Development and Characterization of Toxic-element free and Biocompatible Ti-based Metallic Glasses

Metallic glasses exhibit unique physical properties like large elastic strain, high strength and hardness, high wear and corrosion resistance thanks to their disordered atomic structure, which lacks crystal defects, dislocations and grain boundaries. If a metallic alloy can be sufficiently fast cooled down to avoid crystallization, it solidifies in such a way that its atoms cannot occupy their thermodynamically stable lattice positions and the resulting amorphous structure is referred to as metallic glass (MG). The necessity of a high cooling rate, however, also confines their physical shape to be a narrow form like a thin film, a ribbon, or a few millimeters thick rod. The latter can only be achieved from some specific metallic alloys, which possess a high glass forming ability (GFA) and preserve their amorphous structure in a bulk form.
It has been 18 years since the discovery of the famous Ti-based Ti40Zr10Cu36Pd14 bulk metallic glass (BMG), which was intended to be applied in biomedical applications by replacing polycrystalline commercially pure (cp) Ti or well known Ti−6Al−4V. Yet, although great scientific effort was put into proving that this alloy is biologically safe and has great potential to be utilized in medical implants, the application of cp-Ti and Ti−6Al−4V polycrystalline alloy still persists today. This is party due to the expensive price of Pd, but the main reason is the high Cu content of this BMG and the trivial cytotoxicity of Cu. Moreover, due to the high Cu and low Ti content of the alloy, this BMG exhibits a mediocre corrosion resistance compared to the traditional polycrystalline Ti-based medical alloys, which undermines its superior mechanical properties. The risk of harmful Cu ion release to the surrounding body tissues, which can cause inflammatory reactions after the surgical operation, is the main impeding factor that preventing the application of Cu-bearing Ti-based BMGs in the medical implant industry.
The primary goal of this thesis is to discover new truly biocompatible Ti-based metallic glasses (Ti-based MGs) with the highest possible glass forming ability (GFA), which exhibit absolute long-term safety in the human body due to the fact that they only consist of individually biocompatible elements. This limitation in the choosing of alloying candidates excludes the most formidable glass-forming elements like Be, Ni, Cu and Al, which are indispensable in Ti-based BMG production. Thus, it was sought after new toxic element free Ti-based MGs in the transition metal - metalloid type (TM-M) glass-forming systems. Even though this type of Ti-based MGs possesses inherently low GFA, which confines their physical shape to thin melt-spun ribbons, the GFA of the new alloys is comparatively evaluated and its improvement is pursued, without sacrificing the corrosion resistance for the sake of a high GFA.
The decisive parameter of a medical alloy regarding its biocompatibility is the corrosion resistance. Therefore, maintaining a high corrosion resistance is considered a priority and the Ti-content in the alloy compositions kept as high as possible. Moreover, in order to induce a synergistic effect of multiple vital elements on the corrosion resistance and also to promote the GFA via increasing the configurational entropy and widening the atomic size distribution, a metalloid admixture was utilized as the M component of the TM-M type glass forming alloys. In-depth analyses were carried out on selected alloy groups.