The micro-analytical study of cellular precipitation in a high-nitrogen austenitic steel (HNS) was carried out by means of XPS, AES, photoelectron microscopy and EDS microanalysis. The use of photoelectron microscopy with high lateral resolution at the ESCA microscopy beamline of the Elettra synchrotron in Trieste permitted to determine the concentrations of N and Cr in the supersaturated matrix and inside the lamellae of the transformed zones. N in interstitial solution has a strong effect on solid solution strengthening, austenite stabilization and pitting corrosion resistance. Owing to their properties FINS can be used in different industrial fields, such as petrol plants, energy generation, production of paper and biomedicine. However, the precipitation of chromium nitrides, occurring in the temperature range of 550 - 1050 degrees C, can restrict the high-temperature applications of HNS. Cellular precipitation in the austenitic HNS after heat treatments has been already investigated by various techniques [1,2] and it was found that the transformation is accompanied by the modification of lattice parameter and micro-hardness. The results suggested that long-range diffusion of N from the untransformed gamma(s) matrix to the transformed regions may occur [2-4]. To complete the micro-structural study and verify if diffusion of N from gamma(s) matrix towards transformed regions really occurs, the experiments of AES and photoelectron microscopy have been carried out. This information was not available from the data of conventional XPS, where the diameter of investigated area is much higher than the inter-lamellar distance (about 250 nm) [1]. Moreover, internal friction (IF) and dynamic modulus measurements have been performed for investigating the distribution of N and other alloying elements on atomic scale. The examined HNS steel was prepared by Pressurized Electroslag Remelting; its nominal composition is reported in Tab. 1. The samples were isothermally heated at 850 degrees C in successive steps up to 23 hours. Auger spectra were collected by using an Escalab Mk 1I spectrometer (VG Scientific) equipped with 5-channeltron detection system and electron gun LEG 200, operated at 10 keV and 1-10 nA beam current. The spectra were acquired in the constant retard ratio (1 : 2) mode. The photoemission measurements with high lateral resolution were performed by means of the Scanning Photoelectron Microscopy (SPEM) at the Elettra synchrotron. The SPEM can be operated in imaging and spectroscopic modes with a spatial resolution of about 150 nm and resolution in energy of 0.2 eV, respectively. More experimental details on this technique have been reported previously [5,6]. IF and elastic modulus measurements were carried out by means of the VRA 1604 analyzer [7] on the reeds of 13.4 mm x 6.0 mm x 0.26 mm. The samples have been heated up to 800 degrees C at constant rate of 1.7 x 10(-2) degrees C s(-1). The modulus was determined from the resonance frequency f (Eq. 1) being m a constant, L and h length and thickness of the vibrating reed, rho material density. Fig. 1 shows the cellular precipitation. The structure consists of the N supersaturated s matrix and of the transformed regions, containing secondary gamma austenite and chromium nitrides. The average concentration of N in different regions can be determined from Auger spectra (Fig. 2). By using previously defined sensitivity factors of Cr LMM and N KLL, peaks [8], the Cr/N ratios of 5.9 and 2.9 for untransformed and transformed regions, respectively, were determined. The multipoint analysis of SPEM (points A - D in Fig. 3) permitted to determine the N concentration in the Cr(2)N lamellae (C), the interlamellar region (A), the interface (B) and the untransformed matrix (D). From these data, reported in Tab. 2, it is evident that the average concentration of N in transformed regions (6.25 %) is much higher than that in gamma(s) matrix (0.85 %). The results are compatible with the random walk of N calculated by Eq. (2) [9]. Therefore, the data of AES, SPEW. support the model of Srinivas et al. [3], which identifies the gradient of N between gamma(s) and gamma regions as the driving force for cellular transformation. Results of EDS microanalysis (Figs. 4-5) testify that also the Cr content is different in transformed and untransformed regions and that such difference tends to increase for longer treatment times up to 15 hours, then it remains substantially stable. In general, the literature is poor of IF studies on HNS [10-11]. The IF results of as-prepared and 25 % transformed steels are reported in Figs. 6 and 7, respectively. IF curves show the overlapping of an exponentially increasing background and a broad peak. This peak shifts its position with a change of frequency, indicating that it is a relaxation peak. In fact, the broad peak consists of several Debye peaks with the same activation energy H = 1.76 eV, close to that of N diffusion in austenite [12], but with different relaxation times tau. Eqs. (3-4-5) describe the general characteristics of a Debye peak. The peak, observed in present tests, has been attributed to the re-orientation of interstitial-substitutional (i-s) pairs. In the case of as-prepared steel, C-Mn and N-Mn pairs present in the supersaturated austenite are involved in the distributions of relaxation times reported in Fig. 9. For C-Mn, the activation energy was taken from ref. [13]. As shown in Fig. 8, in the case of partially transformed steel, also N-Mn pairs in the secondary austenite should be considered [14]. The corresponding distributions of relaxation times are displayed in Fig. 11. The distribution of relaxation times for the same physical processes evidence chemical fluctuations on atomic scale in both supersaturated and secondary austenite. In conclusion, the results of present study can be summarized as follows: 1- long-range N diffusion from the untransformed regions to the transformed ones occurs. This confirms that the driving force for cellular precipitation is the gradient of N concentration between supersaturated and secondary austenite; 2- diffusion of Cr from the untransformed regions towards the transformed ones takes place; 3- the anelastic behaviour with distribution of relaxation times for the same physical processes indicates that fluctuations of chemical composition on atomic scale are present in both supersaturated and secondary austenite.
