Structural and thermodynamic properties of nanocrystalline fcc metals prepared by mechanical attrition
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Nanocrystalline fee metals have been synthesized by mechanical attrition. The crystal refinement and the development of the microstructure have been investigated in detail by x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The deformation process causes a decrease of the grain size of the fee metals to 6-22 nm for the different elements. The final grain size scales with the melting point and the bulk modulus of the respective metal: the higher the melting point and the bulk modulus, the smaller the final grain size of the powder. Thus, the ultimate grain size achievable by this technique is determined by the competition between the heavy mechanical deformation introduced during milling and the recovery behavior of the metal. X-ray diffraction and thermal analysis of the nanocrystalline powders reveal that the crystal size refinement is accompanied by an increase in atomic-level strain and in the mechanically stored enthalpy in comparison to the undeformed state. The excess stored enthalpies of 10-40% of the heat of fusion exceed by far the values known for conventional deformation processes. The contributions of the atomic-level strain and the excess enthalpy of the grain boundaries to the stored enthalpies are critically assessed. The kinetics of grain growth in the nanocrystalline fee metals are investigated by thermal analysis. The activation energy for grain boundary migration is derived from a modified Kissinger analysis, and estimates of the grain boundary enthalpy are given.
I. INTRODUCTION Nanocrystalline materials with interesting physical and mechanical properties have been prepared almost exclusively by the inert gas condensation technique since first being reported by Gleiter and co-workers.1'2 Such nanocrystalline materials exhibit a crystallite size in the range of a few nanometers (typically 5-20 nm), so that as much as 20-50% of the material consists of incoherent interfaces between crystals of different orientation. Hence, the properties of nanocrystalline solids are expected to be strongly influenced by the structure and properties of the grain boundaries. This is different from both crystalline and amorphous materials and results in physical properties differing from those of crystals and glasses with the same chemical composition.3 The preparation of ductile ceramics,4 materials effective for hydrogen storage,5 and alloys that are otherwise immiscible6 are only a few examples of the interesting technological applications promised by this new class of materials. Recently it has been shown that nanocrystalline metals and compounds can also be synthesized by mechanical attrition.7"15 Transmission electron microscopy revealed that by ball milling, a nanocrystalline structure with random orientations of individual grains evolves from dislocation cell structures within shear bands. By J. Mater. Res., Vol. 7, No. 7, Jul 1992
further deformation the dislocation cells/low-angle grain boundaries disappear, leading finally to a fully nanocrystalline powder with
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