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E (nc) materials are of interest due to their unique combinations of physical and mechanical properties.[1–4] The issue of thermal stability in nc materials is important for two primary reasons. First, because on the basis of a Gibb’s free energy argument, such fine grains may be metastable, and hence prone to coarsening with a low energy threshold. Second, engineering applications require the consolidation of nc materials, which typically involves exposure to temperature and pressure. Inspection of the scientific literature reveals that the goal of maintaining a stable nc structure through consolidation at elevated temperatures has thus far remained elusive, with few exceptions reported. A number of investigations on the thermal stability of nc materials have been carried out, and the findings reveal that in pure metals, significant grain growth occurs at room temperature, e.g., Sn, Pb, Al, Mg,[5] Cu, and Pd.[6,7] This is particularly noteworthy in the case of Pd since room temperature corresponds to a homologous temperature of only 0.16 TM. In the case of multicomponent systems, however, it is possible to attain a degree of thermal stability via a reduction in grain boundary mobility of the nc grains. This is accomplished, primarily, through the introduction of fine dispersed phases and grain boundary segregation. Recent interest in cryomilling (e.g., milling in a FUSHENG SUN, formerly Postdoctoral Researcher with the Department of Chemical Engineering and Materials Science, University of California, is Senior Technical Specialist, RTI International Metals, Niles, ´N ˜ IGA, formerly Graduate Student with the DepartOH. ALEJANDRO ZU ment of Chemical Engineering and Materials Science, University of California, is Instructor, Departamento de Ingenieria Meca´nica, Universidad de Chile. PAULA ROJAS, formerly Postdoctoral Researcher with the Department of Chemical Engineering and Materials Science, University of California, is Postdoctoral Researcher, Instituto de Fı´sica, Pontificia Universidad Cato´lica de Valparaiso, Chile. ENRIQUE J. LAVERNIA, Dean, is with the College of Engineering, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Contact email: [email protected] Manuscript submitted November 2, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A

liquid nitrogen media) stems from reports that it is possible to increase the stability of nc grains in metallic systems such as Al, Zn, Ni, and Fe, by introducing a dispersion of nanometer nitride phases.[8–13] The presence of grain boundary segregation,[14,15,16] solute drag,[17] impurities,[18] second-phase drag,[19] pore drag,[20] and chemical ordering[21,22] can improve the thermal stability of nc materials via a drag-force mechanism on the grain boundaries. In related studies, several researchers have examined the thermal stability of nc powders generated by mechanical alloying or mechanical milling.[11,13,23–25] Significant stabilization of nc grain structures was reported, and a generalized criterion appears to be that t