| Author: | Li, Tian |
| Title: | The influences of glass-glass interfaces on mechanical properties of metallic nanoglasses : insights from experimental studies and molecular dynamics simulations |
| Advisors: | Zheng, Guangping (ME) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Metallic glasses Nanocomposites (Materials) Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xvi, 162 pages : color illustrations |
| Language: | English |
| Abstract: | Metallic glasses (MGs), also known as amorphous alloys, are a class of materials composed of randomly arranged atoms that can be simply obtained by quenching their melts rapidly. In comparison to those crystalline alloys, there is a lack of long-range order in MGs, whose atomic structures are fully disordered and do not contain any crystalline defects, e.g., vacancies, dislocations and grain boundaries. As a result, defect-free MGs behave much stronger than the former, making them an ideal candidate to be built as mechanical components for the engineering applications. Nevertheless, the brittleness issue arising from localized shear bands still remains and cannot be well resolved over the last several decades, which poses great challenge to their practical use as structural materials. Recently, two-dimensional defects, i.e., glass-glass interfaces (GGIs), have been introduced into amorphous alloys, resulting in a new type of non-crystalline materials called metallic nanoglasses (NGs). Analogous to the nanocrystalline alloys, metallic NGs compose of glassy grains interconnected by amorphous GGIs. The current dual-phase model suggests glassy grains are similar with the rapidly-quenched states of MGs. By comparison, atomic structures of GGIs can be less dense and they are in a new glass phase. More importantly, metallic NGs are promising in overcoming the catastrophic brittle failure observed in MGs, largely attributing to the presence of GGIs that could improve the ductility. Nonetheless, metallic NGs have a much lower mechanical strength, as caused by the excess free volumes of GGIs, which has led to an unsolved issue of tradeoff between strength and ductility. Up to now, a lot of efforts have been made to address this dilemma related to the GGIs. It is worth noting that NGs may have a remarkable mechanical strength once if the glass phase of GGIs is reinforced. Nevertheless, the application of this strategy requires a comprehensive understanding on the atomic and electronic structures of GGIs while current knowledge of them is still lacking. In other words, reinforcement of GGIs is yet not feasible and, as a result, NGs cannot be strengthened effectively. Therefore, in-depth investigations on analyzing the correlation between glass phase of GGIs and NG property could be necessary, which are critical to improving the understanding on the glass phase of GGIs, thereby overcoming the tradeoff between strength and ductility of metallic NGs. Herein, (Co, P)-based metallic NGs prepared through pulse electrodeposition are selected for a systematic investigation on the aforementioned issues by a combination of experimental and simulation studies. The obtained results show that the significant changes on properties of NGs are attributed to the unique glass phase of GGIs, which cannot exist independently and are strongly dependent on the adjacent glassy grains. To be specific, the magnetism of Co-Fe-Ni-P NGs much differs from that of Co-Fe-P NGs, resulting from electronic structures of GGIs as altered by additions of alloying element Ni. The enthalpy change in glass transition of Co-Fe-Ni-P NGs is greater than that of Co-Fe-P NGs, which is attributed to atomic structures of GGIs since they have become more thermodynamically stable after alloying with element Ni. An additional sub-peak observed in Mössbauer spectrum of Co-Fe-Ni-P NGs confirms that the atomic and electronic structures of GGIs are related to the additions of Ni. The glass phase of GGIs is also grain-size dependent and the atomic structures of GGIs are found to be more stable in Co-Fe-Ni-P NGs with reduced mean grain size. Therefore, Co-Fe-Ni-P NGs show better glass forming ability when the average size of glassy grains decreases. Similarly, glass phase of GGIs can be stabilized in Co-Fe-Ni-Zn-P NGs, which is thermodynamically unstable in Co-P or Co-Fe-P NGs. The results suggest that the atomic structures of GGIs have been tailored by the additions of Fe, Ni or Zn, since the entropy of mixing at GGIs increases with increasing number of alloying elements. Such entropy stabilized GGIs would lead to decent improvement in the ductility and strength of Co-Fe-Ni-Zn-P NGs. Furthermore, it is found that the glass phase of GGIs observed in the Co-P NGs is dominated with Co atoms, which has been studied by molecular dynamics simulations and confirmed by atomic probe tomography analysis on elemental contents at the GGIs. The atomic structures of GGIs with increased Co segregation would have lower shear resistances than that inside glassy grains. Consequently, they promote the shear banding under plastic deformation, and the ductility of Co-P NGs could be improved through enhancing elemental segregation at GGIs. |
| Rights: | All rights reserved |
| Access: | open access |
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