Papers by Author: Nadine L. Baluc

Abstract: We describe a comprehensive ab initio investigation of phase stability and mechanical properties of W-Ta and W-V alloys, which are candidate materials for fusion power plant applications. The ab initio density functional calculations compare enthalpies of mixing for alternative ordered atomic structures of the alloys, corresponding to the same chemical composition. Combining the ab initio data with large-scale lattice Monte-Carlo simulations, we predict several low-energy intermetallic compounds that are expected to dominate alloy microstructures, and hence the low-temperature phase diagrams, for both alloys. Using the predicted ground-state atomic alloy configurations, we investigate the short-range order, point defect (vacancy and self-interstitial atoms) energies, and thermodynamic and mechanical properties of W alloys as functions of their chemical composition. In particular, we evaluate the anisotropic Young modulus for W-Ta and W-V alloys from ab initio elastic constant calculations, with the objective of comparing the predicted values with experimental micro-cantilever measurements. Also, using the calculated Poisson ratios for binary W alloys, which combine tungsten with more than 40 different alloying elements, we investigate if alloying improves the ductility of tungsten-based materials.

Abstract: A new ODS composition (Fe-14Cr-2W-0.3Ti-0.3Y2O3) developed in the ExtreMat integrated Project has been produced by mechanical alloying techniques and consolidated by hot extrusion. This study summarizes some results of characterization and cold workability tests carried out at CEA and EPFL. It appears that the microstructure is fine and uniform after hot extrusion. According to microprobe analysis, solute elements are homogenously distributed in the matrix. However, the relatively high hardness level measured after hot extrusion and heat treatment may be detrimental in case of additional cold processing which is required to produce final shape like thin plates or cladding tubes. An assessment of the cold workability and the effect of the degree of cold work by rolling on recrystallisation temperature are addressed here. It is found that this material can be successfully cold rolled with a high degree of cold work (up to 60% of thickness reduction) without any damage. According to optical micrographs and Differential Scanning Calorimetry (DSC) measurements, it seems that the recrystallisation temperature remains always very high (above 1400°C) even though cold work level increases (up to 66% of thickness reduction). However, the hardness values begin to decrease for heat treatment temperatures above 1200°C for hot worked conditions and below 1000°C for cold worked conditions, respectively.

Abstract: Six oxide dispersion strengthened (ODS) ferritic steels, with the composition of Fe-(12-14)Cr-2W-(0.1-0.3-0.5)Ti-0.3Y2O3 (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.