Nanocrystallization and magnetic properties of Fe-30 weight percent Ni alloy by surface mechanical attrition treatment
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newly developed technique, surface mechanical attrition treatment (SMAT),[1,2] accomplished usually by surface shot peening treatment with the shot velocity of ;1 to 20 m/s,[3] has been shown to be an effective approach to create localized plastic deformation resulting in grain refinement progressively down to the nanometer region in the surface layer of metal materials. It has been successfully applied in many systems[4–8] to achieve nanostructure. SMAT can produce a nanocrystalline surface layer with a thickness of ;15 to 50 mm,[3,6] which can be considered as a nanocrystalline bulk, and the sample exhibits the same chemical composition before and after SMAT, unlike other methods of surface nanocrystallization.[9,10,11] The nanocrystallization mechanism of SMAT has been classified into three types according to the stacking faults energy (SFE) by Lu and Lu¨.[3] In materials with high SFEs, such as Fe (;200 mJ/m2), the refinement process is that with increasing strain the dense dislocation walls and dislocation tangles first develop, then transform into subboundaries with small misorientations, and finally subboundaries evolve to highly misoriented grain boundaries. For materials with medium SFEs, such as Cu (;78 mJ/m2), the refinement process is through the development of equiaxed dislocation cells, formation of twins and subboundaries, to the evolution of highly misoriented grain boundaries. For materials with low SFEs, such as AISI 304 stainless steel (;17 mJ/m2), the refinement process is through formation of planar dislocation arrays and twins, subdivision by twins and martensitic transformation, and formation of nanocrystallites. CHUNSHENG WEN, Ph.D. Student, ZI CHEN, Graduate Student, BAOXU HUANG, Ph.D. Student, and YONGHUA RONG, Professor, are with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted October 8, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS
Measurement of mechanical properties and friction and wear properties indicated a significant improvement in the nanocrystalline surface layer by SMAT.[12–15] However, few reports have been presented on the probable change of physical properties. Magnetic properties, as an important physical property, have attracted many researchers in ferromagnetic nanocrystalline materials, and largely show dependence on the composition, microstructure, and particles (or grains) size. The coercivity (Hc) of superfine particles has a striking dependence on their size;[16–19] in addition, it can be increased with the decrease of grain size[20] or with the change of microconstituents[21] or texture.[22] As to saturation magnetization (Ms), it can be hardly changed for its intrinsic feature in a given material.[23] However, some investigations in nanocrystalline particles[24] and nanoplatelets[25] have shown that it decreased due to oxidation. Yoshizawa et al.[26] reported that Ms increased by adding other elements in ultrafine structure FINEMET. Fe-Ni allo
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