Grain Boundary Strengthening in High Mn Austenitic Steels
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the strengthening mechanisms in metallic alloys, grain boundary strengthening is well described by the Hall–Petch relationship: ry ¼ r0 þ kHP d0:5 , where ry is the yield strength, r0 is the friction stress, and d is the grain size.[1,2] kHP is the Hall–Petch coefficient, which indicates the effectiveness of the grain boundary strengthening; with a larger kHP, ry increases by a higher degree with reducing the grain size. Therefore, kHP is essential information for the heat treatments which control the grain size in order to adjust ry . In general, kHP is determined via the experiment where the ry is measured for the alloys sharing the chemical composition with different d. Although kHP was reported to be similar for the same class of alloys,[3] its sensitively depends on the concentration of interstitial atoms at grain boundaries, which was proved by a field ion atom probe study.[4] Accordingly, kHP of Fe-0.003 wt pct C alloy varied from 10 to 23 MPa mm0.5 by changing the cooling procedure, since slow cooling enhances C segregation at the grain
JEE-HYUN KANG, Research Scientist, is with the Steel Institute, RWTH-Aachen University, Intzestraße 1, 52072, Aachen, Germany, and also with the Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology (POSTECH), Pohang, 37673, Korea. Contact e-mail: [email protected]. SHANGHONG DUAN, Student, and WOLFGANG BLECK, Professor, are with the Steel Institute, RWTH-Aachen University. SUNG-JOON KIM, Professor, is with the Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology (POSTECH). Manuscript submitted November 22, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
boundaries.[5] Takaki et al. attributed such features to the stabilization of the dislocation sources at the grain boundaries by the interstitial atoms.[6] The above-mentioned dependence of kHP on the interstitial concentration has motivated the study in austenitic steels. Owing to their fcc structure at room temperature, the C solubility is higher compared to the conventional ferritic steels. Hence, it suggests that the C concentration at the grain boundaries can vary in a larger degree than that in the ferritic steels, which is likely to allow a larger range of kHP for the austenitic steels. Moreover, the complete austenitic structure without precipitates allows to control the grain size easily and to study the strengthening solely via the grain boundaries. A number of studies have been reported for the austenitic stainless steels,[7–15] and the reported kHP are shown in Figure 1 with respect to the interstitial concentration. Despite the scatter of the data, it is obvious that kHP is approximately 10 to 20 MPa mm0.5 with the low interstitial concentration, and significantly increases with the interstitial concentration. In the case of high Mn austenitic steels which have been studied intensively for various carbon concentrations (0 to 1.2 wt pct) during the last decade,[16] the study about the grain boundary strengthening is only limited to Fe-22
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