An Investigation Into 6-Fold Symmetry in Martensitic Steels
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useful high strength structural steels have a dislocated martensite structure and as such have been studied extensively. Recent work has focused on the transition from austenite to martensite in order to better understand the visually complicated martensitic structure, which turns out to have a simple, hierarchical pattern.[1] The purpose of this paper is not to describe this structure, as it has been detailed extensively elsewhere,[1–9] but rather to describe an interesting experimental observation and determine the crystallographic origin of this observation. When examining prior austenite grains using electron backscatter diffraction (EBSD), the image that is most commonly produced is the inverse pole figure map, or IPF map, shown in Figure 1(a). These images use color to depict the orientation of the lattice; after examining a number of IPF images, one can start to see where the prior austenite grain boundaries are and segment the boundaries as shown in Figure 1(a). However, to truly
CHRISTOPHER KINNEY, Research and Design Engineer, KEN PYTLEWSKI, Graduate Student, and ARMEN G. KHACHATURYAN and J.W. MORRIS, Jr., Professors, are with the Department of Materials Science and Engineering, University of California, Berkeley, CA, 94720. Contact email: [email protected] LIANG QI, Professor, is with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, 48109. Manuscript submitted April 11, 2016. Article published online August 15, 2016 5266—VOLUME 47A, NOVEMBER 2016
determine if the martensitic transformation has gone to completion, one must observe the (001) pole figure. As shown previously, a complete transformation will result in the formation of the 24 Kurdjumov–Sachs (KS) variants, and all these variants can be identified on the (001) pole figure.[1,2,4–9] The (001) pole figure of a grain which has completely transformed also exhibits a 3-fold symmetry, which results from the choice of the three different Bain axes available for the martensitic transformation (Figure 1(b)). In the course of examining a number of prior austenite grains from a variety of steel compositions (notably Fe-9Ni, Fe-12Mn, AISI 1030 and 4140 steels), some martensitic prior austenite grains were found that superficially looked very similar, such as grains 1 and 2 in Figure 1(a), but exhibited different symmetries present in their (001) pole figures (Figures 1(b) and (c)). It is important to note that there are no visual clues as to which grain, 1 or 2, should have this 6-fold symmetry or why it should have this extra symmetry, and in fact, there would be no reason to guess that these two grains differ in any substantial way at all from observing the IPF map (Figure 1(a)). In this paper, we will examine why this 6-fold symmetry is present across a variety of martensitic steels, including samples that have a conventional martensite structure (Fe-9Ni) and those that have an unusual dislocated martensite structure (Fe-12Mn), and we will find that there is a simple explanation for this additional symmetry. There were
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