Aged hardened martensitic stainless steels are basically low carbon, high chromium and nickel steels. They are characterized by high strength and toughness combined with a good corrosion resistance. In the 15-5PH stainless steel, the mechanical strength is due to the precipitation of copper rich phases, the toughness is optimised by controlling the volume fraction of reversed austenite while the corrosion resistance is attributable to the high chromium content. The 15-5PH has been reported to be readily weldable and weldments generally show good mechanical properties. The present study is conducted to investigate the mechanical and metallurgical properties of electron beam welded 15-5PH alloy. The fusion zone has a dendritic microstructure and subsequently to welding, heat affected zone (HAZ) extends on both sides of the fillet. The HAZ shows a different microstructure compared to both the molten zone and the base metal. To investigate the mechanical properties, tensile specimens are extracted from the various zones of the welded junction in order to characterize the local properties of the heat-affected zone (HAZ) and the fusion zone (FZ) and to compare them to those of the base metal.

High-purity polycrystalline nickel foils have been oxidized at 1000 degrees C in laboratory air before being analyzed in secondary ion mass spectrometry to locally measure the oxygen content in solid solution. The values obtained in metallic grains are surprisingly the same before and after the oxidation treatments (between 5 and 10 atom ppm) and they are much lower than the ones predicted from the literature solubility and diffusion coefficient data at 1000 degrees C. It is shown that this discrepancy could have its origins in the purity level of the samples but also in the exclusive oxygen diffusion in nickel grain boundaries. This last assumption is supported by the occurrence of nickel oxide particles on the walls of voids located in grain boundaries.

Nickel foils have been oxidised at 1000 °C on one side only in laboratory air, the other side being protected from oxidation by a reducing atmosphere. After the oxidation treatment, the unoxidised face was carefully examined by using an atomic force microscope. Grain boundaries grooves were characterised and their depth were compared to the ones obtained on the same sample heat treated in the reducing atmosphere during the same time. Grain boundaries grooves are found to be much deeper in the case of the single side oxidised samples. It is shown that this additional grooving is directly linked to the growth of the oxide scale on the opposite side and that it can be explained by the diffusion of the vacancies produced at the oxide scale–metal interface, across the entire sample through grain boundaries. Moreover, the comparison between single side oxidised samples and samples oxidised on both sides points out that voids in grain boundaries are only observed in this latter case proving the vacancies condensation in the metal when the two faces are oxidised.

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.

A multiscale study of the high temperature oxidation effects on high purity polycrystalline nickel behaviour is presented. In situ tensile tests have been carried out at room temperature on preoxidised thin foils. The results obtained lead to a better understanding of the strengthening effect of the oxide layer.Detailed examinations of the metal microstructure are also made after high temperature oxidation treatment. Atomic Force Microscope (AFM), Secondary Ion Mass Spectroscopy (SIMS), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) are used to characterise the microstructure. Formation of voids at grain boundaries as well as vacancy injection are then discussed.