Enhanced strength and ductility by optimizing B2-NiAl precipitation shape in ferritic alloys

B2-NiAl strengthened ferritic alloys show promising potential for high-temperature applications, but their room-temperature brittleness under high precipitate volume fraction remains a major limitation. To overcome this shortcoming, a ferritic alloy with two variants was produced by rationally designing the austenite phase region and altering the sequence of solution treatment and hot rolling. The main microstructural difference between the two variants lies in the morphology of B2 phase, while other characteristics remain largely consistent. Mechanical tests demonstrated that the alloy with 30.4 vol% of spherical B2 precipitates exhibits higher yield strength (1104 ± 46 MPa vs. 1058 ± 16 MPa), tensile strength (1725 ± 35 MPa vs. 1597 ± 16 MPa), and uniform elongation (6.5 ± 0.2% vs. 4.8 ± 0.3%) compared to the variant containing rod-shaped precipitates. Further crystal plasticity finite element modeling confirmed that the high symmetry and isotropy of the spherical B2 precipitates promote stress delocalization and higher strain gradients, thereby enhancing back-stress strengthening and work hardening capacity. These findings align with observations of deformed microstructures, which revealed higher dislocation density and uniformly distributed nanopores in the alloy with spherical B2 precipitates. Calculations of the theoretical yield strength indicated that the strength improvement primarily stems from the stronger back-stress strengthening effect induced by the spherical B2 phase. • Rod-shaped and spherical B2-NiAl precipitates were introduced in ferrite comparably. • Spherical B2 precipitates enhance the strength and uniform elongation simultaneously. • High-symmetry spherical B2 precipitates promote stress delocalization.