Publication Date
2021
Document Type
Dissertation
Committee Members
Raghavan Srinivasan, Ph.D. (Advisor); Ahsan Mian, Ph.D. (Committee Member); Henry D. Young, Ph.D. (Committee Member); Joy Gockel, Ph.D. (Committee Member); Vijay Vasudevan, Ph.D. (Committee Member)
Degree Name
Doctor of Philosophy (PhD)
Abstract
Formation and growth mechanisms of intermetallic compounds in the aluminum/tungsten system were studied during consolidation and subsequent heat treatment. Three different processes, spark plasma sintering (SPS), laser powder bed fusion (LPBF), and metal inert gas welding (MIG) were used, and each of these processes were followed up with heat treatment at different conditions. Al12W, Al5W and Al4W are the three intermetallics that are stable in the system in room temperature, and they were expected to form at the interface between tungsten and aluminum at different temperature ranges. Contrary to expectations, the Al5W phase was not observed during this study and the Al4W phase-only formed when molten aluminum reacted with tungsten for a sufficiently long time, for example during welding or during re-melt of LPBF samples. The Al12W phase formed on the Al/W interface during solid-state heat treatment starting in the temperature range from 500 ˚C to the melting point of the matrix. This phase was observed on the interface of Al4W/Al on the side of Al4W during heat treatment of the welded sample, heat treatment below the aluminum melting point for a long time led to the consumption of Al4W to form more Al12W. During solid state heat treatment, the Al12W phase formed at the Al/W interface and grew into the Al matrix outside the W particles border. The Al12W particles had a hexagonal multi-faceted interface with the Al matrix, with a 120o angle between the facets. No composition variation was observed in the Al12W phase over the entire distance from the W/Al12W interface to the Al12W/Al interface. These observations pointed to the possibility that the growth of Al12W during solid state transformation was interface-controlled, with rapid transport of Al and W through the ordered Al12W. The reason for the faceted interface morphology of the Al12W is due to the high ratio of the latent heat of fusion to melting point, i.e. the entropy of fusion. The equilibrium Al12W precipitates have hexagonal shape with internal angle of 120°, this is due to expanding the highest planner density {110} plane family during growth and becomes the dominant planes of the crystal due to the high planner density of atom in this family of planes compared with other families of planes. As in other intermetallics, the tungsten aluminides have complex crystal structures, where cages of aluminum atoms surrounding a single tungsten atom represent the common trait among these structures. The Al12W has a BCC-based crystal structure, where it has Al12W units at the corners and the body center. The Al12W crystal structure has a more open structure and lower density as compared with Al4W and Al5W. Despite the open structure of the Al12W, the structure cavities have small size where none of the tungsten atoms nor aluminum can be fitted in, so that there is no interatrial diffusion can occur. The constant composition along the Al12W intermetallic phase implies an interface-controlled growth mechanism with rapid diffusion. There are interstitial sites on the cube faces that are the potential fast diffusion paths for both Al and W atoms.
Page Count
123
Department or Program
Ph.D. in Engineering
Year Degree Awarded
2021
Copyright
Copyright 2021, all rights reserved. My ETD will be available under the "Fair Use" terms of copyright law.
ORCID ID
0000-0002-9637-9945
