Atom probe tomography analysis of Ti-Y-O clustering during processing of nanoscale oxide particles in 14%Cr ODS ferritic
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Atom probe tomography analysis of Ti-Y-O clustering during processing of nanoscale oxide particles in 14%Cr ODS ferritic steels Ceri A.Williams1, Paulina Unifantowicz2, Zbigniew Oksiuta3, Nadine Baluc2, George D. W. Smith1, and Emmanuelle A. Marquis1 1 Department of Materials, University of Oxford, UK, OX1 3PH. 2 Ecole Polytechnique Federale de Lausanne (EPFL), 5232 Villigen PSI, Switzerland. 3 Bialystok Technical University, Faculty of Mechanical Engineering, 15-351 Bialystok, Poland. ABSTRACT Atom probe tomography is used to investigate the clustering of Y-Ti-O in a 14%Cr2%W-0.3%Ti & 0.3% Y2O3 ODS steel. The clusters in the consolidated material are compared to clusters observed in the powder prior to consolidation. A higher density of smaller clusters is observed in the powder, and the clusters are found to contain more O and less Y. INTRODUCTION Reduced activation oxide-dispersion-strengthened ferritic steels are currently regarded as the most promising group of materials for structural applications in fusion power plant and the next generation of fission reactors. The oxide nanoclusters improve high temperature strength and act as trapping sites for helium and point defects introduced during irradiation [1]. Understanding how the nanoclusters form and develop is key to refining the mechanical alloying (MA) process and optimizing the microstructure. In similar ODS alloys, it was proposed that during MA, the Y2O3 is broken down completely, and the Y and O enter into solid solution in the matrix [2]. Atom probe tomography (APT) of powder after MA has recently shown that clustering of Y-Ti-O begins during the mechanical alloying process [3]. In the analysis of clusters in powder after MA and after MA followed by annealing for 5 minutes at 800°C, Brocq et al. report ‘slight solute enrichment’ of Y, Ti and O in the clusters. They also identify some difficulties in measuring the composition of the matrix. In this present study, a combination of solute nearest neighbour distance distributions, and the maximum separation method [4] are used to analyse the clusters and the surrounding matrix composition within APT data. This addresses some of the issues associated with the analysis of clusters in a material where there is a relatively high concentration of solute atoms in the matrix, and where the levels of solute enrichment in the clusters are likely to change. This study investigates the behaviour of individual solute elements during the formation of nanoclusters in an Fe-14Cr-2W-0.3Ti alloy mechanically alloyed with 0.3wt% Y2O3 developed by Baluc et. al. [5]. APT is used to analyse the alloy powder after MA, and the sizes and compositions of the nanoclusters are presented. The clusters observed in the final consolidated material are quantitatively compared to those observed in the powder after MA. EXPERIMENTAL Elemental powders of Fe, 14wt%Cr, 2wt%W and 0.3wt%Ti were combined with 0.3wt% Y2O3 and milled in a planetary ball mill at 300rpm for 50hrs in an H2 atmosphere. Focussed ion beam milling using a Zeiss NVision 40
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