Development and characterisation of a new ODS ferritic steel for fusion reactor application

This paper describes the microstructure, tensile properties and Charpy impact resistance of a reduced activation oxide dispersion strengthened ferritic steel Fe-14Cr-2W-0.3Ti-0.3Y(2)O(3) produced by mechanical alloying of a pre-alloyed, gas atomised steel powder with Y2O3 particles, compaction by hot extrusion at 1100 degrees C, hot rolling at 700 degrees C and heat treatment at 1050 degrees C for 1 h. At room temperature the material exhibits a high ultimate tensile strength of about 1420 MPa and high yield strength of about 1340 MPa in the transverse direction. In the longitudinal direction the values are about 10% lower, due to the anisotropy of the microstructure (elongated grains in the rolling direction). At 750 degrees C the material still exhibits relatively high yield strengths of about 325 MPa and 305 MPa in the longitudinal and transverse directions, respectively. The material exhibits reasonable uniform and total elongation values over the temperature range 23-750 degrees C, in both transverse and longitudinal directions. The material exhibits weak Charpy impact properties in the transverse direction. Charpy impact properties are slightly better in the longitudinal direction, with upper shelf energy of about 4.2 J and a ductile-to-brittle transition temperature of about 8.8 degrees C. (C) 2009 Elsevier B.V. All rights reserved.

Influence of hot rolling and high speed hydrostatic extrusion on the microstructure and mechanical properties of an ODS RAF steel

Nadine Baluc, Zbigniew Oksiuta

An argon gas atomized, pre-alloyed Fe-14Cr-2W-0.3Ti (wt.%) reduced activation ferritic (RAF) steel powder was mechanically alloyed with 0.3wt.% Y2O3 nano-particles in an attritor ball mill and consolidated by hot isostatic pressing at 1150 °C under a pressure of 200 MPa for 3 h. In the aim to improve its mechanical properties the ODS steel was then submitted to a thermo-mechanical treatment (TMT): hot rolling (HR) at 850 °C or high speed hydrostatic extrusion (HSHE) at 900 °C, followed by heat treatment (HT). Transmission electron microscopy (TEM) observations of the ODS alloys after TMT and heat treatment revealed the presence of elongated grains in the longitudinal direction, with an average width of 8 μm and an average length of 75 μm, and equiaxed grains, a few microns in diameter, in the transverse direction. Two populations of oxide particles were observed by TEM: large Ti-Al-O particles, up to 250 nm in diameter, usually located at the grain boundaries and small Y-Ti-O nanoclusters, about 2.5 nm in diameter, uniformly distributed in the matrix. Charpy impact tests revealed that the HSHE material exhibits a larger upper shelf energy (5.8 J) than the HR material (2.9 J). The ductile-to-brittle transition temperature of both alloys is relatively high, in the range of 55-72 °C. Tensile mechanical properties of both ODS alloys were found satisfactory over the full range of investigated temperatures (23-750 °C). The HSHE material exhibits better tensile strength and ductility than the HR material. These results indicate that HSHE can be considered as a promising TMT method for improving the mechanical properties of ODS RAF steels. © 2010 Elsevier B.V. All rights reserved.

Elsevier2011

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

Nadine Baluc, Robin Schäublin, Lyubomira Veleva

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.

Elsevier2011

Microstructure and mechanical properties of an ODS RAF steel fabricated by hot extrusion or hot isostatic pressing

Nadine Baluc, Zbigniew Oksiuta

Ingots of an oxide dispersion strengthened reduced activation ferritic steel with the Fe-14Cr-2W-0.3Ti-0.3Y(2)O(3) chemical composition (in wt.%) were synthesized by mechanical alloying of elemental powders with 0.3 wt.% Y2O3 particles in a planetary ball mill, in a hydrogen atmosphere. and compacted by either hot extrusion or hot isostatic pressing. The microstructures of the obtained materials were characterized by means of light microscopy, transmission electron microscopy and chemical analyses. The mechanical properties were evaluated by means of Vickers microhardness measurements and tensile tests. It was found that the microstructure of both materials is composed of ferritic grains having a submicron size and containing nanometric Y-Ti-O oxide particles with a mean size of about 10 nm, uniformly distributed in the matrix. The oxide particles in the hot extruded steel were identified as YTiO3 phase. In larger (>10 nm) oxide particles Cr was found next to Ti, Y and O. The steel produced by hot extrusion exhibits much higher tensile strength and hardness at low to moderate temperatures, as compared to the steel fabricated by hot isostatic pressing, which was mainly attributed to smaller pores but also to more severe work hardening in the case of the hot extruded steel. (C) 2011 Elsevier B.V. All rights reserved.

2011