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

Element addition is an effective way to improve the properties of alloys. In this work, (CoCrNi)100-xYx (x = 0, 0.1, 0.3, 0.7 and 1.0, named Y-0, Y-0.1, Y-0.3, Y-0.7 and Y-1.0 accordingly) medium entropy alloys (MEAs) were fabricated by arc melting, and the effects of yttrium addition on the microstructures and mechanical properties of CoCrNi MEAs were studied. For Y-0 and Y-0.1, the alloys remained an FCC solid solution. Due to the most negative mixing enthalpy of Y-Ni element pair in the alloy, the HCP YNi5 precipitates formed in Y-0.3, and the volume fraction of precipitates increased with the increasing Y content. Both Y in the solid solution matrix and YNi5 precipitates resulted in grain refinement, and the grain size decreased with the increasing Y content. Annealing twins were observed; however, the twin boundary fraction showed only slight increase with the increasing Y content (from 69.2% for Y-0 to 73.1% for Y-1.0). The nanoindentation tests showed that the hardness of the HCP precipitates was about two times of the FCC matrix. Compared to Y-0, Y-0.1 showed only slight improvements in microhardness and tensile strength. However, in the presence of precipitates, the microhardness, yield strength and tensile strength showed significant enhancements with the increasing Y content in the alloy. Both the microhardness and strength scaled with the volume fraction of precipitates in the alloy. While precipitation strengthening had the major contribution, both the solid solution strengthening and grain boundary strengthening had only the minor contribution to our system.