Analysis of Phase Distribution in Thin Surface Layers Comparable to the Penetration Depth of X-Rays

Abstract

After hydrogen concentration, gradients in austenitic-type stainless steels, formed during electrochemical charging and followed by hydrogen loss during aging at room temperature, surface stresses, and martensitic phases α′-bcc and ε-hcp, developed. Phase quantitative X-ray surface analysis of distributions of martensitic phases in a thin layer, comparable to the penetration depth of X-rays, based on diffraction data taken for various diffraction reflections (2θ, Bragg’s angles) and with various radiations (λ-wavelengths) was applied for various degrees of the type steel in the surface layers. An examination of the relationships between γ-phase transitions in a number of stainless steels and their γ stability revealed that the stability of the γ phase increased (S stability factor changed from 26.5 in AISI 321 to 44 in AISI 310), the amount of α′-martensites (from 25 pct in AISI 347 to 0 pct in AISI 310) decreased, and ε-martensites (from 48 pct in AISI 310 to 77 pct in AISI 321) increased, while the depth (from 6.2 μm in AISI 321 to 3 μm in AISI 310) of the martensitic phases decreased. Deformation and fracture experiments were carried out at room temperature in a high-resolution transmission electron microscope with single-axis tilt tensile stage and environmental cell. The principal effect of hydrogen was to decrease the stress required for dislocation motion, for phase transformation of the austenite, and for crack propagation. Formation of ε- and α′-martensite was noted along the fracture surfaces and in front of the crack tip. The cracks propagated through the ε-martensite plates, which formed along the active slip planes, while α′ phase was always found in the high stress regions.