Exploring effects of molybdenum and titanium contents on microstructure and mechanical properties of CoCrNi medium entropy alloy films

Abstract

Medium entropy alloys (MEAs), such as CoCrNi, have demonstrated a unique combination of high hardness and excellent ductility, surpassing many high entropy alloys reported to date. In this study, we investigated the effects of Mo and Ti additions on the microstructures and mechanical properties of (CoCrNi)_100–x–yMo_xTi_y MEA films (MEAFs). Mo/Ti-doped CoCrNi MEAFs were deposited on Si substrates using magnetron three-target co-sputtering. While the power applied on CoCrNi target was fixed at 200 W, the same power was applied on Mo and Ti targets which varied from 0 to 90 W. For simplicity, the film was denoted as MoTi80 when Mo and Ti targets were subjected to 80 W power, and so on. X-ray diffraction and TEM SAED results revealed a structural transformation from FCC to FCC + HCP at MoTi50 (x = 3.52, y = 1.36) due to HCP Ni₃Ti precipitation. With further increases in Mo and Ti contents, MoTi80 (x = 6.48, y = 2.52) exhibited a mixed nanocrystalline and amorphous structure, while MoTi90 (x = 8.98, y = 3.59) became fully amorphous. The mechanical properties of the films were investigated using nanoindentation and micropillar compression tests. With the additions of Mo and Ti, the maximum yield strength of 5.87 GPa and hardness of 11.96 GPa were obtained at MoTi70 (x = 6.27, y = 2.44) and then decreased with the higher Mo and Ti contents. The addition of small amounts of Mo and Ti strengthened CoCrNi MEAFs mainly due to solid solution strengthening and grain boundary strengthening.