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Development of amorphous metallic alloys for biomedical applications and understanding of the plasticity phenomena

  • Baulin, Oriane
Publication Date
Oct 12, 2018
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Metallic glasses exhibit improved properties compared to pure crystalline metals, as for example, a high strength and a high corrosion resistance, due to the absence of microstructure and also a low Young modulus. This last feature is interesting for use as biomaterials to prevent bone osteolysis. However, these materials exhibit two main drawbacks: a lack of ductility and a small critical size. To improve these points, most parts of the glasses for biomedical applications still contain toxic elements, such as Be or Al. This work aims to find new fully biocompatible compositions of metallic glasses and suggest three solutions to remedy some issues, based on micro-alloying, powder metallurgy and deformation mechanisms understanding. In the first part of the work, two new compositions for metallic glasses were elaborated: a Mg-Ca-Au-Yb system for use as bioresorbable materials and a Zr-based glassy system for use as reinforcement materials. A complete study on the processing and the characterization of the samples has been conducted: thermal stability, also corrosion properties, cytotoxicity and mechanical properties are also crucial to characterize for a use as biomaterials. However, trying to use only biocompatible elements considerably reduces the possibility to obtain fully amorphous large diameter samples. In that respect, the other part of the work consists to study the possible ways of increasing the samples size, using an Al-free, Ni-free well-known system: Cu-Zr-Ti. First, yttrium additions in the Cu-Zr-Ti system has been investigated. The optimum amount of yttrium to add and the characterization of this material was conducted. 1 at. % of Y in the Cu-Zr-Ti leads to an increase of 2% of plastic strain, of the corrosion resistance, and biocompatibility. The microstructure was precisely studied using Transmission Electron Microscopy (TEM) observations and some explanation about this improvement can be discussed. To the author knowledge, for the first time, yttrium nano-precipitates with a core-shell structure were observed. This leads to an improvement of the ductility of the material, due to the nano-crystallized areas induced by the precipitates. Moreover, a new process, the powders metallurgy with Spark Plasma Sintering (SPS) allows the creation of larger sintered samples. Indeed, the high cooling rate of the atomization allows to obtain fully amorphous powder, even for low GFA systems. Processing of ex-situ composites samples, adding some ductile particles in the amorphous matrix. At last, the role of the mechanical pre-cycling and of the strain rate on the elementary deformation mechanisms were investigated using both atomistic simulation and compressive mechanical tests. A homogenization of the deformation, caused by the pre-cycling, seems to improve the ductility.

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