Constituent phases and their stability in a multi-principal element alloy filler for brazing of nickel-base superalloys
dc.contributor.advisor | Yu, Zhenzhen | |
dc.contributor.author | Schneiderman, Benjamin T. | |
dc.date.accessioned | 2023-05-03T17:26:55Z | |
dc.date.available | 2023-05-03T17:26:55Z | |
dc.date.issued | 2022 | |
dc.identifier | Schneiderman_mines_0052E_12536.pdf | |
dc.identifier | T 9474 | |
dc.identifier.uri | https://hdl.handle.net/11124/176632 | |
dc.description | Includes bibliographical references. | |
dc.description | 2022 Fall. | |
dc.description.abstract | Brazing is one alternative for joining and repair of nickel-base superalloys, in which only a filler material is melted, and filling of a joint-gap or crack is accomplished via surface wetting and capillary action while the base material remains solid. Design constraints on melting temperature and chemical compatibility with the base material guide the selection of a filler alloy composition. An ideal filler should minimally alter the microstructure of the nickel-base superalloy component at the braze site, particularly avoiding the introduction of potentially detrimental non-native constituent phases. The most common filler alloys in contemporary industrial use employ boron and silicon as melt-point depressants, which often introduce embrittling boride and/or silicide particles, and therefore fall short of this objective. A multi-principal element alloy (MPEA) filler was investigated in this work as a novel alternative. Through a suite of computational strategies considering melting temperature, calculated phases, and interactions with nickel-base superalloy substrates, this MPEA was selected from a composition subspace that is robust in the stability of a single, disordered FCC solid solution. It was therefore hypothesized that use of this filler would render the introduction of detrimental non-native phases to the braze-repair site avoidable. To evaluate this hypothesis, it was important to consider the filler’s tolerance to compositional change driven by dilution (i.e., mixing with the base material) at the immediate point of brazing, as well as the thermal phase stability following both short-term and long-term exposures to high-temperature operating conditions. Synchrotron X-ray diffraction is ideally suited to assess and map constituent phases in a complex brazed microstructure because it offers site-specific structure characterizations at the micron-scale and can detect low volume-fraction constituents. Nevertheless, an MPEA filler as a complex, concentrated alloy exhibits deviations from powder-pattern intensities in as-solidified microstructures and stoichiometric deviations from pure phases, leading to significant challenges in assessing the microconstituents using a conventional Rietveld refinement or strategies based in the powder diffraction file. To overcome these challenges, a novel analytical methodology was developed that synthesizes thermodynamic simulations, existing literature data for crystal structures, and experimental diffraction data coupled with supplemental scanning electron microscopy characterizations. Employing this methodology, experimental investigations were conducted using both Alloy 600 and Alloy 738LC as base materials, to respectively highlight brazing of a solid solution strengthened nickel alloy and a precipitate strengthened nickel alloy with greater industrial relevance. The major constituent phase in either braze was a single, disordered FCC solid solution. The absence of intermetallic phases indicated tolerance of the filler to dilution-driven compositional changes, despite the introduction of seven new elemental species to the filler in the case of the Alloy 738LC braze. The only non-native phases introduced to the as-brazed microstructures of either base material were oxides of Al, Cr, and Mn. The source of oxygen introduction was traced back to the laboratory-scale manufacturing conditions at the original MPEA casting step, indicating that oxide introduction is mitigable by employing more advanced manufacturing processes with better control of atmospheric elements. The only phase evolution detected during short-term high-temperature exposures was the precipitation and growth of additional Cr2O3 oxides, indicating that the MPEA filler possesses short-term thermal phase stability if its dissolved oxygen content can be reduced in manufacturing. Predictive assessments of the long-term phase stability of the MPEA filler using thermodynamic simulations indicated that the filler material is expected to outperform the base material in the long-term suppression of topologically close packed intermetallic phases. Despite the presence of contaminant oxides, the as-brazed ductility of Alloy 600 brazes using the MPEA filler was ten times greater than that offered by a conventional boron-containing filler, with comparable strengths between the two fillers. Variations in total elongation among individual specimens were attributed to variations in the oxide size and distribution. Alloy 738LC brazes using the MPEA filler are also expected to exhibit ductile behavior due to the absence of brittle microconstituents, although the suppression of γ’ is expected to negatively impact strength. A mechanical performance assessment of Alloy 738LC brazes and the re-introduction of γ’ to this microstructure are identified as areas for future work. | |
dc.format.medium | born digital | |
dc.format.medium | doctoral dissertations | |
dc.language | English | |
dc.language.iso | eng | |
dc.publisher | Colorado School of Mines. Arthur Lakes Library | |
dc.relation.ispartof | 2022 - Mines Theses & Dissertations | |
dc.rights | Copyright of the original work is retained by the author. | |
dc.subject | brazing | |
dc.subject | high entropy alloy | |
dc.subject | multi-principal element alloy | |
dc.subject | nickel-base superalloys | |
dc.subject | phase identification methodology | |
dc.subject | synchrotron x-ray diffraction | |
dc.title | Constituent phases and their stability in a multi-principal element alloy filler for brazing of nickel-base superalloys | |
dc.type | Text | |
dc.date.updated | 2023-04-22T22:15:50Z | |
dc.contributor.committeemember | Klemm-Toole, Jonah | |
dc.contributor.committeemember | Clarke, Amy | |
dc.contributor.committeemember | Findley, Kip Owen | |
dc.contributor.committeemember | Tucker, Garritt J. | |
dcterms.embargo.expires | 2024-04-22 | |
thesis.degree.name | Doctor of Philosophy (Ph.D.) | |
thesis.degree.level | Doctoral | |
thesis.degree.discipline | Metallurgical and Materials Engineering | |
thesis.degree.grantor | Colorado School of Mines | |
dc.rights.access | Embargo Expires: 04/22/2024 |