In situ martensitic phase reinforced Fe–Nb–Ni–Mn ultrafine composite with enhanced mechanical properties

► Fe–Nb–Ni–Mn alloy with martensite dendrite embedded in ultrafine eutectic composite. ► Ultrafine eutectic matrix consisting of α′-Fe and Fe 2Nb phases, together retained γ-Fe. ► Strain-induced phase transformation of retained γ-Fe to α′-Fe martensite in eutectic region. An in situ formed martensitic dendrite reinforced Fe–11Nb–10Ni–5Mn ultrafine composite was developed by copper mold casting. This new class of ultrafine composite composed of micrometer-scale martensitic α′-Fe dendrites and ultrafine-scale alternating eutectics exhibits a high fracture strength of 1.6 GPa and a large compressive plasticity up to 10.5%. These excellent mechanical properties originate from the precipitation of the martensitic α′-Fe dendrites containing numerous nanometer-scale laths which are effective to generate multiple shear banding and restrict the rapid propagation of localized shear bands.