Three-dimensional analysis and classification of grain-boundary-nucleated proeutectoid ferrite precipitates
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NTRODUCTION
STEELS represent by far the greatest volume of metals in use (e.g., Reference 1). As a principal microconstituent of hypoeutectoid steels, proeutectoid ferrite has understandably received considerable attention over the years. Clearly, proeutectoid ferrite has a strong influence on the hardenability and mechanical properties of low- and mediumcarbon steels. As just one example, it has been shown that the mechanical properties of steel weldments used in ship construction are particularly sensitive to the amount and morphology of proeutectoid ferrite that forms.[2,3] In recognition of it’s technological importance, numerous models have been devised to predict the development of proeutectoid ferrite, (e.g., References 4 through 6), along with complementary experimental studies on morphological development[6–10] and growth kinetics.[11–17] Nucleation sites and morphologies of proeutectoid ferrite grains are often critical features of these microstructural-evolution models, but are often dealt with by employing numerous simplifying assumptions. In this vein, investigations of solid-state precipitate morphologies in materials, including proeutectoid ferrite in steels, have typically been based on observations of twodimensional (2-D) cross sections, whether by optical microscopy (e.g., References 6, 9, 18, and 19), transmission electron microscopy (TEM) of thin foils (e.g., References 7, 20, and 21), or other techniques (e.g., Reference 12). More than 50 years ago, in an effort to develop a better understanding of possible precipitate shapes, Dubé developed a morphological classification system of ferrite precipitates in steels[9,18] based on 2-D observations. This classification system, as later modified by Aaronson,[10] is shown in Figure 1. Furthermore, it has been suggested that the Dubé M.V. KRAL, Senior Lecturer, is with the Department of Mechanical Engineering, University of Canterbury, Christchurch, New Zealand. Contact e-mail: [email protected] G. SPANOS, Section Head, is with the Phase Transformations Section, Physical Metallurgy Branch, Materials Science and Technology Division, United States Naval Research Laboratory, Washington, DC 20375-5000. Manuscript submitted August 11, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
system is generally applicable to other products of diffusional phase transformations as well.[10] However, threedimensional (3-D) analyses of a high-carbon Mn steel by serial sectioning and deep etching[22,23] showed that many aspects of the 3-D morphology and connectivity of proeutectoid cementite precipitates are quite different from what had previously been assumed based on 2-D microscopy observations alone (e.g., Reference 24). It thus seems clear that 3-D analyses are critical to understanding the actual morphologies and distributions of proeutectoid ferrite precipitates as well. In this regard, some enlightening computer-assisted 3-D reconstruction studies of degenerate ferrite[25,26] and of ferrite nucleation on inclusions[27] have recently been reported. The em
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