Effect of Tantalum Content on Microstructure and Mechanical Properties of Cocrnitax Medium Entropy Alloys

CoCrNi medium entropy alloy (MEA) has potential applications in cryogenic environment. Its strength and plasticity simultaneously enhanced with decreasing temperatures. However, the poor yield strength ( YS ) at room temperature (RT) ts its widespread application. In this work, the alloying element Tantalum (Ta) was chosen to improve the YS at RT of CoCrNi MEA. The CoCrNiTa x (x=0.2, 0.4, 0.6 and 0.8 in molar ratio) MEAs were prepared and the effect of Ta content on the microstructure and mechanical properties of the corresponding alloys were investigated. It was found that the Ta-enriched Laves phase was formed with the addition of Ta element. And its microstructure experienced from hypoeutectic to eutectic, and then to hypereutectic morphology evolution in turn. The CoCrNiTa 0.4 alloy was a fully eutectic microstructure with fine lamellar colonies. Its YS was reach ed at »2150 MPa, which was obviously improved after the Ta alloying. Furthermore, the strength contribution of the individual phase to the lamellar colonies was investigated by the high-throughput nanoindentation mapping technique. The research results indicated that the Laves phase had highest hardness increasing from »16.12 GPa to »18.14 GPa, which was almost »3 times than that of FCC phase (»6.03 GPa). The hardness of lamellar colonies had a medium hardness, which ranged from »9.23 GPa to »10.12 GPa