Development of ultra high strength nano-Y2O3 dispersed ferritic steel by mechanical alloying and hot isostatic pressing

The present investigation aims to develop ultra high strength ferritic steels through consolidation of mechanically alloyed powders of 1.0 wt% nano-Y2O3 dispersed 83.0Fe–13.5Cr–2.0Al–0.5Ti (alloy A), 79.0Fe–17.5Cr–2.0Al–0.5Ti (alloy B), 75.0Fe–21.5Cr–2.0Al–0.5Ti (alloy C) and 71.0Fe–25.5Cr–2.0Al–0.5Ti (alloy D) alloys (all in wt%) by hot isostatic pressing (HIP) at 600, 800 and 1000 °C using 1.2 GPa pressure for 1 h. Following this mechano-chemical synthesis and consolidation, extensive effort has been undertaken to characterize the microstructural evolution by X-ray diffraction, scanning and transmission electron microscopy and energy dispersive spectroscopy. Mechanical properties including hardness, compressive strength, Youngʹs modulus and fracture toughness were determined using nano-indentation and universal testing machine. The present ferritic alloys record extraordinary levels of compressive strength (2012–3325 MPa), Youngʹs modulus (230–295 GPa), fracture toughness (4.6–21.8 MPa √m) and hardness (15.5–19.7 GPa), and measure up to 2–3 times greater strength with a lower density (∼7.4 Mg/m3) than that of other oxide dispersion strengthened ferritic steels (<1200 MPa) or tungsten based alloys (<2200 MPa). The novelty of these alloys lies in the unique microstructure comprising uniform dispersion of 20–30 nm Y2O3 (ex situ) or Y2Ti2O7 (in situ) particles in higher volume fraction in high-Cr ferritic matrix.