First-Principles Calculations of Y-Si-O Nanoclusters and Effect of Si on Microstructure and Mechanical Properties of 12Cr ODS Steel in Vacuum Sintering System

High density of thermally stable Y-Si-O nanoparticles dispersed in the Fe matrix play a primary role in oxide dispersion strengthened (ODS) steel. In this study, the binding energies of solutes Y, O and Si with vacancies have been calculated in the framework of first-principles density functional theory. According to the calculations, any two solutes of Y, O and Si bound with each other strongly in the second nearest neighboring (NN) sites while not in 1NN. A vacancy (v) bounds strongly with Y and O in 1NN site. The binding sequence of solutes with v followed O-v → Y-v → Si-v, and the affinity of Y, Si and v with O followed O-Y → O-v → O-Si. The nucleation mechanism of Y-O-Si nanoclusters was determined, which gave the feasibility of adding Si to ODS steels. The core (consisting of Si and O)-shell (enriched Fe and Cr) structure of the microparticles was found in ODS steels containing Si, fabricated by mechanical alloying (MA) and vacuum sintering. Moreover the nanoparticles of monoclinic cubic Y2O3, Y2SiO5 and Y2Si2O7 with sizes of 5 ~ 12 nm were observed in ODS steel. Si reduced the sintering temperature by maximizing densities and mechanical properties at a lower sintering temperature. The steel with 3 wt% Si was sintered at 1280 °C, exhibiting the best comprehensive mechanical properties. The tensile strength, hardness and relative density were 1025 MPa, 442.44 HV and 95.3%, respectively.

SANS study on the stability of oxide nanoparticles in long-term thermally aged 9Cr-ODS steel

A 9Cr-oxide dispersion strengthened (ODS) steel was thermally aged at 873 K for up to 5000 hours. The size distribution and chemical composition of the dispersed oxide nanoparticles were analyzed by small angle neutron scattering (SANS) method under magnetic field. Combined with transmission electron microscopy (TEM), Vickers micro-hardness tests and electron backscattered diffraction (EBSD) measurements, all the results showed that the thermal treatment had little or no effect on the size distributions and volume fractions of the oxide nanoparticles in the ferromagnetic matrix, which suggested excellent thermal stability of the 9Cr-ODS steel.

The Precipitated Particle Refinement in High-Cr ODS Steels by Microalloying Element Addition

Two oxide-dispersion-strengthened (ODS) steels with different compositions (14Cr-ODS and 14Cr-Zr-ODS) were investigated to reveal the influences of microalloying element addition on the microstructure and to clarify the refining mechanism of precipitated particles. TEM and HRTEM results indicated that precipitated particles in the Zr-containing ODS steel had finer sizes and dispersed more homogeneously within the grains. It was found that rhombohedral Y4Zr3O12 particles with complex lattice structures were formed and could pin the migration of the grain boundaries during heat treatment due to their high thermal stability. In addition, the Zr-containing ODS steel exhibited a finer and more uniform grain morphology. Tensile tests showed that microalloying element addition could significantly improve the comprehensive mechanical properties of 14Cr ODS steels at room temperature.

Technological Processes for Steel Applications in Nuclear Fusion

Plasma facing components for energy conversion in future nuclear fusion reactors require a broad variety of different fabrication processes. We present, along a series of studies, the general effects and the mutual impact of these processes on the properties of the EUROFER97 steel. We also consider robust fabrication routes, which fit the demands for industrial environments. This includes heat treatment, fusion welding, machining, and solid-state bonding. Introducing and following a new design strategy, we apply the results to the fabrication of a first-wall mock-up, using the same production steps and processes as for real components. Finally, we perform high heat flux tests in the Helium Loop Karlsruhe, applying a few hundred short pulses, in which the maximum operating temperature of 550 °C for EUROFER97 is finally exceeded by 100 K. Microstructure analyses do not reveal critical defects or recognizable damage. A distinct ferrite zone at the EUROFER/ODS steel interface is detected. The main conclusions are that future breeding blankets can be successfully fabricated by available industrial processes. The use of ODS steel could make a decisive difference in the performance of breeding blankets, and the first wall should be completely fabricated from ODS steel or plated by an ODS carbon steel.

Experimental Investigation of EU-DEMO Breeding Blanket First Wall Mock-Ups in Support of the Manufacturing and Material Development Programmes

This paper presents the testing campaign of the two First Wall mock-ups in the HELOKA facility, one mock-up having a 3 mm thick Oxide Dispersion Strengthened (ODS) steel layer on its surface and the other featuring a tungsten functionally graded cover. Special consideration is given to the diagnostics used for these tests, in particular, the measurement of the surface temperature of the tungsten functionally graded layer with an infrared camera. Additionally, the paper looks into the uncertainty associated with the calorimetric evaluation of the applied heating power for these experiments.