Influence of carbon addition on microstructure and shape memory properties of Co-Cr-Fe-Mn high entropy alloy produced by L-DED

The Co-Cr-Fe-Mn high entropy alloy, a type of Co-Cr-Fe-Mn-Ni-based high entropy alloy where Ni is replaced by Co, is known to exhibit a shape memory effect (SME) through martensitic transformation between face-centered cubic (FCC) and hexagonal closed-packed (HCP) structure. Recently, it was shown that Co-18Cr-20Fe-19Mn and Co-18Cr-20Fe-19Mn-0.6C (wt. %) alloys could be successfully fabricated by L-DED and exhibited the SME. Notably, the carbon-containing alloy demonstrated enhanced shape memory behavior, achieving a maximum total recovery strain of approximately 3.5 %, compared to about 0.79 % of that from the carbon-free alloy. Carbon addition has been reported to enhance the SME of Fe-Mn-Si alloys, another well-known FCC/HCP-type shape memory alloy. It has been known that the carbon addition enhances the SME by acting as an interstitial atom and by forming carbides. In Fe-Mn-Si alloy, carbon could increase strength of austenite matrix as an interstitial solute element, thereby suppressing permanent slip and facilitating FCC to HCP martensite transformation. Also, when carbide precipitation occurs, stacking faults are formed due to the lattice misfit between the carbide and the matrix. In this study, the effect of carbon content on the microstructure and the shape memory properties of Co-18Cr-20Fe-19Mn alloys was investigated. The Co-18Cr-20Fe-19Mn-xC (x= 0, 0.2, 0.4 and 0.6 wt. %) alloys were fabricated by L-DED. Microstructure and shape memory behavior of the alloys with different carbon contents were investigated.