Effect of Al on the NiAl-Type B2 Precipitates in Ferritic Superalloys
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e Fe-based b/b¢ superalloys (Fe-Ni-Al) have gained considerable interest in recent years as a possible alternative to conventional Cr-ferritic (carbide-strengthened) steels for ultra-supercritical steam-turbine applications.[1–7] The mechanical properties of Fe-based superalloys are improved due to the coherent coplanar B2-type (Ni,Fe)Al precipitates (b¢), which exhibit greater thermodynamic stability and coarsening resistance at higher temperatures. In addition, due to the combined effects of Cr in the Fe matrix and b¢ precipitates, both corrosion and oxidation resistances can be improved under superheated-steam environments. The creep behavior of the particle-strengthened alloys is largely dependent on the precipitate parameters, viz. particle size, interparticle spacing, and volume fraction,
SHENYAN HUANG, formerly Graduate Student, Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200, is now with GE Global Research, Niskayuna, NY. GAUTAM GHOSH, Research Professor, is with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108. XIN LI, Postdoctoral Researcher, is with the Indiana University Center for the Exploration of Energy and Matter, Bloomington, IN 47408, and also with the Physics Department, Indiana University, Bloomington, IN 47405. JAN ILAVSKY, Scientist, is with the X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439. ZHENKE TENG, formerly Graduate Student, Department of Materials Science and Engineering, The University of Tennessee, is now Postdoctoral Researcher, Oak Ridge National Laboratory, Oak Ridge, TN 37831. PETER K. LIAW, Professor, is with the Department of Materials Science and Engineering, The University of Tennessee. Contact e-mail: [email protected] Manuscript submitted March 18, 2012. Article published online August 16, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
which are associated with various dislocation-particle interaction mechanisms.[8] Our previous work[4,5] reported the microstructure determined by transmission electron microscopy (TEM) and atom-probe tomography (APT) on selected alloys. In the present work, we extend into precipitate quantifications on more compositions using a different technique—ultra-small-angle X-ray scattering (USAXS), which is statistically representative, faster, and easier to do on a great number of samples. The microstructural attributes of NiAl-type B2 precipitates are investigated in six model ferritic alloys with Al content varied from 4 to 10 mass pct. The primary precipitate attributes are quantified by USAXS measurements, using a theoretical model combining the polydispersity (a monomodal distribution of primary precipitate size) and interference effects of the precipitates (interference between waves scattered by different particles). The establishment of data modeling, which is validated by complementary TEM results, provides the possibility of replacing TEM with USAXS for fast and reliable measurements on precip
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