On the elusive crystal structure of expanded austenite

Abstract

No consistent structural description exists for expanded austenite that accurately accounts for the hkl-dependent peak shifts and broadening observed in diffraction experiments. The best available description for homogeneous samples is a face-centered cubic lattice with stacking faults. Here Debye simulations of stacking fault effects were compared to experimental data for macro-stress free homogeneous expanded austenite to show that a faulted structure cannot explain the observed peak displacement anomalies. Instead it is argued that the shifts are the combined result of elastic and plastic anisotropy leading to (strongly) non-linear hkl-dependent elastic behavior during composition-induced plastic deformation on synthesis of expanded austenite.

On the Nitrogen-Induced Lattice Expansion of a Non-stainless Austenitic Steel, Invar 36®, Under Triode Plasma Nitriding

“…The XRD peak shifts (to lower 2h angles with respect to substrate peak positions) of c N -Invar are much smaller than those of c N -330 after equivalent thermochemical treatments. More importantly, anomalous (anisotropic) lattice expansion, where the XRD peak shift appears significantly larger for 200 reflections than with other hkl planes, is a known signature for the c N(ii) synthesized on ASSs under low-temperature nitriding [7,11,17] (as observed also for c N -330 [18] ). Compared with c N(ii) , c N -Invar exhibits a seemingly ''isotropic'' lattice expansion-where the observed 111 and 200 XRD peak shifts appear similar after nitrogen insertion ( Figure 3)-for non-stainless austenitic steels under low-temperature nitrogen diffusion treatment.…”

The influence of stacking fault energy on plasticity mechanisms in triode-plasma nitrided austenitic stainless steels: Implications for the structure and stability of nitrogen-expanded austenite

“…Low temperature thermochemical diffusion modification of austenitic stainless steel (ASS) was originally identified as an unanticipated corrosion induced carbon transportation mechanism Ð and thus a topic of interest (and of concern) − by the nuclear power industry in the early 1960s [1][2][3][4][5][6] but, as reviewed recently by Somers and Christiansen [7], was subsequently investigated systematically in the open literature from the 1980s as a potential solution to the poor tribological performance of ASS [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. The unusual crystallographic structures observed after treatment, and the remarkable enhancement of surface mechanical/tribological properties that these structures provided (without loss of corrosion resistance), led to a drive to try to understand their origin.…”

The Effect of C₂H₂/H₂ Gas Mixture Ratio in Direct Low-Temperature Vacuum Carburization

“…Generally, the formation of the carburization layer takes concentration-dependent diffusion with a function of in√ . In the bulk material carburized at low temperature, therefore, the heterogeneous solid solution of carbon atoms may have occurred (or can occurs in initial diffusion regime) due to limited expansion of the lattice by the surrounding one, different of diffusion path, or difference of carbon diffusion rate depending on the crystal orientation [30,31]. At that time, if the carbon species continue to introduce from the surface, the ASS will be able to keep maximum para-equilibrium carbon concentration continuously, and then the internal carbon concentration can be gradually increased as the carbon continues to diffuse to the inward direction.…”