Effects of cerium addition on microstructures and mechanical properties of CoCrNi medium entropy alloy films

In this work, the effects of rare-earth element Ce addition on the microstructures and mechanical properties of (CoCrNi)(100-x)Ce-x (x = 0, 1.05, 2.61, 4.2, 5.96) medium entropy alloy films (MEAFs) were investigated. A series of (CoCrNi)(100-x)Ce-x MEAFs was fabricated by radio frequency magnetron co-sputtering. The XRD and TEM results showed that the structure transformed from FCC to FCC + HCP at x = 2.61. Also, as Ni precipitated with Ce to form CeNi5 HCP phase, the localized depletion of Ni would encourage CoCrNi to transform from FCC to HCP phase. With the further increase in Ce content, Ce-4.2 film became mixed nanocrystalline and amorphous structures and Ce-5.96 film was fully amorphous. The nanoindentation and micro-pillar compression tests were conducted to study the mechanical properties of films. Both the hardness and Youngs modulus showed an initial increase, reached the maxima of similar to 10 GPa and similar to 200 GPa, respectively, at x = 1.05, and then decreased with the increasing Ce content. The trends of yield strength and ultimate strength versus Ce content were similar to hardness. These results demonstrated that small amounts of Ce addition could strengthen CoCrNi MEAFs due to solid solution strengthening. However, excessive Ce addition would reduce the grain size to below similar to 10 nm and weaken the CoCrNiCe system due to the inverse Hall-Petch effect.