Role of Cr and Ti contents on the microstructure and mechanical properties of ODS ferritic steels

Six oxide dispersion strengthened (ODS) ferritic steels, with the composition of Fe-(12-14)Cr-2W-(0.1-0.3-0.5)Ti-0.3Y(2)O(3) (wt.%), have been prepared by mechanically alloying elemental powders of Fe, Cr, W, and Ti with Y2O3 nano-particles followed by hot isostatic pressing. The influence of the chemical composition on the microstructure and mechanical properties of various materials was studied. It was found that the chromium content has a significant influence on the microstructure and mechanical properties of the compacted ingots. The 14Cr ODS steel exhibits slightly higher ultimate tensile strength and yield strength values than the 12Cr ODS steel. The total elongation and uniform elongation of both materials, in general, decrease with raising the test temperature, although in the case of the 12Cr ODS steel the elongation is about 30% higher than that of the 14Cr ODS material. In what concerns the effect of titanium content it can be concluded that variations between 0.1 and 0.3% have no visible effects on the microstructure and Charpy impact properties of compacted specimens. However, the microstructure of specimens with 0.5%Ti contains large TiO2 particles with a size in the range of 50-500nm, which have detrimental influence on the mechanical properties of that material

Reduced activation ODS ferritic steel - recent development in high speed hot extrusion processing

The paper presents the microstructure and mechanical properties of an oxide dispersion strengthened (ODS), reduced activation, ferritic steel, namely the Fe-14Cr-2W-0.3Ti-0.3Y(2)O(3) alloy, which was fabricated by hot isostatic pressing followed by high speed hydrostatic extrusion (HSHE) and heat treatment HT at 1050 degrees C. Transmission electron microscopy (TEM) observations revealed significant differences in the grain size and dislocation density between the as-HIPped and as-HSHE materials. It was also found that the microstructure of the steel is stable after HT. The HSHE process improves significantly the tensile and Charpy impact properties of the as- HIPped steel. The ultimate tensile strength at room temperature increases from 950 up to 1350 MPa, while the upper shelf energy increases from 3.0 up to 6.0J. However, the ductile-to-brittle transition temperature (DBTT) remains relatively high (about 75 degrees C). These results indicate that HSHE is a promising method for achieving grain refinement and thus improving the mechanical properties of ODS ferritic steels. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Effect of vanadium addition on the microstructure and mechanical properties of the ODS ferritic steels

In this work, the effects of vanadium addition in the range of 0.3-3% (in weight percent) for an oxide dispersion strengthened reduced activation ferritic (ODS RAF) steel were investigated. Samples of the V-modified steel have been prepared using elemental (Fe, Cr, W, Ti) and Y2O3 powders with the nominal composition of Fe-14Cr-2W-0.3Ti-0.3Y(2)O(3). Consolidated and heat treated samples were investigated using Scanning Electron Microscopy and Scanning Transmission Electron Microscopy equipped with Electron Energy Loss Spectroscopy detector. Hardness and Charpy impact tests (KLST specimens) were also performed. The microstructure investigations revealed numerous particles of the size up to 0.5 mu m. They are primarily Ti-Cr-V oxides located at the grain boundaries and inside the grains. These particles increase hardness and significantly reduce fracture resistance of the ODS RAF alloys developed here. However, it should be noted that the 0.3% V-ODS steel has unexpectedly the lowest transition temperature of about 282 K and that the 1-3% V-ODS steels, in spite of the transition temperature about 373 K, exhibit almost two times higher the lower shelf energy values in comparison with the 0.3% V-ODS and 0% V-ODS steels. (C) 2012 Elsevier B. V. All rights reserved

Processing and characterization of a W–2Y material for fusion power reactors

A W-2Y material has been produced by powder metallurgy techniques including mechanical alloying of W and Y elemental powders in an argon atmosphere, followed by hot isostatic pressing of the milled powder at 1320 degrees C under a pressure of 200 MPa for 2 h. It was found that the mechanical alloying time should not exceed 40 h in order to achieve a homogeneous distribution of small powder particles and to limit air contamination and carbon/WC contamination by the jar and ball materials. The density of the ingots was found to be about 97% the theoretical one. It was observed that the microstructure of the compacted material is composed of grains having a bimodal size distribution, with mean sizes around 50 and 150 nm. In addition, the material contains an inhomogeneous distribution of oxide particles with a mean size ranging from 2 to 20 nm. In situ TEM chemical analyses revealed that the entire content of yttrium reacted with oxygen to form nanometric oxides whose composition corresponds to Y2O3. Charpy impact tests revealed that the material is brittle at the high temperature of about 1000 degrees C. Tensile tests confirmed that the material is brittle at 1000 degrees C but ductile at 1300 degrees C, indicating that the ductile-to-brittle transition temperature should lie between 1100 and 1200 degrees C. (C) 2011 EURATOM associated institution EPFL CRPP. Published by Elsevier B.V. All rights reserved

The formation and evolution of oxide particles in oxide-dispersion-strengthened ferritic steels during processing

The fabrication of oxide-dispersion-strengthened (ODS) steels is a multi-stage process involving powder ball milling, degassing and consolidation by hot isostatic pressing. Y is introduced by mechanical alloying (MA) with either Y2O3 or Fe2Y so a high density of Y-Ti-O-based oxide nanoparticles is formed. The evolution of ∼2 nm oxide nanoparticles and larger >5 nm grain boundary oxides has been studied at each step of the processing. The nanoparticle dispersions produced in material MA with Fe2Y were comparable to those produced by alloying with Y 2O3. Hence the majority of oxygen which forms the nanoparticles must be incorporated from the atmosphere during MA, rather than being introduced via the alloying additions. During mechanical alloying, a high density of subnanometer particles are formed (2.5 ± 0.5 × 10 24 m-3). The oxygen content of the nanoparticles decreases slightly on annealing, and then the composition of the nanoparticles remains constant throughout subsequent processing stages. The nanoparticle size increases during processing up to ∼2 nm radius, while the number density decreases to 4 ± 0.5 × 1023 m-3. Grain boundary oxides (>5 nm) have a Ti-Cr-O-rich shell, and contain no Y before consolidation, but have similar core composition to the matrix nanoparticles after consolidation. The formation of the larger grain boundary oxides is shown to take place during the degassing and consolidation stages, and this occurs at the expense of the nanoparticles in the matrix. This work provides a mechanistic understanding of the importance of controlling the oxygen content in the powder during MA, and the resulting impact on the formation of the ODS microstructure. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Enhanced precipitate growth at reduced temperatures during chemical ordering in deformed red gold alloys

Manufacturing of red gold components relies widely on controlling the formation of ordered precipitates during thermo-mechanical processing. The ordered phase hardens the material, makes it less workable and introduces residual stresses. We show that an industrial wire straightening process on chemically disordered wires enhances the formation of ordered precipitates in the near surface region during natural ageing. To address the role of plastic deformation during artificial ageing, in situ X-ray experiments are carried out on deformed and undeformed samples. It is shown that pre-deformation decreases the onset temperature for ordering and enhances precipitation growth at the early stages of the transformation. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved