complexity of issues emerging from the development of new steels and steel usage, a scenario that supports further delving fundamental aspects of martensite particularly in steels. Martensite in steels is typically heterogeneous and autocatalytic. The reaction proceeds by the nucleation of new units instead of growth of a few ones. Observation of plates in self-accommodating ‘‘zig-zag’’ patterns is common, as shown in Figure 1. The reaction kinetics responds to mechanical stimuli, hydrostatic pressure, and magnetic fields, as will be discussed in Sections IV through VI. The usefulness of classical homogeneous nucleation concepts to martensite was questioned early as the approach failed in providing a unified view of the perceived characteristics of martensite kinetics. To cope with the issues, it has been postulated that compositional[1] or structural martensite embryos[2] be part of the nucleation process. The second type of embryo has been further developed. In the meantime, acknowledged J.R.C. GUIMARA˜ES, (ret) Researcher at Large, is with the Escola de Engenharia Industrial Metalu´rgica de Volta Redonda, Universidade Federal Fluminense, 27255-125, Volta Redonda, RJ, Brazil, and Mal. Moura 338H/22C, 05641-000, Sa˜o Paulo, SP, Brazil. P.R. RIOS, Professor, is with the Escola de Engenharia Industrial Metalu´rgica de Volta Redonda, Universidade Federal Fluminense. Contact e-mail: [email protected]ff.br Manuscript submitted February 14, 2009. Article published online August 19, 2009 2264—VOLUME 40A, OCTOBER 2009
crystallographic aspects of martensite pointed out that the martensite-austenite interface should be semicoherent.[3] Extension of the concept to the realm of the martensite nucleus yielded a nucleation barrier that would not be overcome by thermal fluctuations alone.[4] This led to relocation of the critical barrier from austenitefimartensite to embryofimartensite. Singledomain embryos with semicoherent interfaces were assumed to propagate into martensite by punching out dislocation loops to maintain semicoherence. The calculated activation energy for this process is much less than the overall nucleation barrier for a semicoherent nucleus. A linear relationship between the activation energy for nucleation and driving force is a characteristic of the model.[4] Later, Olson[5] proposed a reaction path comprising the dissociation of an austenite defect into a bcc fault assembly driven by the available free energy. Propagation into martensite would become barrierless for a negative fault energy. The process requires dislocation motion (the partials bounding the fault); hence, it depends on thermal activation that may lead to timedepe
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