Effects of silicon and neodymium additions on microstructures and mechanical properties of CoCrNi medium entropy alloy films

A series of (CoCrNi)100–x–ySixNdy medium entropy alloy films (MEAFs) with manipulated metalloid element, Si, and rare earth element, Nd, was synthesized using magnetron three-target co-sputtering. A transition from single FCC phase to the coexistence of FCC CoCrNi, HCP CoCrNi and HCP NdNi5 phases was observed in XRD and TEM diffraction patterns with the increasing Nd content for Si content of ∼0.7 at.%, and the SEM and TEM images showed the refined columnar grains. The hardness and compressive yield strength reached the maxima of 10.29 and 4.27 GPa, respectively, in Si0.61Nd5.14 film due to the solid solution strengthening, grain refinement and precipitation strengthening. However, The films exhibited a transition from fully crystalline to crystalline within the amorphous matrix in XRD and TEM results with the increasing Si content for Nd content of ∼2.6 at.%, and the columnar structure disappeared in the SEM and TEM images. The hardness and yield strength increased slightly to the maxima of 9.75 and 4.10 GPa, respectively, in Si0.76Nd2.91 film attributed mainly to the interstitial solid solution strengthening. Discussion on (CoCrNi)100–x–ySixNdy MEAFs showed the optimized mechanical properties in Si0.61Nd5.14 film, indicating that precipitation strengthening and grain refinement could be the major strengthening mechanisms in this system.