Abstract
This study explores carbon addition as a materials design approach for simultaneously improving the hardness, crack resistance, and corrosion resistance of high entropy thin films. CoCrFeMnNi was selected as a starting point, due to its high concentration of weak carbide formers. The suppression of carbides is crucial to the approach, as carbide formation can decrease both ductility and corrosion resistance. Films with 0, 6, and 11 at.% C were deposited by magnetron co-sputtering, using a graphite target and a sintered compound target. The samples with 0 at.% C crystallized with a mixture of a cubic closed packed (ccp) phase and the intermetallic χ-phase. With 6 and 11 at.% C, the films were amorphous and homogenous down to the nm-scale. The hardness of the films increased from 8 GPa in the carbon-free film to 16 GPa in the film with 11 at.% C. Furthermore, the carbon significantly improved the crack resistance as shown in fragmentation tests, where the crack density was strongly reduced. The changes in mechanical properties were primarily attributed to the shift from crystalline to amorphous. Lastly, the carbon improved the corrosion resistance by a progressive lowering of the corrosion current and the passive current with increasing carbon concentration.
Highlights
CoCrFeMnNi, known as the Cantor alloy, is a material in the class of high entropy alloys (HEAs) [1]
For the carbon-free samples, the calculations agreed with several examples in the literature [3], predicting a single ccp phase above 800 °C and phase mixture of a ccp phase, a r-phase, and a bcc phase below this temperature
The carbide consists of 30 at.% C, 50–70 at.% Cr and small amounts of the remaining elements, with Mn and Fe being slightly more abundant than Co and Ni
Summary
CoCrFeMnNi, known as the Cantor alloy, is a material in the class of high entropy alloys (HEAs) [1]. The low solubility of C in CoCrFeMnNi, leading to carbide precipitation, clearly limits the benefits of carbon addition, as both the ductility and the corrosion resistance decrease after adding only a few at.% C due to the Cr-rich carbide This limitation can be overcome by sputter deposition. The atoms are rapidly quenched from the gas to a solid phase, so that the phase formation often is governed by diffusion kinetics, and metastable phases can be obtained This has already been used as a strategy to overcome the trade-off between hardness and crack resistance for both single element [13] and HEA [14,15,16] thin films, where the formation of carbides was suppressed in favor of supersaturated solid solution of carbon in the alloy. Electrochemical tests and analyses of the electrolytes with inductively coupled plasma mass spectrometry are used to evaluate the corrosion resistance
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
