An Assessment of Milling Time on the Structure and Properties of a Nanostructured Ferritic Alloy (NFA)
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INTRODUCTION
NANOSTRUCTURED ferritic alloys (NFAs) have seen a tremendous amount of work over the last decade for their potential applications in the nuclear power industry as an irradiation damage resistant material.[1–5] This high irradiation damage resistance is a result of a dense dispersion of sub 10 nm oxides (NFs) that result from high energy milling, or attrition, processing. This processing step is critical to achieving both the desired irradiation damage resistance as well as the required structural properties. It has been shown experimentally that well-processed NFAs result in the full dissolution of the starting yttrium oxide into the metal matrix during high energy attrition.[1,6,7] Upon hot consolidation, complex oxides comprising titanium from the matrix as well as yttrium and oxygen, precipitate intra- and inter-granularly. This precipitation creates a homogeneous dispersion strengthening network and sets NFAs apart from conventional oxide dispersion strengthened (ODS) steels where full dissolution of the initial oxide does not occur. As a result, conventional ODS materials typically feature coarser oxides that exist more predominantly on grain boundaries or prior particle boundaries.[8–10] These differences in oxide character and location result in reduced mechanical properties for conventional ODS materials when compared to NFAs.[1,4,11] Given the importance of attrition on the final NFA properties, this study seeks to isolate this variable and quantitatively assess the effects of attrition time on NF distribution and tensile properties. Identical consolidaRICHARD DIDOMIZIO, SHENYAN HUANG, LAURA DIAL, and MIKE LARSEN, Staff Scientists, are with GE Global Research, Niskayuna, NY 12309. Contact e-mail: [email protected] JAN ILAVSKY, Scientist, is with the Argonne National Laboratory, Argonne, IL 60439. Manuscript submitted December 10, 2013. Article published online August 27, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
tion processing was used to produce NFA forgings of varying milling times. Milling trials and tensile tests were repeated to build statistical confidence for differentiation. The measured mechanical property differences are related to the microstructure through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and small angle X-ray scattering (SAXS) characterization. A model for tensile yield strength based on the experimentally determined microstructural features is then applied, enabling strengthening mechanisms to be ascertained.
II.
EXPERIMENTAL PROCEDURES
The NFA used for this study has a nominal composition in weight percent of Fe-14Cr-3W-0.4Ti-0.25Y2O3. The ferritic matrix powder (Fe-14Cr-3W-0.4Ti) was argon gas atomized by Carpenter and sieved to a mesh size of 150/+325 prior to attrition. The Y2O3 powder used was purchased from Alfa Aesar and had a quoted D50 of 1.2 lm. A particle size distribution showed that the primary particle size was 1 lm while most particles were weak, 10 lm agglomerates following ultrasonication. A. Processing The attrit
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