Microstructural evolution and oxidation behavior of AlNiCrCoSi multi-component alloys

Abstract

Equiatomic or near-equiatomic multicomponent alloys, often termed high-entropy alloys (HEAs), are an emerging class of metallic materials that are being investigated for a wide range of technical applications. Unlike conventional engineering alloys, HEAs lack a principal component and are instead composed of at least five major metallic elements, each with compositions generally ranging from 5 to 35 at. %. Unlike conventional alloys, HEAs have high entropies of mixing that are said to enhance the formation of solid solutions and other metastable phases resulting in materials exhibiting unique combinations of properties including high thermal stability, high strength/hardness, and high corrosion/oxidation resistance. Most studies of HEAs have focused on optimizing microstructures or mechanical properties. Relatively few have designed an alloy for or investigated oxidation behavior of HEAs. This dissertation describes the microstructural evolution and oxidation behavior of three HEA alloys that were designed with the intent to provide high oxidation resistance whilst offering the possibility for high temperature precipitation hardening. The alloys were intentionally designed to contain four elements that are commonly utilized in high temperature high-temperature Ni-based alloys, Al, Ni, Cr, and Co. A fifth element, Si, was also added. Though not a typical addition to structural Ni-based superalloys, it has been show to improve their resistance to oxidation, hot corrosion, and wear, particularly when incorporated into surface layers or protective coatings. All of the alloys investigated were designed using the phase selection rules proposed by Yang and Zhang which predicted that the alloys would consist of a mixture of solid solution and intermetallic phases in the as-cast or as-deposited state. Two alloys were designed to have higher concentrations of Al and Cr, thus maximizing the possibility of forming disordered BCC phases, while one was designed to have lower concentrations of Al and Cr, thus maximizing the chances of forming disordered FCC phases. Detailed microstructural and chemical analysis showed that characterizations have confirmed the presence of the expected solid solution phases, however, the majority phases in all of the alloys were found to be ordered intermetallics which increased in volume fraction upon annealing. The majority of the phases that formed were consistent with thermodynamic predications made using the Thermo-Calc software package; however the alloys were subject to phase transformations that could not be predicted using existing thermodynamic data. Isothermal and cyclic oxidation tests demonstrated that both sets of alloys were capable of forming aluminum oxide (aka alumina) surface scales, but that alloys containing higher Al and Cr were more stable. Coatings with similarly high Al and Cr levels were found to significantly improve the oxidation resistance of pure Ni and to exceed the 250 h oxidation upon the already excellent oxidation resistance of beta-NiAl based coatings. This result was surprising in that the HEA coatings produced in this study were thinner and were deposited on a less oxidation resistant substrate than the beta-NiAl based coatings. Collectively these results have confirmed that complex multicomponent HEAs that are capable of forming protective alumina scales can be designed and processed using existing phase selection rules. These results also reiterate the need for refinement of the phase selection rules for HEA formation and improved thermodynamic databases to facilitate the design of better HEAs.