Characterization of 14YWT As-Atomized, Milled, Milled and Annealed Powders and HIP Consolidated Alloys
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Characterization of 14YWT As-Atomized, Milled, Milled and Annealed Powders and HIP Consolidated Alloys Nicholas J. Cunningham1, Auriane Etienne1, G. Robert Odette1, Erich Stergar1, Yuan Wu1 and Brian D. Wirth2 1 University of California, Santa Barbara, CA 93106, U.S.A. 2 University of Tennessee, Knoxville, TN 37996, U.S.A. ABSTRACT Nanostructured ferritic alloys (NFA) are Fe-Cr based ferritic stainless steels containing an ultrahigh density of very stable Y-Ti-O nanofeatures (NFs) that provide dispersion strengthening and radiation damage resistance for candidate Generation IV and future fusion reactor materials. This work is a small and focused part of a larger collaboration to produce large best practice NFA heats. The powders analyzed were rapidly solidified from a melt containing Fe-14%Cr, 3%W, 0.4%Ti and 0.2%Y by gas atomization in Ar, Ar/O, and He atmospheres. Note this represents a different processing path from conventional NFA production where metallic powders are mechanically alloyed with Y2O3 by ball milling. Electron probe microanalysis (EPMA), atom probe tomography (APT), transmission electron microscopy (TEM) and small angle neutron scattering (SANS) were used to characterize the powders in the as-atomized, ball milled and ball milled and annealed conditions. EPMA showed the Y is heterogeneously distributed and phase separated in all the as atomized powders, but attritor milling for 20 to 40 h is required to mix the Y. Milling also creates a significant quantity of O as well as N contamination. Subsequent powder annealing treatments, typically at 1150°C, result in the precipitation of a high density of NFs. All the annealed powder variants show a bimodal grain size distribution, but TEM and APT show NFs in both large and small grains. Reducing O content added during milling of the Ar atomized powders increased the precipitate size and decreased the number density, adversely affecting the hardness. INTRODUCTION Nanostructured ferritic alloys (NFAs) have the potential to make transformational contributions to developing advanced sources of fission and fusion energy. NFAs are stainless steels containing 12-14 wt% Cr and small additions of Ti and Y. NFAs are dispersion strengthened by a high density of nm-scale Y-Ti-O features (NFs), resulting in a high yield and tensile strength at temperatures well above 600°C, exceptional creep-rupture strength, and radiation damage resistance [1-4]. Typically, NFAs are processed by mechanically alloying (MA) Fe-based alloy powder with Y2O3 powder by ball milling. During MA the yttrium and oxygen dissolve in the Fe-matrix and subsequently precipitate with Ti during hot consolidation, usually by extrusion or hot isostatic pressing (HIPing). A distinction is made here between NFAs and other ODS steels by the size of the dispersed strengthening features. The high number density in excess of 1023/m3 of 2-4 nm precipitates in NFAs more effectively traps helium, increasing creep strength compared to the coarser oxides (>5-10 nm) in ODS steels [3,5,6].
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