Corrosion Performance and Underlying Mechanisms of Multi-principal Element Alloys Prepared by Various Advanced Manufacturing Technologies


Student thesis: Doctoral Thesis

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Awarding Institution
Award date24 May 2022


The wide compositional space of multi-principal element alloys (MPEAs), such as medium entropy alloys (MEAs) and high entropy alloys (HEAs), provides a wealth of opportunity for unique and improved properties including high strength and good ductility. Owing to their superior mechanical properties, MPEAs are promising candidates for engineering applications. Besides, corrosion resistance could be a critical factor that affects the application ranges of MPEAs. The corrosion resistance is related to the strength and life-span of MPEAs. However, most research to date focus on the trade-off between strength and ductility of MPEAs, and the studies on the corrosion of MPEAs remain limited. It is worthy to explore various preparation methods that are feasible to optimize the corrosion resistance of MPEAs by tailoring the microstructure and elemental distribution, etc. Thus, in this thesis, a series of corrosion resistant MPEAs are designed and prepared by several advanced manufacturing technologies including magnetron sputtering, selective laser melting (SLM), hot isostatic pressing (HIP) and HIP combined with cold-rolling (HIP-CR). The corrosion behavior and underlying mechanisms are systematically evaluated.

Firstly, we examined the passivation behavior of the Ti-Ta-Nb medium-entropy alloy (MEA) films prepared by magnetron sputtering. The Ti-Ta-Nb MEA films show significantly improved corrosion resistance compared to pure Ti in the typical corrosive chemical solution of 0.5 M H2SO4. The mechanisms of the passivation behavior and stability of the passive films were extensively explored and discussed. The passivation evolution phenomenon consisting of two passive regions is due to the variation of chemical compositions of passive films. The excellent corrosion resistance of Ti-Ta-Nb MEA films can be ascribed to the stable passive films consisting of the mixture of TiO2/Ti2O3, Ta2O5 and Nb2O5, particularly Ta2O5.

Secondly, many studies have shown that the addition of interstitial elements, such as C and N, could be a powerful approach to improve mechanical properties of the MPEAs. It is also worthy to explore the effect of additional interstitial elements on the corrosion resistance of MPEAs. Considering the extremely low nitrogen solubility in 3d-transition metals, especially Ni and Co, at atmospheric pressure, gas atomization + SLM, has been chosen to manufacture the FeCoCrNi-N0.07, FeCoCrNi-C0.05, and FeCoCrNi MPEAs. Compared with the undoped FeCoCrNi MPEA, the FeCoCrNi-C MPEA shows an inferior corrosion resistance due to the formation of nanosized M23C6-type carbides. In contrast, the FeCoCrNi-N MPEA successfully overcomes the corrosion-strength tradeoff with the help of ammonia and CrN induced by N addition.

Powder metallurgy (PM), which allows superior compositional and microstructural control, has appeared as an interesting alternative processing method for MPEAs. Different PM processing routes may result in totally different microstructures. For example, HIP, with high temperature and high gas pressure simultaneously applied to samples, can result in fine grain and uniform structure. While the MPEAs prepared by SLM usually show the characteristics of the hierarchically heterogeneous structure. In order to reveal the effect of microstructure on the corrosion behavior, the corrosion characteristics of N-doped equiatomic FeCoCrNi MPEAs fabricated by SLM, HIP, and HIP-CR were investigated. In 0.5 M sulfuric acid, the as-SLM FeCoCrNiN0.07 MPEA exhibits a much lower passivation current density than the as-HIP and HIP-CR FeCoCrNiN0.07 MPEAs. Composition analysis of passive films formed on as-SLM FeCoCrNiN0.07 revealed the Cr-enrichment and ultrahigh proportion of Cr2O3. The superb corrosion resistance and improved passivation ability of as-SLM FeCoCrNiN0.07 could be attributed to the dislocation cells and micro-segregation of Cr along the cell boundaries induced by selective laser melting.

Overall, this thesis study has initiated new exploration on the corrosion response and mechanisms of these new MPEAs prepared by different advanced manufacturing technologies, likely to be useful for future applications.