Synthesis and Characterization of Nanocrystalline Zn

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Title: Synthesis and Characterization of Nanocrystalline Zn
Author: Zhang, Xinghang
Advisors: Carl C. Koch, Chair
Jagdish Narayan, Professor
Ronald O. Scattergood, Professor
Robert Kolbas, Professor
Abstract: The goals of this thesis were to synthesize nanocrystalline Zn, to study the mechanical properties of bulk nanocrystalline Zn and try to reveal the deformation mechanisms in nanocrystalline materials. Nanocrystalline Zn powder has been synthesized by a cryomilling method. The average grain size decreased exponentially with the cryomilling time and reached a minimum average grain size of around 17nm. Large numbers of small grains (2~6nm) have been found in the very early stages of cryomilling. Dynamic recrystallization (DRX) was used to explain the observed phenomena. Differential scanning calorimetry (DSC), x-ray diffraction and transmission electron microscopy (TEM) 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. These cryomilled nanocrystalline Zn powders were consolidated into disks with a density of nearly theoretical density by uniaxial compression at room temperature. Cyclic variation of microhardness with milling time has been observed in cryomilled nanocrystalline Zn. Evidence from transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) suggests that the variation of dislocation density and grain size distributions determine the hardness behavior. A model, based on a kinetic reaction-rate model for cyclic amorphous-to-crystalline phase transformations observed during ball milling, simulates the experimental results very well. The model confirms the effect of DRX on modulated cyclic variation of microhardness. Dislocation strain hardening and recrystallization effects are superposed linearly with the intrinsic grain boundary hardening during the simulation. A dislocation density on the order of 10/sup 16/m /sup -2/ is predicted to be necessary to trigger DRX from the model. This prediction is evidenced by HRTEM observation of dislocation density on the same order and consistent with the estimation from thermodynamic calculation. The activation energy for rate controlling step in DRX estimated from the model is around 50 kJ/mol. This estimation indicates that a grain boundary diffusion controlled mechanism could dominate in DRX. Ductility of cryomilled nanocrystalline Zn has been studied by MDBT. The yield strength obtained from MDBT shows modulated cyclic variations with cryomilling time. Three times yield strength is consistent with the microhardness values for the same Zn samples. Ductility of CM2h and CM4h samples are much better than other cryomilled samples as indicated by a much larger ratio of normalized displacement than other cryomilled nanocrystalline Zn samples. However, the ductility of all cryomilled Zn samples is poor or very limited. The poor ductility of cryomilled Zn is presumably due to the remaining flaws as a result of incomplete bonding between particles. The Young's modulus measured from MDBT barely changes for all tested samples. Bulk (spherical balls) ultra-fine-grained (UFG) or nanostructured Zn via in situ consolidation of powders are produced by mechanical attrition at room temperature. The size of these spherical balls increased with the increase of ball milling time. The grain size decreased rapidly to around 80nm after 1h of ball milling and then increased to around 240nm at 3h. The grain size decreased gradually thereafter with the increase of milling time. An average gain size of around 23nm was achieved for Zn bulk samples ball milled for 25h. In situ consolidation of metal powders during mechanical attrition may be a promising method to produce bulk UFG or nanostructured materials with full density and less contamination. The hardness, yield stress measured from MDBT, and tensile tests are consistent with one another. The hardness increased almost linearly with the decrease of grain size. The positive Hall-Petch slope is much smaller compared to the slope for coarse-grained Zn. Except for BM1h Zn sample, all other samples possess good ductility as evidenced from miniaturized disk bend test (MDBT) results and from the observations of fracture surfaces studied by FESEM. A bulged hat shape sample is usually obtained after MDBT test. The Young's modulus almost keeps the same as for conventional coarse-grained Zn. The low temperature ball milling proves to be more efficient in reducing the grain size. A maximum elongation of around 110% is achieved for UFG Zn (around 240nm) under uniaxial tension test, which discloses a superplastic deformation in UFG Zn at room temperature. The elongation of room temperature ball milled Zn decreases with the decrease of grain size. Around 20% elongation is observed for Zn with an average grain size of around 23nm. Tension tests at elevated temperature result in a reduction of yield stress. The significant drop of yield stress at 200 centigrade degree or above may be due to recovery or recrystallization as evidenced from FESEM images. A strain rate sensitivity value of around 0.14 is usually found for Zn tested at 20 centigrade degree - 40 centigrade degree.
Date: 2001-11-30
Degree: PhD
Discipline: Materials Science and Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/4345


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