Influence of laser-based powder bed fusion processing parameters on the microstructure of a metastable HEA

Typ
Examensarbete för masterexamen
Master's Thesis
Program
Materials engineering (MPAEM), MSc
Publicerad
2022
Författare
Anilkumar, Vishnu
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High entropy alloys (HEAs) constitute a novel class of alloys containing multiple principal elements in nearly equiatomic proportions. This compositional complexity increases the configurational entropy and hence stabilizes the formation of solid solutions. The unprecedented compositional complexity has reportedly enabled HEAs to perform better than conventional alloys in various situations. Manufacturing HEAs using additive manufacturing techniques such as laser based powder bed fusion (LB-PBF) helps in preventing the formation of detrimental phases, owing to high solidification rates compared to conventional casting. Rapid melting and solidification during the layer-by-layer printing could also result in martensitic transformations in some metastable HEAs. This work focuses on the development of non-equiatomic metastable HEAs based on the CoCrFeNi grade, aiming to engineer martensitic transformation via tuning the printing parameters. After evaluating the relative densities of the specimens from the parametric development using the design of experiments methodology, two specimens printed with a significant difference in laser power were selected for further analysis. Scanning electron microscopy analyses of as-printed specimens revealed the presence of banded features spanning across the microstructure. Subsequent analyses using transmission Kikuchi diffraction revealed that these bands correspond to the HCP phase, indicating the occurrence of the martensitic transformation during printing. Tensile testing of as-printed specimens with two different processing parameters exhibited a similar yield strength of 560 ± 10 MPa and an engineering strain of 34 %. This could be due to the similar transformation induced plasticity (TRIP) behavior of the two materials during loading.
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High entropy alloys, laser-based powder bed fusion, metastability, martensite, microstructural characterization
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