Abstract
An oxide dispersion strengthened (ODS) ferritic steel with nanometric grain size has been produced by low-energy mechanical alloying (MA) of steel powder (Fe-14Cr-1W-0.4Ti) mixed with Y2O3 particles (0.3 wt %) and successive hot extrusion (HE). The material exhibits superior mechanical properties with respect to the unreinforced steel up to 400 °C; then such differences tend to progressively decrease and at 700 °C yield stress (YS) and ultimate tensile strength (UTS) values are very close. The microstructure and mechanical behaviour have been compared with those of ODS steels prepared by the most common process, high-energy MA, consolidation through hot isostatic pressing (HIP) or hot extrusion (HE), annealing around 1100 °C for 1–2 h. The main strengthening mechanisms have been examined and discussed to explain the different behaviour. In addition, heat treatments in the range 1050–1150 °C were carried out and a microstructural evolution with a relevant hardness decrease has been observed. TEM observations evidenced defect recovery and partial grain coarsening owing to the not perfectly homogeneous distribution of oxide particles.
Highlights
Oxide dispersion strengthened (ODS) ferritic steels are candidate materials for applications in fission and fusion nuclear reactors [1,2,3,4]
Steels, the target is achieved by an uniform dispersion of very fine oxide particles, in general Y2 O3, and by the addition of a small amount of Ti which has the effect to reduce oxide particles to a size of few nanometres; the physical mechanism has been explained by Ukai et al [1]
(produced by Plasma & Ceramic Technologies (PCT) Ltd., Salaspils, Latvia), smaller than 50 nm have been mixed to the steel powder, the mixture has been mechanically alloyed in vacuum (10−4 mbar)
Summary
Oxide dispersion strengthened (ODS) ferritic steels are candidate materials for applications in fission and fusion nuclear reactors [1,2,3,4]. In order to improve the thermal efficiency of reactors, their operational temperature (up to ~800 ◦ C) [5] must be increased good high temperature mechanical properties and creep resistance are strict requirements for structural materials. In the case of ODS steels, the target is achieved by an uniform dispersion of very fine oxide particles, in general Y2 O3 , and by the addition of a small amount of Ti which has the effect to reduce oxide particles to a size of few nanometres; the physical mechanism has been explained by Ukai et al [1]. An optimal solution is the combination of the effects of oxide dispersion strengthening and ultra-fine grains [6,7]. Oxide nanoparticles in the steel matrix act as sinks for point defects improve the resistance
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