Irradiation-Induced Nanoprecipitation in Ni-W Alloys
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NI alloys are presently considered as realistic candidate materials for advanced nuclear reactors operating at elevated temperature.[1,2] The challenges, however, remain large as these materials suffer from both a loss of ductility and significant swelling, due to the formation of large He bubbles and blocky carbides.[1,3,4] Nanostructuring offers a potential means to overcome these problems.[2] Inclusion of a fine dispersion of nanoprecipitates has indeed been successfully employed to increase radiation resistance in ferrite/marternsitic steels by providing a high number density of defect sinks. Improvement in the mechanical properties of these materials, in particular strength and creep resistance, was found to be an additional benefit of these nanoprecipitates.[5] One particular approach to nanostructuring relies on nano-oxides dispersion strengthening (ODS), as for instance in nano ODS Ni alloys[6] and nano ODS austenitic steels.[5,7,8] The negligible solubility of oxides in Ni and Ni-rich fcc structures is advantageous as it results in small coarsening rates, although it does create challenges for the synthesis of these microstructures, and generally non-equilibrium processing techniques such as mechanical alloying must be employed.[6] In this case, JAEYEL LEE and CALVIN R. LEAR, Graduate Students, PASCAL BELLON, Professor, and ROBERT S. AVERBACK, Hamer Professor, are with the Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Champaign, IL 61801. Contact e-mail: [email protected] XUAN ZHANG, formerly Graduate Student with the Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, is now Post-Doctoral Researcher with the Argonne National Laboratory, Lemont, IL. Contact e-mail: [email protected] Manuscript submitted June 24, 2014. Article published online December 19, 2014 1046—VOLUME 46A, MARCH 2015
plastic deformation is employed to force the dissolution of large oxides or other highly immiscible phases into the matrix,[9] although achieving homogeneity remains a significant problem. An alternative non-equilibrium technique for inducing nanoscale precipitation has been proposed based on the tendency of two-phase alloys to self-organize during ion irradiation.[10,11] In moderately immiscible alloys such as Cu-Ag, Cu-Fe, and Cu-Co, it was demonstrated that nanoscale self-organization results from a competition between finite-range ballistic mixing and thermally activated decomposition.[12] It thus requires irradiation temperature sufficiently elevated for thermal diffusion to compete with ballistic mixing. In contrast, nanoscale self-organization has also been observed in highly immiscible alloys such as Cu-Mo and Cu-W[13,14] irradiated at room temperature (RT), where thermal diffusion is negligible. In this case, atomistic simulations[14] suggest that, during the thermal spike phase of displacement cascades, W and Mo undergo clustering by Brownian motion in a molten Cu matrix. These nanoclusters grow until they reach the typical
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