In Situ Neutron-Diffraction Studies on the Creep Behavior of a Ferritic Superalloy
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THE Fe-Ni-Al alloys strengthened by B2-type precipitates are candidates for high-temperature (HT) structural applications [e.g., ultra-supercritical (USC) steam turbines] because of their improved resistance to creep, precipitate coarsening, and oxidation, compared to Cr steels at elevated temperatures.[1–5] Like c/c¢ Ni-based superalloys, the body-centered-cubic (bcc) Fe-matrix (b) with coherent (Ni,Fe)Al B2-type (b¢) precipitates exhibit a cube-on-cube orientation relationship, providing the possibility of achieving an Fe-based analog to the Ni-based superalloys. However, USCturbine applications need highly creep-resistant structural materials, with a tolerable creep rate of 3 9 1011/s.[6] Subsequent optimization of the creep properties of Fe-Ni-Al alloys requires a fundamental understanding of the deformation micromechanisms, particularly at the local b and b¢ levels. Although mechanical behaviors of single-phase solid-solution ferritic alloys[7,8] and NiAl intermetallics[9] have been investigated extensively, few studies have explored the HT deformation of two-phase SHENYAN HUANG, PhD Candidate, ZHENKE TENG, PhD Student, and PETER K. LIAW, Professor and Ivan Racheff Chair of Excellence, are with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996. Contact e-mail: [email protected] DONALD W. BROWN and BJØRN CLAUSEN, Scientists, are with the Lujan Center, Los Alamos National Laboratory, Los Alamos, NM 87545. YANFEI GAO, Associate Professor, is with the Department of Materials Science and Engineering, University of Tennessee, and is also with the Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Manuscript submitted March 15, 2011. Article published online November 15, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
alloy systems with b and b¢ phases present in the constrained state.[1,2,5] Neutron diffraction (ND) provides the crucial structural information at mesoscopic scales (less than 1 lm), specifically the accurate determination of lattice strains in differently oriented grains and/or multiple phases. Large beam size and large penetration of neutrons into matters enable the illumination on a substantial quantity of grains, from which structural mechanisms in the bulk can be determined with better statistics. In contrast to the residual stress/strain state measured with ex situ characterization techniques, in situ measurements under loading at high temperatures probe the dynamic latticestrain evolution during deformation. Because diffraction allows the precise determination of interplanar spacings, it can be used to measure the elastic lattice strains. On the contrary, diffraction is not directly sensitive to plastic deformation because the shear strain parallel to crystallographic planes induced by slip deformation cannot be detected directly by diffraction. However, local plastic strain of differently oriented grains is generally incompatible and, thus, is compensated by elastic strains, which create local residual stre
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