Thermally tailored cellular microstructure enhances ductility in additively manufactured maraging steel via austenite network reconstruction

Overcoming the strength-ductility trade-off remains a challenge for both traditional and emerging high-strength steels. The unique thermal history during additive manufacturing (AM) of metals and alloys includes cellular solute enrichment of alloying elements, enabling the formation of metastable austenite with a cellular morphology after intercritical (α + γ) annealing. Intercritical heat treatment reconstructs the trace retained austenite in the as-printed microstructure, together with reverted austenite from the martensitic matrix, into an interconnected cellular austenite network (~40 vol.%). Yet solute-gradient partitioning can over-stabilize this cellular austenite, suppressing transformation-induced plasticity (TRIP) and work hardening under room-temperature tensile straining and making its stability difficult to tune using conventional heat-treatment schedules. Here, we effectively tune the thermal stability of cellular austenite in laser powder bed fusion (L-PBF) 18Ni300 maraging steel through a multi-step intercritical annealing strategy, so that TRIP can be readily activated during room-temperature tensile loading. Serial ex-situ electron backscatter diffraction (EBSD)/X-ray diffraction (XRD) measurements at comparable locations indicated a substantially larger austenite-to-martensite (γ → α’) transformation (41.5% → 22.3%) as compared to the conventional single-step annealed counterpart (37.7% → 28.1%), consistent with an earlier TRIP onset. Consequently, the optimized condition exhibits significantly enhanced ductility and an extended uniform work-hardening regime (over 50% improvement), attributable to the reduced stability of reverted austenite, which facilitates earlier and more extensive strain-induced γ → α’ transformation. Finally, we propose a microstructure design concept of “cellular structure - retained austenite - cellular austenite network” for strength - ductility synergy, providing a transferable paradigm for strengthening and toughening of other AM metals.