Formation of ZrCu-based Bulk Metallic Glasses and Composites with Enhanced Glass Forming Ability and Mechanical Properties

Bulk metallic glasses (BMGs) have shown a unique combination of mechanical, chemical, and physical properties, but their room-temperature brittleness has been the stumbling block for real structural applications. To respond to this challenge, the concept of developing composite microstructures by combining the glassy matrix with crystalline phases at different length scales has been developed, through which an improvement in compression and tensile ductility has been obtained in several Zr- and Ti-based BMG composites. However, these BMG composites showed a macroscopic strain-softening phenomenon with an early onset of necking (i.e., the maximum strength occurs at the yield point) because of a lack of work hardening mechanisms (endows the materials with minute damage tolerance), which would give rise to serious engineering problems therefrom. In this thesis, three series of ZrCu-based BMGs and Bulk Metallic Glass Matrix Composites (BMGMCs) were designed which are Zr-Cu-Al-Ag, Zr-Cu-Al-Ag-Ti and Zr-Cu-Al-Nb. All these alloys were prepared and characterised in terms of thermal behaviour, phase formation and mechanical properties. The Zr50Cu45-xAl5Agx (x = 0, 0.5, 1 and 2.0 at. %) alloy systems were designed based on the Zr-Cu-Al BMGMC system. The effect of the Ag element addition to the base alloy system has been studied. It is found that the cooling rate (sample size) strongly affects the phase formation and the mechanical properties of the alloy, exhibiting big differences due to the formation of the brittle phases, the volume fraction and size of the B2-CuZr phase. By investigating these alloys, it also provides a scope for preparing medium/large-sized single B2-CuZr phase BMGMCs in the following chapters. In addition, a new series composition of the Zr50-xCu44Al5Ag1Tix (x = 0.5, 1, 2, 3, 4 and 5 at. %) alloy has been designed based on the Zr-Cu-Al-Ag alloy system. By carefully controlling the Ti content of the alloy compositions, the 6 mm Zr49.5Cu44Al5Ag1Ti0.5 and Zr49Cu44Al5Ag1Ti1 alloys have exhibited good work-hardening ability and plasticity during compression tests. The martensitic transformation from the metastable B2-CuZr phase to the monolithic B19’ martensitic phase was observed after the compression tests. The deformation-induced martensitic transformation process leads to a significant improvement of compressive plasticity, with plastic strain of 12.3 % and obvious work hardening behaviour. Finally, the glass forming ability (GFA), thermal properties, kinetics and mechanical properties of Zr50Cu45-xAl5Nbx (x = 0.2, 0.4, 0.6, and 0.8 at. %) BMGs have been investigated. The alloy displayed a significant compressive strain of 7.1 % at room temperature. The results showed that the GFA is enhanced with an increase of Nb content. The plastic strain exhibits a trend of increasing with the Nb content. The current findings offer a new paradigm for developing BMGMCs with improved ductility for practical engineering materials. These studies and observations provide an understanding of the formation, deformation and microstructural optimisation of the ZrCu-based BMGs and BMGMCs.