Effect of the alloy/mould contact on surface crystallisation of a biocompatible ZrCoAl bulk metallic glass.

Casting of metallic glasses (MG) sometimes induces surface crystallisation despite the fact that the surface is expected to be the region where the cooling rate is the highest. This phenomenon has been observed on various MG, even for those with large critical diameters. Such surface crystallisation can be detrimental when the target applications are focused on surface properties, such as corrosion resistance for biomedical applications. In this paper, a Zr56Co28Al16 bulk metallic glass (BMG) with a large critical diameter was used. We reveal that samples processed using common copper-mould suction casting present surface crystallisation up to 20 µm in thickness, greatly deteriorating corrosion resistance. Using in-house highly reproducible suction casting and injection micro-casting processes, the influence of the processing parameters (mould material and temperature, working atmosphere, applied pressure) were investigated. The origin of surface crystallisation was found to arise from the complex thermal history of the alloy depending on the alloy/mould contact quality. By ensuring a tight contact between the solidifying alloy and the mould, BMG samples without crystalline surface defects were obtained.

The influence of temperature during water-quench rapid heat treatment on the microstructure, mechanical properties and biocompatibility of Ti6Al4V ELI alloy.

This study investigates the influence of a rapid heat treatment followed by water-quenching on the mechanical properties of Ti6Al4V ELI alloy to improve its strength for use in implants. Prior to the experiment, a dilatometry test was performed to understand the progressive α-to ß-phase transformation taking place during heating. The results were then used to carry out heat treatments. Microstructure was analysed using SEM, EBSD, EDX and XRD techniques. Vickers micro-hardness, tensile and high cycle rotating bending tests were used to analyse the influence of the $\alpha'$-phase fraction on the strength of the studied alloy. Results show that this process can provide a Ti6Al4V ELI alloy with a better Yield Strength (YS)/uniform deformation (εu) ratio and improved high cycle fatigue strength than those observed in the current microstructure used in medical implants. Lastly, cytotoxicity tests were performed on two types of human cells, namely MG63 osteoblast-like cells and fibroblasts. The results reveal the non-toxicity of the heat-treated Ti6Al4V ELI alloy.

Wear study of Total Ankle Replacement explants by microstructural analysis.

The implantation of Total Ankle Replacement (TAR) prostheses generally gives satisfactory results. However, a high revision rate is associated with the Ankle Evolutive System (AES) implant, due to periprosthetic osteolysis that generates significant cortical lesions and bone cysts in the periprosthetic region. Radioclinical and histological analyses of peri-implant tissues show the presence of numerous foreign particles that may come from the implant. It is known that a precocious wear of materials may lead to an important rate of foreign body in tissues and may generate osteolysis lesions and inflammatory reactions. Thus the objectives of this retrospective study of 10 AES TAR implants (recovered after revision surgeries) are to understand how the prostheses wear out, which part is the most stressed and to determine the nature and size of foreign body particles. A better understanding of friction mechanisms between the three parts of the implant and of the nature and morphology of foreign particles generated was needed to explain the in vivo behavior of the implant. This was achieved using microstuctural and tomographic analysis of both implants parts and periprosthetic tissues.