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Title: Synthesis, microstructure and mechanical behaviour of CuZr-based bulk metallic glass composites
Author: Wang, Haiyun
ISNI:       0000 0004 7964 5134
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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Bulk metallic glasses (BMGs) have attracted lots of attention owing to their unique physical, chemical and mechanical properties. However, BMGs are still far from practical application due to their catastrophic failure under loading. Substantial efforts have been made to improve their plasticity. CuZr-based bulk metallic glass composites (BMGMCs) have stood out by introducing transformation induced (TRIP) plasticity to the amorphous matrix. So far most developed BMGMCs are available only with limited dimensions (less than 4 mm in diameter). Furthermore, size and distribution of the second phase in the amorphous matrix are readily influenced by the manufacturing conditions and the purity of the raw materials. Zirconium in high purity is favoured, otherwise, unexpected brittle crystalline phase will precipitate, thereby reducing deformability. Casting temperature will determine the melt status in the mould and the resultant microstructure. High melting temperature and short melting duration in the casting process are preferred, ensuring homogeneous melting and hindering the absorption of the residual oxygen from the atmosphere. Minor Sn alloying addition (= 1 at%) can effectively improve the glass forming ability (GFA) of the ternary metallic system Cu48Zr48Al4, and thereby modifying the microstructures and corresponding mechanical performance. BMGMCs with 0.25 at% Sn addition can reach up to 7 mm in diameter. The B2-CuZr crystalline spheres ranged in size from 10 µm to 40 µm with a volume fraction of ~ 10% in the glassy matrix in the specimens with a size of 4 – 7 mm without sacrificing compressive strength and plasticity. BMGMCs with 0.75 at% Sn addition could be made up to 8 mm in diameter. The size and volume fraction of B2-CuZr crystalline spheres increased with the increase in specimen dimension. The corresponding compressive mechanical property and work hardening capability were improved. Minor Ni alloying addition (= 1 at%) to the ternary metallic system Cu48Zr48Al4 can effectively adjust the morphology and distribution of in-situ formed CuZr crystalline spheres in the glassy matrix and thus improve the deformation performance. Ni addition results in the formation of spear-like martensitic phase dispersed in the B2- CuZr crystalline phase, which may act as nucleation sites for martensitic transformation upon deformation. With the deformation proceeding, martensitic transformation zone gradually extended from the interface to the inside of crystalline phase (mainly B2). Interaction between untransformed crystalline phase and amorphous matrix, between transformed crystalline phase and amorphous matrix, between untransformed and transformed crystalline phase contributed to the improved plasticity and workhardening ability. Phase separation phenomenon was observed in the Cu47.5Zr48Al4Co0.5 bulk metallic glasses. It is different from the literature, however, AFM and TEM results provided straightforward evidence for the occurrence of the phase separation.
Supervisor: Rainforth, W. Mark ; Todd, Iain Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available