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

In this project, we aim to synthetise novel ZrCu thin film metallic glasses (TFMGs) with controlled thickness, composition and microstructure, while investigating the relationship with the mechanical behaviour focusing on the nanometre scale deformation mechanisms. Moreover, we aim to study the mechanical properties of film with complex architectures such as multilayers and amorphous-nanocrystalline composites.

Metallic glasses (MGs) represent a peculiar class of metallic alloys characterised 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 applications such as hard coatings, MEMS (Micro Electro Mechanical Systems), biomedical tools.

Despite of these very appealing properties, bulk metallic glasses (BMGs) suffer from a lack of plasticity due to shear banding phenomena during deformation [1]. However, mechanical size effects have been reported when reducing specimen dimensions down to the sub-micrometre scale. 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 challenges arise. First of all, understanding of the physical origin of size effects is far from being achieved. Here, the elementary deformation mechanisms are still under debate and open questions involve the nucleation and propagation of shear bands for small film thicknesses. In addition, little attention is reported to the synthesis of novel nanostructured TFMGs involving the effect of the composition and annealing treatments (below and above the glass transition temperature), and how they affect the local atomic arrangement and the mechanical behaviour.

In this context, we studied a large variety of binary ZrxCu100-x TFMG compositions (24 < x at.% < 61) deposited by magnetron sputtering, investigating their mechanical and thermal behaviour. Evolution of the amorphous structure with temperature was monitored through in-situ X-Ray diffraction (XRD) heating, showing that crystallisation temperature depends on composition.

Mechanical characterisation through nanoindentation shows that hardness increases with content of Cu content (at.%) from 5.5 up to 7.7 GPa. Elastic modulus follows the same trend, increasing from 82 to 112 GPa. In addition, dependency of loading rate was studied, highlighting a different serrated plastic flow behaviour in relation with film composition.

By collaborating with Laboratoire des Sciences des Procédés et des Matériaux (LSPM) of University Sorbonne Paris Nord (Profs. Philippe Djemia and Damien Faurie), we carried out surface Brillouin light scattering (BLS) and tensile test on films deposited on flexible polymeric substrates. We show that all elastic constants (Young’s, shear and bulk moduli) increase at high Cu content (at. %), while crack density evolution show that Cu-rich samples have the largest onset for crack nucleation.

The next steps for the project involved the investigation on micromechanical behaviour of freestanding ZrxCu100-x TFMG deformed by on-chip tensile test technique developed within Prof. Thomas Pardoen’s group (Université catholique de Louvain, Belgium).
Structural analysis will be carried out by HRTEM and Atom Probe Tomography (APT), in order to identify a link between mechanical properties and atomic structure.

In parallel, we will also develop synthesis of more complex film architectures, such as multilayers alternating different ZrxCu100-x compositions and composite films featured by presence of crystalline phases dispersed in an amorphous matrix. Mechanical characterisation will be carried out to study how these morphologies, could interfere with shear bands propagation enhancing the overall film mechanical properties.

SEM cross-section image of an amorphous Zr52Cu48 thin film metallic glass showing a columnar nanostructured morphology

SEM cross-section image of a multilayer thin film metallic glass, alternating layers with two different compositions: Zr24Cu76 and Zr52Cu48

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