Lamellar microstructure enables exceptional fatigue resistance in a medium-entropy alloy manufactured by integrated directed energy deposition with interlayer rolling

Directed energy deposition (DED) offers higher manufacturing efficiency and material utilization, making it suitable for producing large-sized structural components. However, due to columnar coarse grains and manufactured defects, how to remarkably elevate the fatigue resistance of DED-fabricated face-centered cubic (FCC) materials is an important yet technically challenging issue. To address the challenge, this study employed a medium-entropy FCC alloy, (CoCrNi)₉₄Al₃Ti₃, as the base material and adopted a processing strategy that integrates interlayer rolling into DED to controllably introduce lamellar structure, an effective fatigue-resistant microstructure. Through process optimization, the sample with 3-time inter-layer rolling (DED-R3) exhibits a significantly enhanced fatigue resistance and fatigue ratio along the rolling direction (RD), higher than DED sample by 60 % and 48 %, respectively. The lamellar heterostructure introduced by inter-layer rolling consists of alternating coarse and fine grains, with coarse grains accounting for 66.7 % and fine grains for 33.3 %. This lamellar heterostructure resulted from high geometrically necessary dislocation density induced by cold rolling and critical recrystallization temperature through cyclic heating, facilitating columnar-to-equiaxed transition at local positions. The high fatigue resistance of DED-R3 samples was attributed to the simultaneous achievement of cyclic stability and resistance to crack propagation from lamellar heterostructure. On the one hand, quasi-in-situ fatigue experiments were conducted to reveal enhanced crack initiation mechanisms: different from intense plastic strain localization induced grain boundary (GB) or slip band (SB) cracks in DED samples, most cracks in DED-R3 samples initiated from the interaction between SBs and defects. The mitigated surface roughening by lamellar microstructure suppressed the risk of microstructure cracking. On the other hand, the macroscopic deflection induced by the heterostructure interface and the high-frequency deflection by dense GBs collectively reduced the crack propagation rate.