Exceptional thermal stability and the strengthening mechanism of ultrafine-grained Fe_xCrNiAl medium-entropy alloys synthesized via powder metallurgy

Lightweight Fe_xCrNiAl (x = 1.0, 0.5, 0.25) medium-entropy alloys (MEAs) were synthesized via powder metallurgy method combining mechanical alloying (MA) and spark plasma sintering (SPS). This study systematically investigated microstructural evolution, mechanical performance, and strengthening mechanisms. The sintered alloys exhibited ultrafine-grained microstructures with a mean grain size ≤ 0.54 μm, comprising Fe-Cr-enriched BCC and Ni-Al-dominated B2 (ordered BCC) phases, alongside minor oxides (8.05–15.33 vol%). The reduction of Fe content leads to the decreasing BCC while increasing B2 and oxides. The alloys demonstrated exceptional strength-ductility synergy, Fe_0.25 achieved a yield strength of 2092 ± 114 MPa, representing a 67 % improvement over the arc-melted alloy and just a 15 % decrease in fracture strain. The alloys exhibited excellent phase stability after 1000 °C/100 h annealing, showing only 8.6 % hardness reduction and retaining the Hall-Petch relationship. Quantitative analysis revealed that grain refinement and precipitation particles were the main strengthening mechanisms, respectively occupying 28 % and 35 % for the yield strength of Fe_0.25, collectively contributing over 60 % of the strength enhancement.