Morphology of Proeutectoid Ferrite
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INTRODUCTION
THE morphology of proeutectoid ferrite in steels has attracted much attention ever since Sorby[1] first examined a polished and etched section of a steel specimen almost one and a half century ago. Dube´[2] devised a framework for classifying various shapes. A slightly modified system was then published by Aaronson[3] at the 1960 Conference on The Decomposition of Austenite by Diffusional Processes and recently by Kral.[4] At the same conference Hillert[5] presented another system for classifying shapes of grain boundary nucleated particles, two of which had facets to both grains. See Figures 1 and 2. Dube´’s system, as modified by Aaronson, has been widely accepted and in a recent volume on Phase Transformations in Steels[6] there were no less than eight citations. However, they only concern the two-dimensional shapes observed in the plane of polish. In recent years, there have been several reports on the 3D shapes of ferrite particles[7–9] that have yielded valuable information by serial sectioning. The first detailed study of the orientation relationships between grain boundary nucleated particles of ferrite and the austenite grains was carried out by King and Bell[10] using electron diffraction. In recent years, there have been several studies using laser-scanning confocal microscopy (LSCM)[11,12] and electron back scattering diffraction (EBSD).[13–16] The modern techniques have open up new possibilities for efficient studies of crystallographic aspects of various phase interfaces with ferrite particles and also the role of the orientation JIAQING YIN, MATS HILLERT, and ANNIKA BORGENSTAM are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, Brinellvgen 23, 100 44, Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted June 23, 2016. Article published online January 5, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
relationship between two austenite grains and the direction of their grain boundary on the selection of particle shapes. However, a purpose of the present work was to provide a more thorough analysis of the 2D shapes of grain boundary nucleated ferrite particles to be used as a basis for studies with modern techniques. In early studies of particle shapes, there was a practical limit caused by the resolution of light optical microscopy (LOM). To facilitate the comparison with old observations and classification systems based thereon, it was decided again to use LOM as the main tool also for the present study of particle shapes at 973 K (700 °C). scanning electron microscopy (SEM) was adopted when the study was extended to lower temperatures. It is interesting to note that also in recent 3D studies with SEM one has often presented the 2D microstructure with LOM. A second purpose of the present work was to study the role of various particle shapes in the development of a particular microstructure of upper bainite, called feathery bainite.[17] A third purpose was to study the role of nucleation of new ferrite particles in the growth of feather
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