Loïc Malet

Dislocation-mediated relaxation in nanograined columnar palladium films revealed by on-chip time-resolved HRTEM testing

The high-rate sensitivity of nanostructured metallic materials demonstrated in the recent literature is related to the predominance of thermally activated deformation mechanisms favoured by a large density of internal interfaces. Here we report time-resolved high-resolution electron transmission microscopy creep tests on thin nanograined films using on-chip nanomechanical testing. Tests are performed on palladium, which exhibited unexpectedly large creep rates at room temperature. Despite the small 30-nm grain size, relaxation is found to be mediated by dislocation mechanisms. The dislocations interact with the growth nanotwins present in the grains, leading to a loss of coherency of twin boundaries. The density of stored dislocations first increases with applied deformation, and then decreases with time to drive additional deformation while no grain boundary mechanism is observed. This fast relaxation constitutes a key issue in the development of various micro- and nanotechnologies such as palladium membranes for hydrogen applications.

Grain scale analysis of variant selection during the gamma-epsilon-alpha' phase transformation in austenitic steels

Austenitic steels can exhibit a complex transformation sequence during deformation. Indeed, the austenitic phase transforms first into bands of ε (HCP) martensite. This transformation is then followed by the formation of α’ (BCC) martensite. In this study, the crystallography of the transformation together with the occurrence of variant selection is studied at the scale of individual austenite grains. About ten prior austenite grains deformed at different strain levels in uniaxial tension were analysed by means of EBSD techniques. One of the classical approaches to predict the variant selection phenomenon is based on the calculation of the interaction energy between the macroscopic stress and the shape deformation associated with the formation of the product phase. The formation of the α’ variants was observed to lead to a very strong variant selection that cannot be fully explained by energetic criterion. It is suggested that the crystallography of the transformation sequence can account for the unexpected variants.

Quantitative Analysis of Variant Selection and Orientation Relationships during the γ-to-α Phase Transformation in Hot-Rolled TRIP Steels

The orientation relationships that apply to the fcc (γ) – bcc (α) phase transformation in high-performance hot-rolled TRIP-aided steels were characterised by EBSD techniques. A statistical treatment of the experimental data allows the mean orientation relationship to be determined. This mean orientation relationship was compared to the models commonly proposed in the literature and confronted qualitatively to the predictions of the phenomenological theory of martensite crystallography (PTMC). The variant selection phenomenon was also characterized quantitatively at the level of individual austenite grains. The reconstruction of the EBSD maps evidences that bainite grows by packets in which the bainite laths share a common {111}γ plane in the austenite. This growth mechanism is not influenced by the prior hot deformation of the austenite. The hot deformation has a critical influence on the number of packets that forms. The analysis of the crystallographic features of the bainite packets reveals that all possible variants are formed in a packet, though in different proportions.

Synthesis and Characterization of Zinc Oxide Nanoparticles for Application in the Detection of Fingerprints

Stable zinc oxide nanoparticles were prepared by chemical process. Zinc oxide nanoparticles were synthesized using a modified method with zinc nitrate and sodium hydroxide at 60° C. The synthesized nanopowders were characterized in terms of chemical composition (EDS), structure FTIR and XRD, particle size and morphology by TEM. The XRD results confirm that ZnO nanoparticles were obtained with hexagonal arrangement (Wurtzite). The nanoparticles showed sizes between 10 to 30 nm and semispherical forms. The luminescent properties of the synthesized nanoparticles were measured in a photoluminescence assay on a Raman instrument. The samples were irradiated with two laser beams of different wavelengths. The application of the fingerprints on different surfaces was done using deferments surfaces.

Variant Selection and Texture in an AISI 301LN Stainless Steel

There has recently been significant interest in the problem of variant selection in the strain-induced transformation of austenite to α’-martensite in metastable austenitic stainless steels. Previous work has highlighted our poor understanding of the mechanisms leading to this transformation, in particular the role that the macroscopic stress plays in the transformation. In this work, we have sought to perform detailed experiments aimed at developing a statistical grain level view of variant selection in one particular grade of austenitic stainless steel. EBSD measurements made over a large number of grains as well as macroscopic texture measurements made at different levels of imposed plastic strain allow for comparison against various approaches for predicting variant selection based on the Patel-Cohen interaction energy.