Impact of sulfur on the structure and dynamics of glass forming metallic melts
Wilden, Johanna Stephany; Kargl, Florian (Thesis advisor); Roth, Georg (Thesis advisor)
Aachen : RWTH Aachen University (2022, 2023)
Dissertation / PhD Thesis
Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022
Just recently Sulfur was considered as alloying element for bulk metallic glass production. It was found that the Sulfur addition extends the glass forming ability as well as the glass forming region in various alloys. In Titanium-based alloys without Beryllium, Sulfur addition allows the glass formation for the first time. These Titanium-based bulk metallic glasses are particular interesting for biomedical applications. This work aims to understand, why Sulfur improves the glass forming ability in these Titanium-based alloys as well as in Palladium-based alloys. Thus, the melt properties in ternary Pd-Ni-S and Ti-Ni-S melts were examined. In order to process these highly reactive Titanium-based melts, levitation methods were utilized, as electromagnetic levitation and electrostatic levitation. Thus, the structure-dynamics relation in Ti-Ni-S and Pd-Ni-S melts was examined. The levitation methods were supplemented by incoherent, quasielastic neutron scattering in order to examine the atomic motion in the melts. Furthermore, the levitation methods were supplemented by synchrotron X-ray diffraction and neutron diffraction in order to examine the atomic structure and the solidification behavior in the melts.Within the Ti-Ni-S system, the optimized composition with the best glass forming ability contains 8 at. % Sulfur. Here, it was found that the melt packing and the melt dynamics decreases upon Nickel substitution by Sulfur. Thus, there is no direct correlation between the melt packing and the melt dynamics. The decrease in packing fraction indicates chemical interactions, which may promote the glass formation in Ti-Ni-S melts. The melt structure in Ti-Ni melts reveals a preference to form heterogeneous atomic pairs, as well as a mean coordination number ⟨Z⟩ > 12, which implies a more complex short-range order than an icosahedral short-range order. Thus, the short-range order in Ti-Ni melts is composed of topological and chemical contributions. In contrast to binary Ti-Ni melts, ternary Ti-Ni-S melts reveal a primary Sulfur-rich phase upon solidification, which increases the liquidus temperature. This Sulfur-rich phase changes from Ti3S to Ti2S upon Nickel substitution by Sulfur. The microstructure in the quenched Ti-Ni-S alloys shows that the Ti3S phase is surround by metastable, quasicrystalline phases, while the Ti2S phase is surround by stable, crystalline phases. Thus, Ti3S promotes the glass formation in the quenched Ti-Ni-S alloys, while Ti2S promotes their crystallization. Within the Pd-Ni-S system, the optimized composition with the best glass forming ability contains 26 at. % Sulfur. Here, it was found that the melt dynamics barely change with composition. In comparison to Pd-Ni-P melts, the melt dynamics increase upon Phosphorous substitution by Sulfur, whereas the packing fraction decreases. Thus, similar to Pd-Ni-(Cu)-P melts, there is a direct correlation between the melt packing and the melt dynamics. However, in contrast to Pd-Ni-(Cu)-P melts, a dynamic decoupling was already observed above the liquidus temperature. Possibly, the small Nickel and Sulfur atoms diffuse through an immobile Palladium-matrix. Overall, these observations indicate different mechanisms responsible for the glass formation in Ti-Ni-S and Pd-Ni-S melts.
- Division of Materials Science and Engineering 
- Fundamentals of Solidification Teaching and Research Area