Depth-Resolved Phase Analysis of Expanded Austenite Formed in Austenitic Stainless Steel

Abstract

Expanded austenite γN formed after nitrogen insertion into austenitic stainless steel and CoCr alloys is known as a hard and very wear resistant phase. Nevertheless, no single composition and lattice expansion can describe this phase with nitrogen in solid solution. Using in situ X-ray diffraction (XRD) during ion beam sputtering of expanded austenite allows a detailed depth-dependent phase analysis, correlated with the nitrogen depth profiles obtained by time-of-flight secondary ion mass spectrometry (ToF-SIMS) or glow discharge optical emission spectroscopy (GDOES). Additionally, in-plane XRD measurements at selected depths were performed for strain analysis. Surprisingly, an anomalous peak splitting for the (200) expanded peak was observed for some samples during nitriding and sputter etching, indicating a layered structure only for {200} oriented grains. The strain analysis as a function of depth and orientation of scattering vector (parallel/perpendicular to the surface) is inconclusive.

Comparison of Nitriding Behavior for Austenitic Stainless Steel 316Ti and Super Austenitic Stainless Steel 904L

“…Still, the (111) expansion saturated quite fast, wh (200) shows a jump for 904L and a gradual increase for 316Ti. This could be due formation of a layered structure with two la ice expansions [32], which is investig detail below. For 500 °C in Figure 4d, yet another different behavior is observed: for 904L, th evolution is very similar to that for 450 °C, whereas for 316Ti a strongly reduced exp for the (200) oriented grains, even smaller than for the (111) oriented grains at low peratures, is measured.…”

“…While the transition occurred around 1 h after the beginning of the nitriding, at the end of the experiment, after nearly 3 h, only the high-expansion phase was visible in XRD. Subsequent ion etching revealed a surface layer of about 600 nm thickness with the higher expansion (but only in the {200} oriented grain), while no pronounced discontinuity was observed in the nitrogen depth profile [32].…”

“…At the same time, it has to be kept in mind that only the lattice expansion normal to the surface is probed. Yet, concurrent in-plane measurements yield no unequivocal results about the three-dimensional strain [32].…”

“…Still, the (111) expansion saturated quite fast, while the (200) shows a jump for 904L and a gradual increase for 316Ti. This could be due to the formation of a layered structure with two lattice expansions [32], which is investigated in detail below.…”

“…Hence, a general explanation for the layered expansion is currently not available. One possible candidate is a reduced symmetry, e.g., from fcc to fct symmetry [32,39], with a certain depth or stress necessary to induce the transition. Similarly, work for Ni-based Ni-Ti alloys indicate an origin in the strong depth dependency of the level of residual stress [40].…”