Bimodally grained high-strength Fe fabricated by mechanical alloying and spark plasma sintering

Nanocrystalline iron containing a certain fraction of coarse grains with nanosized oxide dispersoids has been processed by mechanically milling Fe powder and subsequent spark plasma sintering. Sintered samples exhibited a high tensile strength of 2100 MPa with 5% ductility; by optimizing the sintering conditions, it was possible to tune the strength–ductility balance. The optimally sintered material showed a tensile strength of 1500 MPa and 15% elongation. The microstructure consists of nanograined (<100 nm) as well as coarse-grained regions (>1 μm) with uniform dispersion of nanosized chromium oxide particles (∼10 nm). The strength and elongation show strong dependence on the volume fraction of the coarse grains, and the high strength can be attributed to the ultrafine grain size of the nanograined regions and precipitation hardening by the oxide dispersoids. The ductility is considered to be due to the presence of coarse grains.