The combined effect of molybdenum and nitrogen on the fatigued microstructure of 316 type austenitic stainless steel

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1 The combined effect of molybdenum and nitrogen on the fatigued microstructure of 316 type austenitic stainless steel M. Murayama, K. Hono, H. Hirukawa, T. Ohmura and S. Matsuoka National Research Institute for Metals, Tsukuba, Japan Keywords: fatigue, austenite steel, field ion microscope, atom probe, TEM Introduction The mechanical properties of austenitic stainless steels, including the low cyclic fatigue behavior have been a subject of numerous studies [1-8]. Nitrogen addition, in general, increases the fatigue strength by decreasing the stacking fault energy of austenitic stainless steels [6]. The stacking fault energy has a strong influence on the cross slip difficulty, thus it affects the saturation dislocation structure formed by the cyclic deformation. However, several studies [9-12] reported that the effect of nitrogen on the dislocation distributions in the deformed microstructures can not be explained by a decrease in the stacking fault energy alone. Swann [9] observed the microstructure of elongated Cr-Ni austenitic stainless steels and suggested that nitrogen atoms associated with the chromium atoms interact with dislocations. Douglass et al. [10] proposed that nitrogen atoms are attracted to the short range ordered Fe/Cr regions and this ordering leads to a change in the dislocation structure. Recently, using X-ray absorption fine structure measurement, Oda et al. [11] suggested the Cr-N interstitial-substitutional (I-S) complex in a 15Cr-15Ni austenitic stainless steel plays an important role in work softening in the low cycle fatigue test. Hence, there is some disagreements on the effect of nitrogen on the dislocation structure of the cyclic deformed austenitic stainless steels. Recently, Vogt et al. [6] reported that nitrogen addition improves the low cycle fatigue resistance of 316 type austenitic stainless steel. They attributed this effect to the reduction of the stacking fault energy by the addition of nitrogen. More recently, Hirukawa et al. [13] confirmed that additions of more than 0.2 wt.% nitrogen to a 316 stainless steel decrease the fatigue crack growth rate and increase the fatigue threshold. In order to understand the origin of the nitrogen effect, Hirukawa et al. [13] measured the fatigues properties of Mo-free 316 stainless steel and reported that the effect of nitrogen addition is not noticeable when Mo is removed from the 316 type stainless steel. Since the role of Mo was not considered in the most of previous studies, the present study aimed at understanding the combined effect of Mo and N on the fatigue properties of 316 type austenitic stainless steels. For this purpose, we have investigated the fatigued microstructure and the distribution of nitrogen by a transmission electron microscope (TEM), a conventional atom probe (1DAP) and a three dimensional atom probe (3DAP). Experimental Procedures The chemical compositions of the alloys used in this study are shown in Table 1. Unlike the standard 316 stainless steels, the experimental alloys used in this study are free of Si, because mass-to-charge ratios of Si 2+ and N + ions overlap in atom probe mass analysis. It was confirmed experimentally that the crack propagation property of the alloys were not affected by the Si contents [13]. The alloys were solution heattreated at 1080 C for 30 min and subsequently water quenched. Table. 1 Chemical compositions of the specimens used in the present study Cr Ni Mo Mn Si Cu N 2.2Mo-0.2N <0.01 < Mo-0.2N <0.01 < Mo-0.0N <0.01 <0.01 < All concentrations are in wt. %. Balance is Fe.

2 For atom probe analyses, a locally built reflectron-type energy compensated time-of-flight atom probe (1DAP) and a three dimensional atom probe (3DAP) equipped with CAMECA tomographic atom probe (TAP) detection system [14] were used. Field ion microscopy (FIM) images were observed at temperatures of K with Ne as an imaging gas, and atom probe analyses were carried out at a specimen temperature of about 45 K, under a vacuum of approximately ~1x10-10 Torr, with a pulse fraction (V p /V dc ) of 20 % and a pulse repetition rate of 1500 Hz. Microstructures of the specimens were examined with a Philips CM200 transmission electron microscope (TEM), operated at 200 kv. Thin foils for TEM observations were prepared by grinding the slices to a thickness of about 100 µm, which were subsequently electropolished by the twin jet technique using a 5 % perchloric acid-acetic acid solution at room temperature. The TEM specimens were prepared from the gage sections of the low cycle fatigue test specimen which were cycled until fracture at the total strain amplitude, ε ta =0.6 % and 1.25 % and were cut parallel to the axis of loading. Results and Discussion The fatigue crack propagation rate, da/dn, as a function of the effective stress intensity range, K eff, of the modified SUS316 stainless steels measured by Hirukawa et al. [13] is reproduced in Fig. 1. The crack propagation rate of the 2.2Mo-0.24N specimen is improved more than 50% in the entire range of K eff compared with that of the nitrogen-free specimen. Nitrogen addition to the Mo-free specimen is effective only in the low K eff region, and da/dn of the 0.0Mo-0.23N specimen is almost the same as that of the N-free specimen in the large K eff region. This indicates that nitrogen addition is effective to improve the fatigue crack propagation properties only when it is added in combination with Mo. Such a combined effect of N with Mo has not been addressed in any of the previous works. It should be noted that the low cycle fatigue life was also improved by combined addition of molybdenum and nitrogen [13]. Figures 2 (a) - (c) show TEM micrographs of the fatigued microstructures of 2.2Mo-0.0N, 0.0Mo-0.2N and 2.2Mo-0.2N specimens at ε ta =0.6 %. The microstructure of the nitrogen free 2.2Mo-0.0N alloy, Fig. 2 (a), shows a typical dislocation cell structure with the interior of the cells almost free of dislocations. For the evolution of such a dislocation cell structure, cross slip is required, thus it is believed that the stacking fault energy of this material is high. The drastic effect of nitrogen addition on the dislocation structure can be seen in Fig. 2 (b) and (c). In the 0.0Mo-0.23N specimen, dislocations form bands, but the cell structure is not Mo-0.0N 0.0Mo-0.23N 2.2Mo-0.24N / m cycle -1 da dn Keff / MPa m 1/2 Figure 1. Fatigue crack propagation properties at room temperature (after Hirukawa et al.).

3 (a) (b) (c) g g g nm Figure 2. Fatigued microstructures of cyclically deformed (a) 2.2Mo-0.0N, (b) 0.0Mo-0.2N and (c) 2.2Mo-0.2N alloy at room temperature ( ε ta = 0.6%). (a) (b) (c) g200 g200 g nm Figure 3. Fatigued microstructures of cyclically deformed (a) 2.2Mo-0.0N, (b) 0.0Mo-0.2N and (c) 2.2Mo-0.2N alloy at room temperature ( ε ta = 1.25%). developed. Several stacking faults are observed with a thickness fringe contrast. This indicates that nitrogen addition decreases the stacking fault energy significantly as reported by Stoltz and Vander Sande [5]. Although the stacking fault energy is believe to be low in this specimen, planar array of dislocations is not evident. In contrast, the microstructure of the 2.2Mo-0.2N alloy shows planar array of dislocations. The planar structure of dislocations were often reported from the specimens with a low stacking fault energy, because the cross slip difficulty increases as the stacking fault energy decreases. If the cross slip is difficult, the dislocations are observed along their slip planes and they do not form bands of tangled dislocations. It should be noted that extended stacking faults are not observed in the 2.2Mo-0.2N specimen at all, although many stacking faults are observed in the 0.0Mo-0.2N specimen. This suggests that the planar dislocation structure may not be explained only by the low stacking fault energy. Figure 3 shows fatigued microstructures of 2.2Mo-0.0N, 0.0Mo-0.2N and 2.2Mo-0.2N specimens at ε ta =1.25 %. In addition to the nitrogen-free 2.2Mo-0.0N alloy, the microstructure of the 0.0Mo-0.2N specimen shows the dislocation cell structure. However, the microstructure of the 2.2Mo-0.2N specimen still shows planar bands of dislocations. These results indicate that the dislocation structure in the fatigued microstructure of the 2.2Mo-0.2N specimen is completely different from those of the 2.2Mo-0.0N and 0.0Mo-0.2N specimens. This again indicates that the reduction of the stacking fault energy alone cannot explain the extended fatigue life.

4 (a) (b) (c) Figure 4. Ne field ion images of the (a) 2.2Mo-0.0N, (b) 0.0Mo-0.2N and (c) 2.2Mo-0.2N alloy observed at 40-60K.The arrows in photograph (c) indicate the diffused bright spots correspond to the Mo-N pair. In order to examine how Mo and N are dissolved in the austenitic phase, the distributions of atoms were observed by FIM and APFIM. Figures 4 (a) - (c) show Ne field ion images of 2.2Mo-0.0N, 0.0Mo-0.2N and 2.2Mo-0.2N specimens. In Figs. 4 (a) and (b), all bright spots exhibit the same imaging feature. This suggests that major alloying elements protruding on the specimen surface are contributing to the formation of the bright spots with the same manner. In Fig. 4 (c), diffuse bright spots are observed (indicated by arrows) in addition to the normal bright spots observed in Fig. 4 (a) and (b). These diffuse bright spots are observed only in the specimen containing both Mo and N, thus it is believed that the diffuse spots are attributed to Mo-N pair or clusters. Wada et al. [15] reported the presence of such diffuse spots in Ne field ion images of Cr-Mo- C steel and concluded that these spots are due to Mo-C clusters. They proposed that the diffuse nature of these spots is due to the protrusion of Mo-C clusters on the specimen surface, which causes higher electric field than the best image field of Ne. If the bonding of Mo-N is strong, the evaporation field of Mo-N cluster will be higher than that of the surrounding atoms, and this would also cause diffuse bright spots as seen in Fig. 4 (c). Figures 5 (a) - (c) show atom probe mass spectra obtained from the 2.2Mo-0.0N, 0Mo-0.2 N and (a) (b) Mo ++ N + (c) Cr ++ Fe ++ Ni ++ Mo +++ N + Mo ++ MoN ++ Figure 5. Atom probe mass spectra obtained from the (a) 2.2Mo-0.0N, (b) 0.0Mo-0.2N and (c) 2.2Mo-0.2N alloy.

5 2.2Mo-0.2N specimens. From the nitrogen-free specimen, all metallic ions are detected as doubly-charged single metallic ions. All Mo atoms are detected as Mo ++. When nitrogen is added in the Mo-free specimen, nitrogen atoms are exclusively detected as N +. Although Cr has a strong affinity with N, no CrN compound ions are detected. On the other hand, in the 2.2Mo-0.2N alloy, nitrogen atoms are detected as MoN ++ ions as well as N + ions. In this specimen, approximately 30 % of N atoms are detected as MoN ++ ions. CrN compound ions are not detected in this alloy as well. However, it should be noted that this does not rule out the possibility of the presence of Cr-N pairs in the specimen, since they may be detected as separate Cr and N ions. In other words, the detection of MoN ++ ion indicates that there is a very strong interaction between Mo and N, because Mo-N pairs are preserved as molecular ions even after the field evaporation process. Therefore, it is concluded that the Mo and N form strong I-S atomic pairs in the alloy. From this atom probe result, the diffuse bright spots in the FIM image can be attributed to Mo-N pairs. Detection of MoN ++ ions was also reported by Lundin et al. [16] in an 9Cr-1Mo martensitic steel, and they also interpreted this as evidence for Mo-N clusters. In order to investigate whether or not these Mo-N pairs form clusters in the austenite phase, the distributions of the alloying elements were examined by 3DAP. Figure 6 shows 3DAP elemental maps of N, Mo, Ni and Cr in the 2.2Mo-0.2N alloy. In these maps, each dot corresponds to the positions of the individual atoms. The uniformly distributed N atoms indicate that nitrogen does not form their own clusters. In addition, the uniformly distributed Mo, Cr and Ni atoms suggest that austenite phase is a uniform solid solution. It is known that the affinity between nitrogen and chromium is high. Douglass et al. [10] and Oda et al. [11] reported the presence of Cr-N clusters and they proposed that these clusters influence the dislocation structure, thereby changing the cycling deformation mode. Since Cr concentration in the alloy is high (~20 at.%), there is no doubt that many nitrogen atoms have Cr as the nearest neighbor atoms. Assuming that N atoms occupy octahedral sites, 1.2 Cr atoms are around a nitrogen atom in average. Therefore, N atoms can move freely throughout the lattice maintaining the contact with at least one Cr atom. Thus, Cr-N clusters, if present, may not work effectively as obstacles of dislocation motion. In the atom probe analysis, no nitrogen atoms were detected as CrN molecular ion, suggesting that their bonding is not strong as that of Mo-N. On the other hand, approximately 30% of nitrogen atoms are detected as Mo-N pairs in the atom probe mass spectrum obtained from the 2.2Mo-0.2N specimen, Fig. 5 (c). 3DAP results suggest that these Mo-N pairs are uniformly distributed in the austenite phase. In this case, the densities of Mo-N atom pairs in the matrix is estimated to be ~ 0.2 nm -3. This means the average separation distance between the adjacent Mo-N pairs is 2.28 nm, which roughly corresponds to 7.9 Burgers vectors. Dislocations moving through the lattice would have to destroy the strong Mo-N bonding and this would work as strong obstacle for the dislocation motion. This is in line with the proposal that short range order (SRO) or precipitate particles is more important factor for the formation of planar dislocation structures rather than the stacking fault energy [12]. Summary The mechanism of the improved fatigue crack growth properties of SUS316 austenitic stainless steels by nitrogen addition has been investigated by comparing the fatigue properties of the modified SUS316 N Ni Mo Cr ~25nm Figure 6. 3DAP elemental mapping of N, Mo, Cr and Ni obtained from the 2.2Mo-0.2N alloy. ~10nm

6 based steels with and without Mo and N. TEM observations of the dislocation structures and APFIM analysis results suggest that this effect is not only due to the single addition of nitrogen, but also is due to the interactions between Mo and N atoms. FIM and atom probe results strongly suggest that there are Mo-N pairs in the nitrogen containing SUS316 stainless steel, and TEM observation results show the planar array of dislocations only in the Mo-N containing specimens. These results suggest that the stacking fault energy alone cannot explain the change in the fatigue properties, but the presence of Mo-N atomic pairs would also influence the dislocation microstructure. Acknowledgements The authors wish to thank Dr. O. Umezawa at NRIM and Prof. W. T. Reynolds Jr. at Virginia Polytechnic Institute and State University for valuable discussions. This work was supported by the Frontier Research Center for Structural Materials, NRIM. References 1. M. Fujikura, K. Takada and K. Ishida, Trans. ISIJ 15, 464 (1975). 2. S. Horibe, Y. Seki, T. Fujita and T. Araki, Tetsu-to-Hagane 64, 94 (1978). 3. T. Kato, M. Fujikura, S. Yahagi and K. Ishida, Tetsu-to-Hagane 67, 587 (1981). 4. K. Shibata, N. Namura, Y. Kishimoto and T. Fujita, Tetsu-to-Hagane 69, 134 (1983). 5. R.E. Stoltz and J.B. Vander Sande, Metall. Trans. A. 11A, 1033 (1980). 6. J.B. Vogt, S. Degallaix and J. Foct, Int. J. Fatigue 6, 211 (1984). 7. S. Degallaix, J. Foct and A. Hendry, Mater. Sci. Technol. 2, 946 (1986). 8. J.-B. Vogt, J. Foct, C. Regnard, G. Robert and J. Dhers, Metall. Trans. A. 22A, 2385 (1991). 9. P.R. Swann, Corrosion 19, 102t (1963). 10. D.L. Douglass, G. Thomas and W.R. Roser, Corrosion 20, 15t (1964). 11. K. Oda, N. Kondo and K. Shibata, ISIJ Int. 30, 625 (1990). 12. V. Gerold and H. P. Karnthaler, Acta metall 37, 217 (1989). 13. H. Hirukawa, E. Takeuchi, S. Matsuoka, K. Yamaguchi, T. Ohmura and K. Tsuzaki, CHAMP ISIJ 12, 457 (1999). 14. D. Blavette, B. Deconihout, A. Bostel, J.M. Sarrau, M. Bouet and A. Menand, Rev. Sci. Instrum. 64, 2911 (1993). 15. M. Wada. K. Hosoi and O. Nishikawa, Acta Metall., 30, 1005 (1982). 16. M. Klesnil and P. Lukás, Fatigue of Metallic Materials, Elsevier Scientific Publishing, New York (1980). 17. L. Lundin and H.-O. Andren, Appl. Surf. Sci. 94/95, 320 (1996).