Microstructure and mechanical properties of nano-Y2O3 dispersed ferritic alloys synthesized by mechanical alloying and consolidated by hydrostatic extrusion

Abstract

The present study reports synthesis of 1.0wt% nano-Y2O3 dispersed high strength ferritic alloys with nominal compositions of 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) (all in wt%) by mechanical alloying using planetary ball mill followed by consolidation of alloyed powders by hydrostatic extrusion at 1000°C and 550MPa pressure with a strain rate ~10−1s−1. The products of mechanical alloying and extrusion have been characterized by X-ray diffraction, scanning and transmission electron microscopy, energy dispersive spectroscopy and image analysis. Mechanical properties in terms of hardness, compressive strength, yield strength and Young's modulus have been determined using nano-indenter and universal testing machine. The present ferritic alloys record significantly high levels of compressive strength (850–2226MPa) and yield strength (525–1505MPa), Young's modulus (240–265GPa) and hardness (14.7–17.8GPa) with an impressive true level of strain (5.0–22.5%). These superior mechanical properties measure about 1.5 times greater, albeit with a lower density (~7.4Mg/m3) than that of standard oxide dispersion strengthened ferritic alloys (<1200MPa). Furthermore, the extent of plastic strain before failure in the present routine surpasses all previous attempts of identical synthesis but different consolidation routes for the same set of ferritic alloys. In general strength is higher along transverse than longitudinal direction of extrusion. Thus, it is concluded that uniform dispersion of nanometric (10–20nm) Y2O3 (ex-situ) or Y2Ti2O7 (in-situ) in high volume fraction along boundaries and within the grains of high-Cr ferritic matrix is responsible for this unique combination of high strength and ductility in the present alloys developed by powder metallurgy route.