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Novel nanostructured ZrCu thin film metallic glasses with superior mechanical properties and thermal stability

In this project, we aim to synthetize novel ZrCu thin film metallic glasses (TFMGs) with controlled thickness, composition and morphology, while investigating the relationship with the main mechanical properties and focusing on the nanometer scale deformation mechanisms. Moreover, we aim to investigate the thermal stability and the evolution of the atomic order performing dedicate annealing treatments.  

Metallic glasses (MGs) represent a peculiar class of metallic alloys characterized by an amorphous atomic structure which gives them superior mechanical strength, large elastic deformability, high wear and corrosion resistance making them very attractive materials for such as hard coatings, MEMS (Micro Electro Mechanical Systems), biomedical tools or flexible electronics.

Despite of these very appealing properties, bulk metallic glasses (BMGs) suffer from a lack of plasticity due to shear banding phenomena involved during deformation [1].

However, mechanical size effects have been reported when reducing specimen dimensions. Specifically, TFMGs have shown a mutual combination of large plastic deformation (above 10%) and superior yield strength (~3500 MPa) reaching the theoretical limit [2].

Nevertheless, many scientific challanges arise. First of all, the physical origin of size effects is far to be achieved. Here, the elementary deformation mechanisms are still under debate and open questions involve the nucleation and propagation of shear bands at the sub-micrometer scale. In addition, little attention in literature is reported to the synthesis of novel nanostructured TFMGs involving the effect of annealing treatments (below and above the glass transition temperature), the nanoporosity and the composition and how they affect the local atomic arrangement and the mechanical behaviour.

In this project, the relationship between atomic structure and mechanical properties of ZrxCu100-x TFMGs will be investigated. Films will be deposited by magnetron sputtering enabling an accurate control of composition, microstructure and thickness through regulation of process parameters. Then, the micro-mechanical behavior will be studied with different techniques among which nanoindetation, in-situ SEM compression test of micropillars and on-chip tensile tests of freestanding specimens.

In addition, for selected compositions, special care will be devoted to the study of the effect of thermal annealing treatments to identify the change of the local atomic arrangement, free volume content and how it affects the formation of shear bands, reflecting on the mechanical behaviour. The crystallization kinetics will be analysed as well with the idea to develop a composite film in which crystalline nanocrystals are embedded in an amorphous matrix, representing a novel strategy to further enhance TFMGs mechanical properties.

Finally, structural analysis will be carried out by HRTEM, in order to identify a link between mechanical properties and atomic structure.

[left] SEM cross-section image of nanoporous thin metallic glass film. [right] TEM cross-section image of thin film metallic glass embedded with crystalline phase. In the insets, selected area diffraction patterns.
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