Revealing the role of microstructure architecture on strength and ductility of Ni microwires by in-situ synchrotron X-ray diffraction.

Deformation mechanisms of cold drawn and electropolished nickel microwires are studied by performing in-situ monotonous and cyclic tensile tests under synchrotron radiation. X-ray diffraction tests allow probing elastic strains in the different grain families and establishing a link with the deformation mechanisms taking place within the microwires. The measurements were carried out on several microwires with diameters ranging from as-drawn 100 µm down to 40 µm thinned down by electropolishing. The as-drawn wires exhibit a core-shell microstructure with <111> fiber texture dominant in core and heterogeneous dual fiber texture <111> and <100> in the shell. Reduction of specimen size by electropolishing results in a higher yield stress and tensile strength along with reduced ductility. In-situ XRD analysis revealed that these differences are linked to the global variation in microstructure induced by shell removal with electropolishing, which in turn affects the load sharing abilities of grain families. This study thus proposes a new way to increase ductility and retain strength in nickel microwires across different diameters by tuning the microstructure architecture.

The mechanical behavior of nacre across length scales.

Nacre achieves excellent mechanical properties with a relatively simple hierarchical structure. Analyses suggest that a significant gain in toughness is realized with a modest reduction in strength, with increasing levels of hierarchy. This study probes the role of different hierarchical length scales in governing the strength and modulus of nacre using a combination of bulk compression tests, microindentation and nanoindentation tests. The variability in the measured properties is assessed through Weibull analyses. The transition from elastic deformation is characterized using spherical indentation tests at the micro and nano scales together with a Herztian analysis. The modulus of the organic phase at different scales was deduced using indentation data and appropriate micromechanical models. The results show a minimal influence of length scales on elastic-plastic transitions, suggesting that initiation of plasticity occurs through a common biomineral sliding mechanism across length scales. However the ultimate strengths follow the trends of models for hierarchical materials, with the strength reducing by a factor of ~2 with each increase in level of hierarchy. The modulus of the organic phase was higher at the lowest scale, in contrast to an earlier study, indicating that confinement significantly modifies the effective properties of the organic.