Effect of rare-earth element Y addition on microstructure and mechanical properties of CrFeNi2 medium entropy alloy

In present work, a novel Co-free (CrFeNi2)100-xYx (x = 0, 3, 6, 9) medium entropy alloys (MEAs) were successfully prepared by vacuum arc-melting method. The results revealed that the addition of rare-earth element Y had a great effect on the phase composition of CrFeNi2 MEA, which was transformed from the single FCC phase structure to the duplex structure of the FCC and HCP (YNi5 type) phases. Meanwhile, the phase evolution of the current alloy system was evaluated using the previous criteria (dHmix-S, D-S, Delta chi, VEC and A). In addition, the volume fraction of the HCP phase increased significantly with the increasing Y content, which caused the obvious grain refinement. On the other hand, the nanoindentation test results revealed that the nanohardness of the HCP phase (-10.7 GPa) was more than three times of the FCC phase (-3.4 GPa). The dislocation nucleation types of the FCC phase (3.68 GPa) and the HCP phase (6.46 GPa) were discussed by calculating their maximum initial plastic shear stress. With the increasing Y content, both the hardness and strength of the alloy were significantly improved, but the plasticity deteriorated. The second phase strengthening played a dominant role in the strength improvement of (CrFeNi2)100-xYx MEAs, while both the solid solution strengthening and grain boundary strengthening contributed slightly to the strength increment.