First Principles Method for Complex Cement Structure and High Entropy Alloys
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The ultimate objective of this dissertation contains two different areas of materials – secondary of Portland cement structure and high entropy alloys (HEAs). Similar concept of the computational method was applied to obtain properties of the two materials. Two based DFT packages are used, VASP and OLCAO. Both materials possess a complex structure. Ettringite and thaumasite are two of the relevant by-products in Portland cement minerals with high water content. The calculations focus on the comparison of their structures and properties to gain insights that can reveal the minute difference in properties due to differences in their structure, composition and water content. Detailed analysis of interatomic bonding shows the similarities and subtle difference between them even though thaumasite has the Si tetrahedron as the backbone unit in contrast to ettringite with Al-columns and no Si atoms. Moreover, thaumasite has strong C–O bonds that are absent in ettringite, and ettringite has far more water molecules with a substantial contribution from the hydrogen bonding. The role played by relatively strong S–O bonds in both crystals has a large impact on the hydration in these crystals. On the basis of calculated total bond order density (BOD), it is concluded that thaumasite is slightly more cohesive than ettringite consistent with the calculated mechanical properties. HEAs have attracted great attention due to their many unique properties and potential applications. Their exceptional properties are attributed to the disordered random solid solution of multiple alloying elements. The nature of interatomic interactions in this unique class of complex multicomponent alloys is not fully developed or understood. Here, three types of single-phase crystal structures are investigated to understand the fundamental properties of HEAs. Large supercells of body centered cubic (bcc), face centered cubic (fcc), and rock-salt fcc HEAs are used to examine their chemical defect such as local lattice distortion (LLD). One part of this work is to investigate TiNbTaZrMo HEAs as a key model compare its properties with 12 bcc HEAs. In this study porosity models were also constructed to reduce the Young modulus for biomedical applications. The second part is to examine four Cantor alloys related HEAs with the focus on LLD that is validated experimentally. The last part of this work adding interstitial elements into HEAs to achieve better properties. The results show that the single-phase bcc is the most ductile, least hardness, and lower elastic moduli compared to the fcc based HEAs. Also, a severe LLD can happen in the bcc structure, while this distortion is too small in the fcc system, depending on their local chemical environment. A theoretical modeling technique is also reported to enable in-depth analysis of the electronic structures and interatomic bonding and predict new HEAs properties based on the use of the quantum mechanical metrics, the total BOD and the partial BOD. These findings will shed light on understanding different type of HEAs and can be used as database for machine learning to design new HEAs and can be also extended for other complex structure materials.
Table of Contents
Introduction and background -- Methodology -- Comparative study of thaumasite and ettringite minerals -- Fundamental understanding of biocompatible entropy alloys -- Lattice distortions and mechanical properties in FCC high entropy alloys -- Effect of carbon content on structural and mechanical properties of high entropy alloys -- Conclusion remarks
Ph.D. (Doctor of Philosophy)