Macrostressed polycrystalline thin surface layers will in case of intrinsic elastic anisotropy and variable grain orientation also exhibit microstrain. The microstrain distribution will lead to microstrain broadening of the diffraction peaks, which may provide valuable information about the overall state of stress/strain in the layer (see e.g. [1]). However, most macrostress-studies by diffraction techniques, e.g. using the sin²ψ method, do not consider the line broadening of the diffraction peaks. That is partly due to the often relatively large instrumental broadening of the diffractometers dedicated to (macro)stress analysis, masking the structural line broadening.

The present contribution deals with synchrotron X-ray powder diffraction measurements (B2, HASYLAB Hamburg) on cementite layers (thickness 4.6 µm) grown on α-iron using gaseous nitrocarburising [2]. The instrumental configuration was chosen to combine the possibility for specimen tilting with high instrumental resolution. Analysis of the average reflection positions as a function of the specimen-tilting angle ψ reveals compressive stresses in the cementite. The slopes of the largely linear sin²ψ plots depend strongly on the reflection indices hkl, i.e. the maximum and minimum slopes differ by a factor of about 1.7. This hkl-dependence can be explained by the strong elastic anisotropy of cementite [3].

The microstrain broadening of the different reflections is also strongly hkl dependent. In fact, those reflections showing large widths exhibit also large (in absolute terms) slopes in the sin²ψ plots (cf. Figure). Methods for a systematic comparison of the hkl-dependent slopes and line widths will be presented.

[1] C. M. Sayers, Phil. Mag. A 49 (1984) 243.

[2] T. Gressmann, M. Nikolussi, A. Leineweber, E. J. Mittemeijer, Scr. Mater. 55 (2006) 723.

[3] M. Nikolussi, S. L. Shang, T. Gressmann, A. Leineweber, E. J. Mittemeijer, Y. Wang, Z.-K. Liu, submitted for publication.

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