Fatigue and Fracture of a Bulk Nanocrystalline NiFe Alloy
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IN recent years, there has been increasing interest in the processing, structure, and properties of bulk nanocrystalline (nc) metallic materials with grain sizes typically ranging from 2 to 100 nm.[1–18] Such interest has been stimulated by the remarkable combinations of to 6 pct),[1–4,6] strength (1 to 2 GPa),[1–6] ductility pffiffiffiffi(3 [7] fracture resistance (20 to 120 MPa m*), and superior *Results were obtained under plane stress conditions.
wear resistance[1] that can be engineered in such systems. However, although there has been much focus on the deformation mechanisms in nc metals and their alloys,[1–4,8–18] there have been relatively few studies of the mechanisms of fracture and fatigue in nanostructured materials.[7,19–22] There is, therefore, a need for Y. YANG, formerly Graduate Student, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, is Assistant Professor with the Department of Mechanical Engineering, Hong Kong Polytechnic University, Hong Kong, PR of China. B. IMASOGIE, Professor, is with the Department of Metallurgical and Materials Engineering, Obafemi Awolowo University, Ile-lfe 220005, Nigeria. G.J. FAN, Postdoctoral Researcher, and PETER K. LIAW, Professor, are with the Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996. W.O. SOBOYEJO, Professor, is with the Department of Mechanical and Aerospace Engineering, and the Princeton Institute of the Science and Technology for Materials, Princeton University. Contact e-mail: [email protected] Manuscript submitted June 26, 2007. Article published online March 21, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
further research to explore the mechanisms of fracture and fatigue in bulk nanostructured metallic materials that can now be processed in sufficient quantities for experimental studies of mechanical behavior. Engineered Ni-Fe–based nc materials exhibit low thermal expansion, high magnetic permeability, and excellent structural quality.[9,23] Prior research has shown[23] that the mechanical properties and microstructures of electrodeposited nc Ni-Fe materials are related to their microtexture and domain structure, as well as their processing conditions. More generally, however, a wider range of bulk nanostructured metals have been engineered[12,17,24] to impart excellent combinations of both ultra-high yield and fracture strengths, moderate elongation and toughness, and superior wear resistance, compared to their coarse-grained counterparts. Studies by Weertman et al.[25] and Koch et al.[26] have also reported several examples of nanostructured metals and alloys with elevated strengths and good ductility. However, recent studies[27] have suggested the possible occurrence of embrittlement, due to room-temperature aging in a bulk nanostructured Ni-Fe alloy. This trend has been attributed to the possible grain-boundary segregation in nanostructured Ni-Fe alloys. However, in-situ Auger studies of the grain boundary segregation are yet to be performed on the
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