Behaviour of the oxide scale during SEM in situ plastic deformation of pure nickel foil

The present study reports direct observations of the fracture of NiO layers on high purity nickel foils during in situ straining experiments performed in a scanning electron microscope at room temperature. The experimental set-up which has been specifically developed in our laboratory for performing such tests is first presented in details. The evolution of cracking on the oxide scale is followed during the plastic deformation of the substrate and up to the specimen rupture which roughly occurs at a total strain of 16%. The first cracks appear at 0.6% true strain and are perpendicular to the load axis. New oxide cracks are formed all along the test. The oxide scale remains fully adherent to the substrate and no kind of spallation or delamination could be noticed. A statistical study of the inter-cracks spacings is carried out and shows that the inter-cracks spacing decreases with the deformation according to a power law of the plastic strain.