TEM studies of plasma nitrided austenitic stainless steel

Abstract

Summary Cross‐sectional transmission electron microscopy and X‐ray phase analysis were used to study the structure of a layer formed during nitriding the AISI 316L stainless steel at temperature 440°C. It was found that the applied treatment led to the formation of 6‐μm‐thick layer of the S‐phase. There is no evidence of CrN precipitation. The X‐ray diffraction experiments proved that the occurred austenite lattice expansion – due to nitrogen atoms – depended on the crystallographic direction. The cross‐sectional transmission electron microscopy studies showed that the layer consisted of a single cubic phase that contained a lot of defects such as dislocations, stacking faults, slip bands and twins. The high‐resolution electron microscopy observations were applied to study the defect formation due to the nitriding process. It was shown that the presence of great number of stacking faults leads to formation of nanotwins. Weak, forbidden {100} reflections were still another characteristic feature of the S‐phase. These were not detected in the X‐ray spectra of the phase. Basing on the high‐resolution electron microscopy studies it can be suggested that the short‐range ordering of the nitrogen atoms in the octahedral sites inside the f.c.c. matrix lattice takes place and gives rise to appearance of these spots. It is suggested that the cubic lattice undergoes not only expansion but also slight rombohedral distortion that explains differences in the lattice expansion for different crystallographic directions.

High-density stacking faults in a supersaturated nitrided layer on austenitic stainless steel

“…structure. These forbidden spots have been reported in previous studies (Meletis et al, 2002;Lei, 1999;Stró ż & Psoda, 2010;Jiang & Meletis, 2000), and are possibly due to N atoms at specific octahedral sites (Meletis et al, 2002;Stró ż & Psoda, 2010;Jiang & Meletis, 2000). The spots identified by the black spots in Figs.…”

“…Such layers contain nitrogen ranging from about 10 to 35 at.% in the austenite matrix, and consist of a single phase named N (Williamson et al, 1994). The crystallographic features of the phase have been studied by X-ray diffraction (XRD) (Ö ztü rk & Williamson, 1995;Bacci et al, 2001;Meletis et al, 2002;Fewell et al, 2000;Sun et al, 1999;Fewell & Priest, 2008) and transmission electron microscopy (TEM) (Lei & Zhang, 1997;Xu et al, 2000;Lei, 1999;Mitchell et al, 2003;Stró ż & Psoda, 2010;Jiang & Meletis, 2000). However, the detailed atomic structure is controversial.…”

“…lattice structure of the N phase has also been confirmed by selected-area electron diffraction (SAED) (Lei & Zhang, 1997). Moreover, TEM observations provide direct images of stacking and twin faults in the N phase (Meletis et al, 2002;Xu et al, 2000;Lei, 1999;Mitchell et al, 2003;Stró ż & Psoda, 2010;Jiang & Meletis, 2000). In the f.c.c.…”

Atom probe tomography characterization of nitrogen induced decomposition in low temperature plasma nitrided 304L austenitic stainless steel

“…It is difficult to specify if these planar defects are micro-twins or systems of stacking faults, since both appear on 111 f gplanes [26,28]. Previous high-resolution electron microscopy investigations of the γ N phase report high densities of stacking faults with spacings between them sometimes less than few lattice planes [28,29]. In average one every sixth (111) plane contains stacking fault [26].…”

X-ray Diffraction Investigation of Stainless Steel—Nitrogen Thin Films Deposited Using Reactive Sputter Deposition

“…However, this does not appear to have been done in any of the above studies, although stacking faults have been observed in several TEM studies. Xu et al [26] and Stroz and Psoda [27] both examined the microstructure of plasma nitrided samples and observed stacking faults in the S-phase; the high-resolution image in the latter study showed stacking fault bundles with α~0.1. Nonetheless, they proposed the peak shift was due to a slight rhombohedral distortion in the lattice.…”