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Title: Enhanced room-temperature plasticity of bulk metallic glasses by liquid phase separation
Author: Chen, Shuang shuang
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2014
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Bulk metallic glass (BMG) materials have become one of the hot topics in the field of advanced materials because of their unique physical, chemical and mechanical properties. So far, however, BMG alloys have not been utilized much in structural applications, owing to a highly localized nature of plastic flow that leads to catastrophic failure during mechanical loading. In this thesis, 2 new alloy composition groups based on Cu and Zr were developed to synthesize phase-separating BMGs by copper-mold suction casting. The glass-forming ability (GFA), microstructure, and formation mechanism of Cu−Zr−Al−Nb and Zr−Cu−Ni−Al−Nb BMGs were systematically investigated. It was found that the Cu47.2Zr46.5Al5.5Nb0.8 and Zr60Cu17.5Ni13.5Al8.5Nb0.5 alloys can be cast into a fully amorphous rod of 2 mm in diameter. Both alloys had undergone metastable phase separation by nucleation and growth mechanism in the liquid state and subsequently solidified into two distinct amorphous phases. Under uniaxial compressive loading, the designed Cu- and Zr-based BMGs demonstrated a remarkable macroscopic plastic strain of 16.3% and 19.7% respectively at room temperature. Theoretical calculation revealed that the enhanced plasticity in phase-separated alloys may attribute to the small difference in shear modulus (G) between two glassy phases. This may create similar dynamics for shear transformation zone (STZ) formation, permitting shear banding to develop throughout the whole microstructure rather than just in the low G phase. This work may provide guidelines for the improvement of the ductility of BMGs through careful compositional and structural design by microalloying. Besides, we also investigated the GFA, thermal, kinetic and mechanical properties of high-zirconium-based alloys. The results showed the Zr68Cu13Ni10.5Al8.5 alloy exhibits the best GFA with a critical casting diameter of 2.5 mm among the alloy series. The alloy displayed a significant compressive strain of 11.2% at room-temperature. The large plasticity was considered to originate in more free volume and lower icosahedral population induced by increasing the fraction of Zr−Zr atomic pairs. It was also found that the fracture strength of the Zr68Cu13Ni10.5Al8.5 BMG decreased with increasing strain rates in the range of 2×10−4 and 2×10−1. The decrease in fracture strength was mainly attributed to the creation rate of the free volume increases with the strain rate increases.
Supervisor: Todd, Iain Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available