In-Situ Observation of Texture Changes during Phase Transformations in Ultra-Low-Carbon Steel
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TRODUCTION
TEXTURE changes during phase transformations in metals are of longstanding interest, particularly a commonly observed memory effect, when the metal is heated, transforms to a new structure and returns to the original structure upon cooling. This implies a crystallographic relationship between parent and product phases, as well as variant selection, during the forward and reverse phase transformations occurring during heating and cooling. The relationships have been studied extensively in diffusionless transformations such as in martensitic steels.[1] Transmission electron microscopy revealed details on intergrowths between martensite and austenite,[2–4] and the preferred structural relationships have been modeled with molecular dynamics calculations.[5,6] Currently, there is much interest in epitaxial relations in thin films of martensitic systems.[7] While much of this work concentrates on local features, the microscopic processes are also expressed in macroscopic bulk properties such as crystal orientation distributions. There is an extensive literature on textures in steel (e.g., reviews[8–12]), including effects of phase transformations.[13–15] Yet not much direct experimental information about the high-temperature c-phase (fcc) texture exists. The reason is because quantitative in-situ texture measurements above 950 C have been very difficult to achieve. Recently, new methods have become available for such investigations, such as synchrotron X-ray and time-of-flight (TOF) neutron diffraction with vacuum furnaces. The incentive behind this investigation was to test the capabilities of the new TOF diffractometer highH.-R. WENK, Professor, and I. HUENSCHE, are with the Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA. Contact e-mail: [email protected] I. HUENSCHE is with the Institut fuer Strukturphysik, Technische Universita¨t, 01069 Dresden, Germany. LEO KESTENS, Professor, is with the Department of Material Science and Engineering, Delft Technical University, 2628 CD , Delft, The Netherlands. Manuscript Submitted October 10, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
pressure preferred orientation (HIPPO) at Los Alamos Neutron Science Center (LANSCE), and a common metal such as iron was a good example. The results of the study are interesting enough to warrant reporting. The review of Ray et al.[12] on phase transformations in steel and their effect on texture still provides most of the background, and the reader is referred to it for any details on previous work. Based on single-crystal studies and orientation relationships in precipitates, three orientation relationships between low-temperature bcc and high-temperature fcc structures have been proposed.[16] The first suggestion was by Bain and Dunkirk,[17] {001} (bcc) fi {001} , which produces three crystallographically equivalent variants for each parent orientation. Later Kurdjumov–Sachs (KS)[18] introduced a {110} (bcc) fi {111} relationship (Figure 1). In this case, there are 24 variants. Nishiyama–Wasserm
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