Development and relaxation of stress in surface layers; Composition and residual stress profiles in γ′-Fe4N1−x layers on α-Fe substrates

Composition-depth and (residual) stress-depth profiles were investigated in two different γ′-Fe₄N_1−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′-Fe₄N_1−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′-Fe₄N_1−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. Formerly Graduate Student, Laboratory of Metallurgy, Delft University of Technology.