Grinfeld instability in the formation of a tweed structure at the Al crystal surface under cyclic tension

Analysis of our research results and relevant available data shows that the formation of a tweed structure at the Al crystal surface under cyclic tension involves the Grinfeld instability at a stress above the yield strength. The tweed structure period and hillock height measured in experiment agree satisfactorily with those predicted in the linear approximation of the Grinfeld model. The variation in the cross-sectional profile of the tweed structure with increasing the number of loading cycles agrees qualitatively with the Grinfeld instability evolution in the nonlinear approximation. It is taken into account that under cyclic tension, a shear-unstable high-defect region arises in near-surface layers of Al crystals beneath the oxide film; the region can be considered as a defect phase in equilibrium with the Al crystal phase. Periodic modulation of elastic energy density in the defect region gives rise to a chemical potential gradient in the field of which the material is redistributed with the formation of a tweed structure, and this provides an additional channel of elastic energy dissipation alternative to dislocation glide in the loaded crystal. Consideration is given to the structural peculiarities of Al crystals responsible for the Grinfeld instability under cyclic tension and to the plausible mechanisms of mass transfer in their near-surface layers. It is concluded that the Grinfeld instability is direct evidence for a peculiar role of the surface layer as an independent structural level in plastic deformation and fracture of materials and this role is valid in the framework of physical mesomechanics.