Grain Cluster Microstructure and Grain Boundary Character Distribution in Alloy 690
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ALLOY 690 has been used as a substitute for Alloy 600 as a steam generator tube material in pressurized water reactors because of its excellent corrosion resistance in a broad range of aqueous environments.[1–3] With prolonged service life and improved performance being demanded by the nuclear energy industry, however, the need to improve the resistance to intergranular failure in Alloy 690 should also be considered. Grain boundary engineering (GBE) has attracted much attention in material science and engineering research[4,5] since the concept of ‘‘grain boundary design’’ was first proposed by Watanabe.[6] With the help of the automatic indexing electron backscatter diffraction (EBSD) technique, GBE research advanced a great deal. In studies of GBE, grain boundary misorientations are often classified according to the coincident site lattice (CSL) model.[7,8] The aim of GBE is to enhance the grain-boundary-related properties of materials by increasing the population of low-R CSL (R £ 29) grain boundaries.[9,10] The boundaries that do not have CSL misorientations are often defined as random grain boundaries. However, the CSL grain boundary with a high R value cannot be identified automatically by EBSD. Hence, in experimental studies, random grain boundaries include general large-angle grain boundaries SHUANG XIA, Assistant Professor, BANGXIN ZHOU, Professor and Academician of Chinese Academy of Engineering, and WENJUE CHEN, Associate Professor, are with the Institute of Materials, School of Materials Science and Engineering, Shanghai University, P.O. Box 269, No. 149 Yanchang Road, Shanghai 200072, People’s Republic of China. Contact e-mail: [email protected]; [email protected] Manuscript submitted December 2, 2008. Article published online September 26, 2009 3016—VOLUME 40A, DECEMBER 2009
and CSL grain boundaries with a R value large enough that it cannot be identified automatically by EBSD. The grain boundary character distribution (GBCD) of fcc metal materials with low to medium stacking fault energy (SFE) can be altered through thermal-mechanical processing (TMP) to improve the grain-boundaryrelated bulk properties.[9–11] Alloy 690 is an fcc material with low SFE; therefore, GBE may increase the resistance of the alloy to intergranular failure. One of the central topics of GBE is how the TMP affects the microstructural evolution that leads to high proportions of low-R CSL grain boundaries. A common conclusion on this topic has not been reached. However, there were two common points that can be summarized, as follows: (1) only the proportion of R3n (n = 1, 2, 3…) grain boundaries could be altered in the GBCD, while other low-R CSL grain boundaries had very low occurrences in spite of the selection of the TMP; and (2) the magnitude of deformation used in TMP was small, often on the order of 3 to 30 pct. Thus, the matter turns to how a small deformation plus different annealing treatments enhance the proportions of R3n grain boundaries. Some articles pointed out that conventional recrystallization introduced random
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