Abstract
High-entropy alloys (HEAs) have emerged as a class of structural alloys with various attractive properties, and their application in additive manufacturing, which enables unprecedented thermal history and geometrical complexity, is promising for realising advanced materials. This study investigates the corrosion behaviour and passive film characteristics of an equimolar AlCoCrFeNi HEA additively manufactured by electron beam melting (EBM). Potentiodynamic polarisation in a 3.5 wt% NaCl solution revealed that the bottom part of the EBM specimen shows better corrosion performance than a conventionally prepared cast specimen in terms of both corrosion and passivation current density, while a continuous increase in the current density without any apparent passivity was observed during the anodic polarisation of the top part. The electrochemical impedance spectroscopic study indicated significant differences in the passive film characteristics between the specimens, and revealed an enhanced charge-transfer resistance and the formation of a more protective passive film of the bottom part. The elemental redistribution, in particular, the enrichment of Cr in the B2 phase during the post-melt high-temperature exposure of the alloy during EBM, was responsible for the improved stability of the passive film, retarding the selective dissolution of the B2 phase in the bottom part. These findings indicate that the microstructural evolution caused by ‘in situ annealing’ during the EBM process significantly influences the corrosion behaviour of the HEA.
Highlights
High-entropy alloys (HEAs) are multicomponent alloys composed of five or more elements in equimolar or near-equimolar proportions[1,2,3]
The modulated structures correspond to the duplex microstructure of the body-centred cubic (BCC) and B2 phases
The modulated structure in the electron beam melting (EBM) specimens was markedly coarsened at the bottom section, while it was finer at the top section, as compared with that of the cast specimen
Summary
High-entropy alloys (HEAs) are multicomponent alloys composed of five or more elements in equimolar or near-equimolar proportions[1,2,3]. It has been claimed that due to the unique alloying concept, the high mixing entropy of these alloys stabilises the solid-solution phases, such as in face-centred cubic (FCC) and body-centred cubic (BCC) structures. It does not necessarily assure the formation of a simple solid solution and, some HEAs show complex microstructures with the formation of secondary or intermetallic phases within the matrix[10,11]. It was demonstrated that as the x value increased to 1.00, the microstructure varied from single-phase FCC to BCC/B2
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