Displacive Phase Transformation and Surface Effects Associated with Confocal Laser Scanning Microscopy
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INTRODUCTION
CONFOCAL laser scanning microcopy (CLSM) is a powerful method for the study of phase transformations at high temperatures because the method avoids the deterioration of the image caused by the thermal radiation.[1] However, observations using this technique may be influenced by the associated lack of constraint at the free surface, especially in the case of transformations that are dominated by strain energy. The martensitic transformation is such a case in which the strain energy resulting from the related shape deformation[2] is a seminal feature that determines not only the shape of the plate[3] but also the magnitude of the free energy change required to drive the transformation.[4] Much of the reported literature in which confocal microscopy has been applied to steels has focused on the nucleation site or the evolution of the growing plate, etc. in the context of martensite.[1,5,6] However, there is a shortage of systematic work to understand the surface effect itself, which was the aim of the current work. Surface martensite has previously been studied in nickel-rich alloys.[7–9] An important interpretation about surface effect was in the work of Klostermann and Burgers,[8] where it was concluded that in the Fe-Ni alloy studied, surface martensite forms at a temperature which is 5 K to 30 K greater than that forming in the bulk of the material, because of the reduced constraint at the surface. The authors talked about hydrostatic JUNHAK PAK, Researcher, and DONG WOO SUH, Professor, are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea. Contact e-mail: [email protected] H.K.D.H. BHADESHIA, Professor, is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology and also with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, U.K. Manuscript submitted September 23, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS A
pressure, but it is the relief of the much greater shear strains that should in fact dominate over volume change effects. We examine, therefore, how an orientation of a plate whose growth is accompanied by a large shear is influenced by the presence of a free surface.
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EXPERIMENTAL DETAILS
The work presented in this study is based on a more extensive study on the coalescence of plates of martensite or bainite, a process that leads to a deterioration in mechanical properties.[10] It was in the course of those studies that the alloy listed in Table I was studied using confocal laser scanning microscopy. Specimens for dilatometry were machined into 3 mm diameter cylinders of 10 mm in length. The specimens for CLSM were machined into 2-mm cubes. The details about the equipment are described in other works.[10,11] In confocal laser scanning microscopy, the sensed temperature that controls the heat treatment differs from that of the specimen surface because of the heat transfer through the body of the alumina crucible on which th
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