Process Metallurgy steel research int. xx (200x) No. x 1 Weak beam TEM study on stacking fault energy of high nitrogen steels M. Ojima 1,2 , Y. Adachi 2 , Y. Tomota 1 , Y. Katada 2 , Y.Kaneko 3 , K.Kuroda 3 and H.Saka 3 1 Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki, 316-8511, Japan 2 National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan 3 Nagoya University, Furo-cho, Chikusa, 464-8603, Nagoya Conatct: ADACHI.Yoshitaka@nims.go.jp Abstarct: The nature of high work-hardening rate of nitrogen bearing steels was examined focusing on stacking fault energy (SFE). Dislocation configuration and dissociated dislocation width in various kinds of austenite stainless steels with and without nitrogen were evaluated by the weak beam method. Nitrogen addition resulted in changing the dislocation configuration from tangled to planar. Nitrogen was, however, found to increase the SFE rather than decrease as reported previously and the SFE can be formulated as a function of chemical composition, SFE(mJ/m 2 )= 5.53-0.16(wt%Cr)+1.40(wt%Ni)+17.10(wt%%N). These results indicate that dislocation planarization by nitrogen addition is unlikely explained in terms of the SFE. Keywords: High nitrogen steel, stacking fault energy, weak beam 1 INTRODUCTION There is an ongoing driving within the steel industry to achieve high strength steels without sacrificing ductility. This has led to the examination of newly graded austenitic steels with relatively low stacking fault energy (SFE). It has been examined extensively that high nitrogen steels (HNS) exhibit a good balance of strength with ductility [1]. This is realized mainly because of high work-hardening of HNS, which seems to be ascribed to the dislocation configuration changed from tangled to planar by adding nitrogen in steels [2]. This planarization of dislocations might contribute to increase the work-hardening rate by suppressing cross slip. Some researchers [2] proposed that the dislocation planarization results from nano-size short range ordering (SRO) [3]. In contrast, others correlated the high work-hardening rate of HNS with the low SFE rather than SRO. Although SFE must also affect the work- hardening, there are two opposing results reported on the effect of nitrogen addition on SFE. Some researchers reported that nitrogen decreased SFE [4-8 1 ], meanwhile the other showed a non-monotonic influence of nitrogen on SFE that nitrogen increased and decreased SFE at lower and higher nitrogen contents, respectively [1, 9, 10, 11 2 ] . The discrepancy on the effect of nitrogen on SFE in 1 [4]In CrNiMn steels, SFE decreases and does not change below and above 0.24mass% N, respectively. [5] In 20Cr20Ni steel, SFE decreases from 48 to 38mJ/m 2 with increasing nitrogen content from 0.005 to 0.05 mass%. [6] In 18Cr13Ni steel, SFE decreases from 16 to 13mJ/m 2 by adding 0.12 mass% nitrogen. [7] In 18Cr10Ni steel, SFE decreases from 10-12 to 9mJ/m 2 with increasing nitrogen content from 0.02 to 0.25 mass%. [8] In CrNi, CrNiMn and NiMn steels, SFE decreases by adding nitrogen. 2 [9]In 13Cr19Mn steel, SFE increases from 32 to 40mJ/m 2 with increasing nitrogen content from 0.05 to 0.23 mass%. [10] In 18Cr10Ni8Mn steel, SFE increases and decreases below and above 0.2-0.3 mass% N, respectively. austenitic stainless steels might be attributed to different chemical compositions of base steels or unreliable measurement technique. Gavriljuk et al. [10] proposed a complicated correlation between the density of states at the Fermi level and SFE. There, based on the different alloying element effects such as Cr and Mn vs. Ni on the electron structure of base steels, different tendency in SFE with nitrogen addition depending on a chemical composition of base steels were explained. However, it is still not fully clear whether nitrogen raises or lowers SFE in austenitic steels. To enhance better understanding of nitrogen addition effect on SFE in austenitic ferrous alloys, further systematic measurement of SFE seems to be required. This study, therefore, aims at clarifying the nature of high work-hardening rate of nitrogen bearing steels with a particular attention to SFE. Dislocation configuration and dissociated dislocation width in various kinds of austenite stainless steels with and without nitrogen were evaluated by the weak beam transmission electron microscope technique. In addition, austenite phase stability was estimated thermodynamically using ThermoCalc. 2 EXPERIMENTAL PROCEDURE Some austenitic stainless steels with and without nitrogen (Table 1) were used in this study. For ternary Fe-Cr-Ni alloys such as 10Cr-20Ni, single crystals were grown by the Bridgeman method. The single crystals were mechanically thinned to the thickness of 150μm, and then were slightly deformed by hand at room temperature to introduce dislocations. A high-nitrogen steel (HNS) with more than 1 mass% nitrogen, but without manganese were prepared using a pressure electro-slag remelting device. Three commercial polycrystalline steels were deformed 5% in tension at room temperature. Thin TEM foils were fabricated by the conventional twin-jet method using a 5% HClO 4 -95% CH 3 COOH solution at room temperature. [11] In 18Cr16Ni10Mn steel, SFE increases and decreases below and above 0.4 mass% N, respectively. In 15Cr17Mn steel, SFE decreases and increases below and above 0.48 mass% N, respectively.