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Title:Effect of roughness, microstructure, and chemistry on the environmental durability of structural alloys
Author(s):Shetty, Pralav P.
Director of Research:Krogstad, Jessica A.
Doctoral Committee Chair(s):Krogstad, Jessica A.
Doctoral Committee Member(s):Braun, Paul V.; Bellon, Pascal; Heuser, Brent J.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):structural metals
oxidation resistance
corrosion resistance
microstructure control
asphaltene deposition
functional coatings
Abstract:Unlike functional materials where one design property is usually optimized, structural materials need to meet several design requirements including but not limited to excellent mechanical properties and environmental tolerance. However, more often than not, the metallurgical principles used to improve mechanical properties result in a decrease in the environmental tolerance. Though it is difficult to design materials that can thrive in extreme environments like high temperature, high pressure, and harsh chemicals, these conditions are often where the biggest scientific and economic opportunities lie. Aeroengines, and oil refineries are two example applications that encounter these issues. This study specifically focusses on improving the environmental tolerance of NiCrAl films a common bond coat used in aeroengines, and ferrous alloys used in petroleum production. The goal was to develop solutions that improve the environmental tolerance while minimally affecting other properties of these alloys. The oxidation behavior of a model sputtered NiCrAl system was studied in various time and oxygen partial pressure (pO2) regimes. Low pO2 conditions seemed to favor the formation of protective oxides. However, tuning the composition of the base alloy was an effective way to limit the oxidation rate in high pO2 conditions. Thermal destabilization of the sputtered microstructure was found to take place on similar timescales to the transient oxide formation. Thus dilute Y additions were made to temporarily stabilize the sputtered microstructure and manipulate the transient stages of oxidation. Yttrium addition not only retarded grain growth through nanoclustering and kinetically pinning grains, but also helped nucleate and grow dense, and slow-growing oxides. In some cases, the improvement in oxidation resistance via. Y addition was similar to reducing the pO2 by several orders of magnitude. Robust α-alumina oxides formed on Y doped NiCrAl films at temperatures as low as 900 oC by oxidation in an air environment which is unprecedented and could be of major commercial importance. An attempt was made to understand this anomalous oxidation behavior by using unconventional diffusion-triples comprising of a sputtered NiCr (undoped and Y doped) top layer, Al middle layer, and a sintered (micrograined) NiCr bottom layer. Annealing experiments conducted on the diffusion-triples proved that Al diffusion in sputtered NiCr is more rapid than that in sintered NiCr. Through the use of profile processing techniques, Al was shown to follow type B kinetics for grain boundary diffusion in sputtered NiCr. It also revealed that Y addition to sputtered NiCr further accelerates Al diffusion through a non-Fickian mechanism involving Al clustering. The baseline fouling and corrosion behavior of ferrous alloys in a high temperature, high pressure, and an asphaltenic environment was also evaluated. Key insights were generated on the interplay between the thermochemical properties of the asphaltene, the environmental conditions, surface preparation of the alloys, and the chemistry of the deposits. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy allowed for the first time to pinpoint mechanisms for high temperature model asphaltene deposition on ferrous alloys. Improving surface roughness alone was found to be a good strategy to mitigate asphaltenic fouling at lower temperatures where the asphaltene remains intact. However, at temperatures where reactive asphaltene decomposition products become present in solution, surface chemistry control becomes important. Specifically, a protective atomic layer deposition alumina chemistry on steels was found to significantly reduce asphaltenic fouling. In order to evaluate whether a protective alumina chemistry could be generated on components with more complex geometries, low temperature pack cementation of ferrous alloys was conducted. Preliminary data did show an improvement in anti-fouling properties with both model asphaltenes in a static environment, as well as with crude oil in a hydrodynamic environment. However, XPS revealed a mixed alumina-hematite oxide on the surface that may be limiting the anti-fouling properties of these surfaces. Finally, new insights gained from developing low temperature pack cementation for ferrous alloys allowed for the modification of low thermal stability, functional metallic structures like Ni inverse opals. It resulted in a thermal stability enhancement by 500 oC, comparable to refractory metals in the same configuration. And also improved both the modulus and hardness of these structures.
Issue Date:2019-02-22
Rights Information:© 2019 Pralav P. Shetty. All rights reserved.
Date Available in IDEALS:2019-08-23
Date Deposited:2019-05

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