The phase evolution, microstructures and mechanical properties of CoCrNiCe medium entropy alloy films (MEAFs) were studied in this work. A series of CoCrNiCe MEAFs containing various Ce content ranging from 0 to 5.96 at.% were fabricated using radio frequency magnetron co-sputtering. The XRD and TEM results showed the structure transformed from FCC, FCC+HCP and to amorphous phase. The nanoindentation and micro-pillar compression tests were conducted to study the mechanical properties of films. Both the hardness and Young’s modulus showed an initial increase, reached the maxima of ~10 GPa and ~ 200 GPa, respectively, at x=1.05 due to the solid solution strengthening, and then decreased. When doping the 4.2 at.% Ce, the hardness decreased to the minimum value of ~ 8 GPa, attributed to the formation of amorphous phase. The trends of yield strength and ultimate strength versus Ce content were similar to hardness.
To enhance the mechanical properties of (CoCrNi)95.8Ce4.2 MEAFs, the as-deposited films were annealed at 250, 350, 450 and 500 °C. The XRD and TEM results revealed the crystallinity increased with the increasing annealing temperatures. Interestingly, plenty of stacking faults were observed at 450 and 500 °C because of the low stacking fault energy and annealing process. The hardness and Young’s modulus increased to 12.60±0.43 GPa and 231.24±4.11 GPa, respectively, due to the fully crystallization and the formation of stacking faults at 450 °C. Hence, both doping small amounts of Ce addition and conducting the post-annealing treatment can strengthen the CoCrNi MEAFs effectively.