Split and Shift of <i>ε</i>-martensite Peak in an X-ray Diffraction Profile during Hydrogen Desorption: A Geometric Effect of Atomic Sequence

Abstract

Cryogenic X-ray diffraction measurements demonstrated a split of the ε-martensite peak at 193 K in a hydrogen-charged austenitic steel. Only the higher angle peak remained after aging at room temperature. This phenomenon can be interpreted by a change in the interstitial hydrogen position. Particularly, the motion of the leading partial involved in ε-martensitic transformation can move interstitial hydrogen from a tetrahedron to an octahedron site, expanding the lattice. Subsequently, the hydrogen can move back to the tetrahedron site, which relatively shrinks the lattice. The two different hydrogen positions cause the peak to split.

Overview of Hydrogen Effects on <i>γ</i>−<i>ε</i> Martensitic Transformation in Steels

“…Microscopic hydrogen mapping confirmed preferential hydrogen diffusion along the c-plane in the Fe-15Mn-10Cr-8Ni alloy with c/a = 1.613 (Fig. 12) 36) . Therefore, hydrogen diffusivity decreases as c/a increases until 1.6 and then increases as c/a increases beyond 1.6, resulting in minimum hydrogen diffusivity at c/a ~ 1.6.…”

“…4(a), the hydrogen atoms in the HCP lattice were set to metastable tetrahedral sites. This is because the motion of Heidenreich-Shockley partials, which is the transformation dislocation of the γ-ε martensitic transformation, crystallographically changes the interstitial atom positions from octahedral sites in the FCC lattice to tetrahedral sites in the intrinsic stacking fault region (HCP lattice) when only substitutional atoms are assumed to move [34][35][36] . In contrast, when the hydrogen position in the HCP lattice was assumed to be the octahedron site, the effect of hydrogen on the energy change became relatively small, as shown in Fig.…”

“…(a) SEM image showing Ag particles deposited along ε-martensite platesin the Fe-15Mn-10Cr-8Ni alloy. The corresponding EBSD-phase and (c) IPF maps36) . The specimen was cooled down to 77 K. Then, the pre-cooled specimen was hydrogen-charged and subsequently immersed in a 4.3 mM K[Ag(CN) 2 ] aqueous solution for 24 h. To promote hydrogen diffusion, the temperature during silver decoration was set at 323 K, which was lower than the A s .…”