Origins of stored enthalpy in cryomilled nanocrystalline Zn
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Magdy Kassem Department of Mat. and Met. Engineering Faculty of Pet. & Mining Eng., Suez Canal University, Suez, Egypt
Jagdish Narayan and Carl C. Kocha) Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695-7907 (Received 11 May 2001; accepted 21 September 2001)
Nanocrystalline Zn was prepared by cryomilling (mechanical attrition at liquid nitrogen temperature). Differential scanning calorimetry (DSC), x-ray diffraction, and transmission electron microscopy were used to study the structural changes and grain size distribution with milling time and subsequent annealing. Maxima in both stored enthalpy (for the low-temperature DSC peak) and lattice strain on the Zn basal planes were observed at the same milling time. Dislocation density on the basal planes is proposed as a major source for lattice strain and the measured stored enthalpy. The released enthalpy that might be due to grain growth is very small.
I. INTRODUCTION
Mechanical attrition, ball milling, has been widely used to synthesize nanostructured materials. 1 This method induces heavy cyclic deformation in powders. It is believed that mechanical attrition produces its nanostructures by the structural decomposition of coarser-gained structures as the result of severe plastic deformation. In many cases, the ball-milling process was performed at room temperature and in certain instances at liquid-nitrogen temperature (77 K); thus, the ball-milling process is a form of cold deformation. Cold deformation of metals and alloys has been studied extensively.2– 6 Most of the work expended in the cold deformation process is given out as heat, and only a very small amount remains as stored energy in the metal. In the case of cold working, this stored energy provides the source of all the property changes7 that are typical of deformed metals. The interpretation of the mechanisms of storage of energy arising from cold work is based on two concepts, namely the presence of lattice strains and of imperfections.8 Direct comparisons of the elastic strain energy and the total stored energy have been made.9–11 The contribution of lattice strain to stored enthalpy is usually less than or around 10% of the measured stored enthalpy. Therefore, the major part of the energy a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 12, Dec 2001
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storage is often ascribed to the presence of lattice imperfections. Point defects might be considered responsible for some energy storage.12 However, the mobility of vacancies and interstitials is so high that, except in the special case of deformation at very low temperatures, point defects do not contribute significantly to the stored energy of deformation. In the common case of deformation at ambient temperature a major fraction of the stored energy is derived from the accumulation of dislocations, and the essential difference between the deformed and the annealed states lies in
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