Effects of La addition on the microstructure and mechanical properties of CoCrNi medium entropy alloy

• Effects of La addition on structures and properties of CoCrNi MEA are studied. • The structure remains FCC solid solution for low La contents (≤ 0.24 at%). • LaNi 5 hexagonal precipitates form in FCC matrix for higher La contents (≥ 0.32 at%). • Precipitates result in La depletion in matrix and reduced lattice distortion/grain refinement. • Optimum mechanical properties obtained for 0.24 at% La due to solid solution/grain refinement strengthening. Ternary CoCrNi medium entropy alloy (MEA) with a face-centered cubic (FCC) structure has been proven to possess superior mechanical properties in many studies. Alloying element addition is a common method to strengthen the alloys; however, the effects of rare earth element addition have rarely been discussed. In this study, different amounts of La were added in CoCrNi MEAs to investigate the microstructure and mechanical properties. (CoCrNi) 100- x La x ( x = 0, 0.1, 0.2, 0.3 and 0.5) MEAs were fabricated using arc-melting, followed by homogenization, hot rolling and recrystallization. While the structures of La-0, La-0.1 and La-0.2 were FCC solid solutions, LaNi 5 precipitates of hexagonal crystal structure formed in the FCC matrix for La-0.3 and La-0.5. The presence of La in FCC matrix not only enhanced the lattice distortion because the atomic radius of La was larger than the other elements in the alloys, but also refined the grain size of the matrix. However, in the presence of LaNi 5 precipitates, the La content was reduced in the matrix and the extents of both lattice distortion and grain refinement diminished which, in turn, resulted in the decreased hardness and tensile strength in La-0.3 and La-0.5. The optimum mechanical properties were obtained with small amounts of La addition in La-0.2 because of both solid solution strengthening and grain refinement strengthening.