Diffraction elastic constants and stress factors; grain interaction and stress in macroscopically elastically anisotropic solids; the case of thin films

Thin films and surface-adjacent material of bulk polycrystals cannot gener- ally be considered as macroscopically elastically isotropic; at most transverse isotropy can occur. However, in this case, the usually applied models for grain interaction (as those proposed by Voigt, Reuss, Neerfeld–Hill and Eshelby– Kr¨ oner) erroneously predict macroscopic elastic isotropy. The present chapter presents an overview of recent work on the diffraction analysis of elastic grain interaction in polycrystalline material. It has been demonstrated for the first time, by X-ray diffraction (XRD) analysis of stresses in thin films, that the elastic grain interaction can lead to macroscopically elastically anisotropic behaviour (as exhibited by non-linear so-called sin2 ψ-plots), even in the ab- sence of texture. A rigorous treatment has been given for the diffraction stress analysis of both macroscopically elastically isotropic specimens and macro- scopically elastically anisotropic specimens. A new model has been proposed for the direction-dependent elastic grain interaction which predicts macro- scopically anisotropic behaviour of thin films. The proposed model has been successfully applied in the X-ray diffraction study of the residual stresses in thin films. The notion surface anisotropy of bulk specimens has been revisited as a special case of direction-dependent grain interaction.