Soundès Djaziri

Deformation-Induced Martensite: A New Paradigm for Exceptional Steels

Martensite steel is induced from pearlitic steel by a newly discovered method, which is completely different from the traditional route of quenching austenitic steel. Both experimental and theoretical studies demonstrate that Fe-C martensite forms by severe deformation at room temperature. The new mechanism identified here opens a paths to material-design strategies based on deformation-driven nanoscale phase transformations.

Comparative study of the mechanical properties of nanostructured thin films on stretchable substrates

Comparative studies of the mechanical behavior between copper, tungsten, and W/Cu nanocomposite based on copper dispersoid thin films were performed under in-situ controlled tensile equi-biaxial loadings using both synchrotron X-ray diffraction and digital image correlation techniques. The films first deform elastically with the lattice strain equal to the true strain given by digital image correlation measurements. The Cu single thin film intrinsic elastic limit of 0.27% is determined below the apparent elastic limit of W and W/Cu nanocomposite thin films, 0.30% and 0.49%, respectively. This difference is found to be driven by the existence of as-deposited residual stresses. Above the elastic limit on the lattice strain-true strain curves, we discriminate two different behaviors presumably footprints of plasticity and fracture. The Cu thin film shows a large transition domain (0.60% true strain range) to a plateau with a smooth evolution of the curve which is associated to peak broadening. In contrast, W and W/Cu nanocomposite thin films show a less smooth and reduced transition domain (0.30% true strain range) to a plateau with no peak broadening. These observations indicate that copper thin film shows some ductility while tungsten/copper nanocomposites thin films are brittle. Fracture resistance of W/Cu nanocomposite thin film is improved thanks to the high compressive residual stress and the elimination of the metastable beta-W phase.

Maintaining strength in supersaturated copper–chromium thin films annealed at 0.5 of the melting temperature of Cu

The thermal stability of evaporated copper–chromium alloy films was studied by correlating hardness trends from nanoindentation to nanostructural–compositional changes from transmission electron microscopy. In particular, the hardness evolution with ageing time at ambient and elevated temperatures of two compositions, dilute (Cu96Cr4) and chromium-rich (Cu67Cr33) solutions, was studied. Due to the negligible mutual miscibility of copper and chromium, the chosen solid solutions are trapped in metastable states as supersaturated solid solutions with face-centred cubic and body-centred cubic phases. Nano-mechanical probing of the nanostructural evolution as a function of temperature provided interesting insights into the phase separation of these systems.