La−modified CrFeNi2Lax (x = 0, 0.05, 0.15, 0.25) medium−entropy alloys were investigated to reveal La's effects on phase evolution and mechanical properties. Thermodynamic and experimental analyses demonstrated that La addition transformed the single FCC phase into dual FCC + HCP phases, with HCP volume fraction increasing alongside La content. Grain refinement occurred from 412 to 245 μm with higher La concentrations. Mechanical properties improved significantly: hardness rose from 125 to 325 HV, yield strength surged from 110 to 839 MPa, and optimal fracture strength (1317 MPa) was achieved at 3.6 at.% La. These enhancements stemmed from combined effects of HCP phase formation, grain refinement, and La−induced strengthening mechanisms.
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