Flash-Annealing of Copper-Zirconium-Aluminium based Bulk Metallic Glasses (PDF)

Doctoral Thesis / Dissertation from the year 2017 in the subject Chemistry - Materials Chemistry, grade: 1,0, Dresden Technical University (Fakultät für Maschinenwesen), language: English, abstract: The first aim of the present work is to design, built and test a device being capable of rapidly heating and subsequently immediately quenching bulk metallic glass (BMG) specimens in a uniform and reproducible manner. The heating and ejection process of the BMG specimen is temperature-controlled, so that the heating rate and temperature to which the BMG is annealed to, can be varied.

BMG composites consisting of B2 CuZr crystalline particles embedded in the glass shall be prepared. The occurrence of the shape-memory B2 CuZr phase and a good glass-forming ability limit the selection of the alloy system to Cu-Zr-Al. The high-temperature B2 CuZr phase is metastable at room temperature, yet rapid cooling of Cu-Zr-Al-based alloys enables to cast B2 CuZr BMG composites. One could speculate that the B2 CuZr phase forms as well during the devitrification of Cu-Zr-Al-based metallic glass at high heating rates. Indeed, a recent work yields hope rendering the preparation of B2 CuZr BMG composites by devitrification possible. Therefore, a rapid heating or flash-annealing device as it is termed here, is developed first. Thereby, flash-annealing is defined as rapid heating to a predefined temperature followed by immediate quenching. Cu-Zr-Al-based BMGs are flash-annealed at different heating rates to temperatures above the crystallization temperature and the influence of the heating rate on the phase formation shall be unravelled. The exact ejection temperature of the BMG specimen determines the volume fraction of the crystalline phase(s) and the crystal size distribution of the so-obtained composite. The immediate subsequent quenching at a high cooling rate is crucial here, since the partially crystallized supercooled liquid must be frozen in. The mechanical properties of the BMG composites and deformation mechanisms involved are studied. Moreover, the microstructure of these BMG composites allows to gain knowledge about crystallization kinetics of highly supercooled liquids during rapid heating.