Strengthening Effect of Multiscale Second Phases in Reduced Activation Ferrite/Martensitic Steel

Abstract

Owing to their excellent resistance to irradiation, good creep property, and extraordinary structural and chemical stability in harsh, high-temperature environments, oxide dispersionstrengthened (ODS) reduced activation ferritic martensitic (RAFM) steels are considered prime candidates for advanced structures, including nuclear fusion reactors. [1,2] The excellent properties of ODS RAFM steel can be attributed to the fine grain size of the steel and the dispersed nano strengthening phases, such as Y 2 O 3 , MX (M ¼ Ta, V, X ¼ C, N), and M 23 C 6 (M ¼ Fe, Cr). [3,4] Dispersed Y 2 O 3 particles can not only effectively block dislocation movements but also absorb the vacancies produced by irradiation and the helium generated by transmutation in the nuclear fusion reactor, thereby improving the radiation resistance of the alloy. [5] Y 2 O 3 particles could be refined by the addition of Ti and Zr to the Fe matrix to form Y-Ti-O and Y-Zr-O clusters. [6,7] Ti and Zr elements could also form TiC, (Ti, W) C, (Ti, Ta) C, and V 3 Zr 3 C, which have strengthening effects similar to that of MX with Ta/V, to improve the high-temperature mechanical properties of steel alloys. [8,9] Ti and Zr are important alloying elements often used to prepare ODS steel. ODS RAFM alloys are commonly manufactured via powder metallurgy (PM), which involves mechanical alloying, hot powder consolidation (e.g., hot isostatic pressure, hot extrusion, or sparking-plasma sintering), and heat treatment. However, PM presents several inherent defects for the preparation of ODS alloys; for example, the technique could introduce impurities (e.g., O 2 ) to its complex processes, thereby inducing anisotropy in the microstructure and mechanical properties of the resulting alloys. [10] The technology also requires large amounts of energy and, at present, cannot be scaled up to industrial production.A demonstration nuclear fusion reactor requires over 11 000 tons of ODS RAFM steel as its structural material, [11] but PM technology for fabricating large-scale ODS steels has not yet been developed. This shortcoming is a basic defect of ODS RAFM steel production. Research on alternative preparation technologies for ODS steel is ongoing globally.Shi and Han directly added micron Y 2 O 3 powders to molten Fe alloy to prepare ODS steel by vacuum casting, forging, and heat treatment. [12] Analysis of the resulting alloys revealed complicated oxides containing Cr, Ti, Ta, Mn, V, Y, and O, which could prevent dislocations from gliding to improve the yield strength of the steel. Zhan et al. prepared 9Cr-ODS alloys by adding Y and Ti alloys to 9Cr steel through vacuum induction melting (VIM). [13] A large number of Y-Ti-O particles smaller than 0.5 μm were found in the steel, and the density of these particles reached 2.69 Â 10 19 m À3 ; this value is close to the density reported for 9Cr-ODS steel prepared by chemical reduction and mechanical milling (i.e., 5.7-8.1 Â 10 20 m À3 ). Oxygen carrier casting technology has been adopted to prepare ODS RAFM alloys by introducin...

Microstructures and Mechanical Properties of Alumina and/or Yttrium Oxides Strengthened 9Cr Steel Fabricated by Vacuum Casting

“…[16] More recently, the casting (or liquid metal) route, which simply adds fine oxide powder directly to liquid steel, is suggested to be a promising approach to producing low-cost ODS steels with larger volumes and higher throughput. [17] Using the casting route, many researchers mainly focused on the investigation of Y-containing oxides for ODS F/M steels, such as Y 2 O 3 , [18,19] Y-Ti-O, [20][21][22][23] and Y-Zr-O [23][24][25][26] To the authors' knowledge, other types of oxides are rarely introduced into the ODS F/M steels fabricated through the casting route.Al is generally used for the deoxidation of steel in the smelting process and Al 2 O 3 inclusions inevitably appear in the steel after solidification. Furthermore, Al 2 O 3 , as an oxide dispersion strengthened phase, has been successfully introduced in carbon structural steel [27] and martensite stainless steel [28] by casting route.…”

“…[16] More recently, the casting (or liquid metal) route, which simply adds fine oxide powder directly to liquid steel, is suggested to be a promising approach to producing low-cost ODS steels with larger volumes and higher throughput. [17] Using the casting route, many researchers mainly focused on the investigation of Y-containing oxides for ODS F/M steels, such as Y 2 O 3 , [18,19] Y-Ti-O, [20][21][22][23] and Y-Zr-O [23][24][25][26] To the authors' knowledge, other types of oxides are rarely introduced into the ODS F/M steels fabricated through the casting route.…”