Structure and Properties of Nano-Scale Oxide-Dispersed Iron

Bulk samples of pure iron and yttria dispersed iron with and without titanium (i.e., Fe, Fe-Y₂O₃, and Fe-Y₂O₃-Ti) were prepared by hot extrusion of high-energy ball-milled powders. An examination of the microstructure using TEM revealed that the addition of titanium resulted in the reduction of the dispersoid size with a concomitant increase in the volume fraction of the dispersoids. As a result, Fe-Y₂O₃-Ti exhibited a substantial increase in hardness and tensile properties as compared to Fe and Fe-Y₂O₃. The higher hardness and strength of Fe-Y₂O₃-Ti is shown to be due to the presence of finer and higher number density of Y-Ti-O complex oxides. Dynamic strain aging in the temperature range of 423 K to 573 K (150 °C to 300 °C) was observed in all the compositions studied.     

Role of Microstructure and Temperature on the Tensile Fracture Behavior of Oxide Dispersion Strengthened 18Cr Ferritic Steel

The tensile fracture behavior of oxide dispersion strengthened 18Cr (ODS-18Cr) ferritic steels milled for varying times was studied along with the oxide-free 18Cr steel (NODS) at 25, 200, 400, 600, and 800 °C. At all the test temperatures, the strengths of ODS–18Cr steels increased and total elongation decreased with the duration of milling time. Oxide dispersed 18Cr steel with optimum milling exhibited enhanced yield strength of 156 pct at room temperature and 300 pct at 800 °C when compared to oxide-free 18Cr steel. The ductility values of ODS-18Cr steels are in the range 20 to 35 pct for a temperature range 25 to 800 °C, whereas NODS alloy exhibited higher ductility of 37 to 82 pct. The enhanced strength of ODS steels when compared to oxide-free steel is due to the development of ultrafine grained structure along with nanosized dispersion of complex oxide particles. While the pre-necking elongation decreased with increasing temperature and milling time, post-necking elongation showed no change with the test temperature. Fractographic examination of both ODS and NODS 18Cr steel fractured tensile samples, revealed that the failure was in ductile fracture mode with distinct neck and shear lip formation for all milling times and at all test temperatures. The fracture mechanism is in general followed the sequence; microvoid nucleation at second phase particles, void growth and coalescence. The quantified dimple sizes and numbers per unit area were found to be in linear relation with the size and number density of dispersoids. It is clearly evident that even nanosized dispersoids acted as sites for microvoid nucleation at larger strains and assisted in dimple rupture.

     

Strengthening Mechanisms in Mechanically Milled Oxide-Dispersed Iron Powders

Nanocrystalline iron and oxide dispersion-strengthened (ODS) iron powders (Fe, Fe-Y₂O₃, and Fe-Y₂O₃-Ti) were prepared by mechanical milling for periods ranging from 1 to 40 hours. The as-milled powders were examined for changes in their particle sizes, crystallite sizes, hardness values, and phases present as a function of milling time. Both the particle and the crystallite sizes of all the three compositions decreased with milling time, while the hardness values of all the three powders increased with milling time because of the crystallite size refinement. At the same crystallite size, the hardness values of Fe-Y₂O₃ and Fe-Y₂O₃-Ti powders were higher than that of the Fe powders. Though, the presence of 40 nm Y₂O₃ could be established for 2-hour milling, such particles were not resolvable in 40-hour-milled powders. However, SAD patterns confirmed the presence of complex oxide dispersoids in the Fe-Y₂O₃ and Fe-Y₂O₃-Ti powders. The variation of hardness value with the crystallite size and as a function of the milling time can be rationalized on the basis of Hall–Petch crystallite size strengthening in combination with dispersion strengthening (in Fe-Y₂O₃- and Fe-Y₂O₃-Ti-milled powders) due to dispersoids. The observed double-positive slopes in the Hall–Petch relationship can be explained in terms of an increase in misorientation angle between the crystallites with increasing milling time due to the crystallite rotation driven by disclination dipoles.