In situ imaging and diffraction studies of the deformation of metallic glasses and composites

Khong, Jia Chuan

Abstract

Three types of experimental studies were done on metallic glass alloys in this PhD project. Firstly, in situ tensile deformation of bulk metallic glass alloy Zr₄₁.₂Ti₁₃.₈Cu₁₂.₅Ni₁₀Be₂₂.₅ (Vit-1), and a bulk metallic glass matrix composite, Zr₃₉.₆Ti₃₃.₉Nb₇.₆Cu₆.₄Be₁₂.₅ (ZrTi composite) were studied using in situ scanning electron microscopy (SEM) and in situ synchrotron X-ray diffraction (SXRD). It was found that the initiation and propagation of the shear band can be controlled through introduction of notch-type stress concentrator and allow it to be observed under in situ SEM. Using this technique, the local stresses/strains in front of the shear band can be measured using in situ SXRD that linked the initiation and propagation of shear band directly to the local stress field.

In Vit-1 sample, the formation of “step” was first observed between 1703MPa and 1992 MPa under in situ SEM imaging after two opposite propagating shear bands met and formed a crossly joined. This then led to formation of stress concentration area at the crossly joined area and led to another bigger “step”. In ZrTi composite, the plastic deformation in the dendrite was captured by in situ SEM. Moreover, in situ SXRD showed reduced axial strain rate in dendrite and increased axial strain rate in matrix at similar tensile stress. This concluded that load was transferred to matrix when yielding occurred in dendrite.

Secondly, in situ high pressure compression studies of a binary metallic glass alloy, Cu₅₀Zr₅₀, and a ternary metallic glass alloy, Fe₆₀Nb₁₅B₂₅ composite was done using Diamond Anvil Cell and SXRD. Cu₅₀Zr₅₀ showed no obvious polyamorphism structural change when pressed until 43.15 GPa but the pair distribution function analysis showed that the 3rd atomic shell started to split into two minor peaks which indicated the atomic coordination number in the 3rd atomic shell start to change. Likewise, Fe₆₀Nb₁₅B₂₅ showed no obvious polyamorphism structural change when pressed until 70.64 GPa but pair distribution function analysis showed that the rate of deformation in 2nd atomic shell become 5 times slower at 35.33 GPa. The difference in atomic size ratio between binary and ternary behaved differently during high pressure compression such that Cu₅₀Zr₅₀ with 30% difference from bigger atom and same atomic concentration of the two elements showed more fluctuation in atom shell movement compared to Fe₆₀Nb₁₅B₂₅ that have 3 different atomic radii with 21% and 56% difference from the bigger atom. The 3rd atomic shell movement fluctuation in Cu₅₀Zr₅₀ might be due to the rearrangement of Cu and Zr to allow further compression.

Thirdly, studies were carried out to design, make and characterise a series of new Fe-based bulk metallic glass alloys that can achieve sufficient glass forming ability and neutron absorption capability. After 3 rounds of iterative studies, Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Gd₂ (Fe-B₆Gd₂) was successfully developed with high glass forming ability of up to Ø5.8 mm and highest neutron absorption cross section with 73.64 cm-1 at thermal neutron 25.3 meV compared to other existing Fe-based metallic glass.

Citation

Khong, J. C. (2016). In situ imaging and diffraction studies of the deformation of metallic glasses and composites. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4219302

• Thesis Type: Thesis
• Deposit Date: Jun 2, 2017
• Publicly Available Date: Feb 23, 2023
• Keywords: Engineering
• Public URL: https://hull-repository.worktribe.com/output/4219302
• Additional Information: School of Engineering, The University of Hull
• Award Date: Jan 1, 2016