X-ray diffraction analysis can be very usefully applied in the field of surface engineering, since a depth of a few micrometres only is usually probed. This paper provides an overview of current X-ray diffraction methods for the analysis of surface layers. The treatment is illustrated by examples taken from recent work on nitrided iron and steels. Distinct ways to characterize the effective depth and layer thickness probed are presented. Composition-depth profiles can be measured accurately by tracing the lattice parameter (taking into account the possible presence of stresses) as a function of depth below the surface by successive sublayer removals. A correction for the penetration effect is required and methods for that are presented and demonstrated. The basis of the determination of (macro) stress (the sin2ψ method) is indicated. Attention is paid to the determination of the strain free lattice spacing and (diffraction) elastic constants, to the effect of a concentration-depth profile, and to ...

Role of x-ray diffraction analysis in surface engineering: investigation of microstructure of nitrided iron and steels

Composition-depth and (residual) stress-depth profiles were investigated in two different γ′-Fe4N1−x surface layers on α-iron substrates. X-ray diffraction analysis was applied. Determination of the actual lattice-spacing depth profiles required correction for the effect of penetration of the X-rays. The dependence of the nitrogen content in γ′-Fe4N1−x surface layers on depth below the surface corresponds with that expected for diffusion-controlled layer growth. The formation of porous grain boundaries (channels) in the surface-adjacent part of the layers provides the possibility of nitrogen uptake at “internal” surfaces during nitriding, leading to the development of concave concentration-depth profiles in this part of the layers. Stress buildup in the massive interface-adjacent part of γ′-Fe4N1−x layers on a-Fe substrates originates from both the presence of a concentration-depth gradient and the difference in thermal contraction between layer and substrate on cooling from the nitriding temperature to the measurement temperature. Channel/pore development along grain boundaries in the surface-adjacent part of the layers provides a mechanism for stress relaxation during nitriding. On cooling, the channels can also accommodate part of the thermally induced misfit. Lateral strain gradients in the most severely porous part of the layers are revealed by the X-ray diffraction analysis.

Development and relaxation of stress in surface layers; Composition and residual stress profiles in γ′-Fe 4 N 1−x layers on α-Fe substrates

Elastic anisotropy of γ'-Fe4N and elastic grain interaction in γ'-Fe4N1-y layers on α-Fe: first-principles calculations and diffraction stress measurements

Pure iron foils were converted into {
 
$$\gamma ' - Fe_4 N_{1 - x} $$

 } foils of various compositions by means of nitriding in selected NH3/H2 gas mixtures at 803 and 843 K. The lattice parameter and the linear thermal expansion coefficient of {
 
$$\gamma ' - Fe_4 N_{1 - x} $$

 } nitride and the surrounding gas atmosphere.

Dependence of the lattice parameter of {$\gamma '$} iron nitride, Fe4N1−x, on nitrogen content; Accuracy of the nitrogen absorption datairon nitride, Fe4N1−x, on nitrogen content; Accuracy of the nitrogen absorption data

Compound layers composed of γ′-Fe4N1-x on α-Fe substrates were oxidized in oxygen-containing gas atmospheres at 603 K. The microstructural changes were analyzed applying X-ray diffraction, dilatometry, and light, scanning electron, and transmission electron microscopy. Compositional changes were traced, in particular, with scanning Auger electron spectroscopy. Dual-phase, Fe3O4 (magnetite) and α-Fe2O3 (hematite), oxide layers formed at the compound layer surface. At the interface between the oxide and nitride layers, e-Fe2N1=x nucleated because of a local nitrogen enrichment caused by the conversion of γ′ nitride into oxide. The volume of each of the three phases formed (hematite, magnetite, and e nitride) increased with the square root of oxidation time, indicating solid-state diffusion-controlled layer growth. No evidence was obtained for the existence of γ′ or e oxynitrides. Within the γ′ layer, ferrite precipitated during oxidation as a consequence of an iron supersaturation of γ′ nitride due to a production temperature higher than the oxidation temperature. During this iron precipitation, stress relaxation occurred, as was concluded from X-ray diffractometric and dilatometric analyses. The stress relaxation was rate-controlled by nitrogen diffusion in γ′ nitride. The present findings were also used to explain microstructural changes during (commercial) oxidation of e-Fe2(N,C)1-x compound layers.

Phase transformations and stress relaxation in γ′-Fe4N1-x surface layers during oxidation

Specimens of pure iron, an iron-carbon (1 wt-%C) alloy, and steels Ck 45 and En 40B (=24CrMo13) were salt bath nitrocarburized. M etallographic examination was performed after Murakami etching. This etchant provides a sensitive discrimination between pure nitrides and carbon containing nitrides ( carbonitrides). Thus the two-phase (e and γ' ) constitution, as well as the carbon concentration profile within the compound layer, was revealed. Nitrogen, carbon, oxygen, and sulphur concentration profiles were quantitatively determined by electron microprobe analysis. Residual stress measurement and phase analysis were carried out by X-ray diffraction methods. Both macros trains (by analysis of line shift using the sin2 Ψ method) and microstrains (by analysis of line broadening using a Voigt single-line method) were determined. In view of a possible interaction between residual stresses in the compound layer and the diffusion zone, residual stresses were also determined in the diffusion zone beneath the...

H C F Rozendaal

Papers

Morphology, Composition, and Residual Stresses of Compound Layers of Nitrocarburized Iron and Steels