Microstructure, texture and mechanical properties of the 14YWT nanostructured ferritic alloy NFA-1

The 14YWT FCRD NFA-1 is a nanostructured variant of ODS ferritic steels. It is processed by ball milling FeO and argon atomized Fe-14Cr-3W-0.4Ti-0.2Y (wt%) powders, followed by hot extrusion, annealing and cross-rolling to produce ≈10 mm thick plates. The plate contains a bimodal distribution of highly textured, pancake-shaped, generally submicron, grains. NFA-1 also contains a large population of microcracks lying in planes normal to the plate thickness direction. The microcracks form on {001} planes and propagate in<110> directions along low angle subgrain boundaries formed during high-temperature deformation. Tensile tests in directions parallel to the extrusion (L) or cross-rolling (T) manifest high strength and good ductility over a wide range of temperatures. In contrast, loading in the short plate thickness (S) direction, perpendicular to the microcrack faces, manifests a much lower strength, and almost zero ductility, with flat, faceted cleavage fracture surfaces up to ≈100 °C. However, tensile ductility in the S orientation increases at higher temperatures above with a brittle-to-ductile transition (BDT). The L, T and S properties are reasonably similar (isotropic) above ≈200 °C. At lower temperatures, deformation in both tensile and fracture toughness tests is accompanied by extensive delamination due to propagation of the microcracks. Delamination has relatively modest effects on tensile properties, but actually improves fracture toughness, either by relaxing triaxial stress in thin delaminated ligaments near the tip, or crack deflection, depending on the specimen orientation. Elastic-plastic toughness (K_Jc) of NFA-1 undergoes a cleavage BDT at ≈−175 °C, with stable crack tearing initiation just beyond general yielding at higher temperatures.