الفهرس 
Chapter 1: IntroductionOnly 14 pages are availabe for public view
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Abstract
It is known that tin has high ductility and can be easily worked. It two alltropic forms where white tin (β-Sn) has a tetragonal crystal tice and the second one is grey tin (α-Sn) that has a diamond type crystal lattice. All impurities expect Sb are insoluble in tin. The addition of Sb, even in small amounts, is sharply reduced both the rate of Transition from pVSn to a-Sn and the equilibrium temperature condition. Such alloy Sn70 Sb2o Cu10 must has a sufficient hardness.
Itis thought that quasicrystal (QC), like many alloy systems, is rbrmed by taking into consideration a modification of the Hume-Rothery rales to produce intermetallic compounds. Thus, one can expect new types of ordering and defect structures by preparing the QC from a high Reality alloy with small amounts of structure disorder. By employing the re details, Cu was added with small amounts to Sn-Sb system as the requirement of the QC production. Here, the role of Cu was to eliminate gation, due to differences in density of the compound Sn-Sb, and to the compound CU3 Sn. Besides, it generated a decoration by atomic •musters.
In the present work, dense, monolithic Sn7oSb2oCuio quasicrystalline ay was prepared from the materials of Sn, Sb and Cu with purity of 5t, 99.98% and 99.99% respectively, using a standard metallurgical le. X-ray diffraction, scanning electron microscope, electrical measurements and positron annihilation lifetime spectra were to characterize the structure of the prepared Sn7oSb2oCu1o
quasicrystal alloy. All of these techniques were expected to be complimentary to one another.
X-ray diffraction experiment was carried out by using Cu Ka ’on at room temperature by the obtained results of the Sn70Sb2oCui0 alloy confirmed the existence of QC phase which are strongly ”ected by temperatures and applied stresses. This might in turn represent a good evidence of a superlattice reflection of the structure near ’ e eutectic temperature (283.3 °C).
Scanning electron microscope (JEOL JSM 840 A) was used to acterize the Sn7oSb2oCui0 alloy by quantitative micro-analysis. It owed the observer to defect the microstructural features such as dislocations or twins that were generated from deformation and could ove interagranularly by glide and climb. Clearly, the development of this microstructure could be attributed to the grain refining effect of the Cu indicating the role of the applied stress at the quasicrystalline phase.
Four-probe method was used to measure the electrical resistivity of the Sn7oSb2oCuio QC alloy. The maximum value of the electrical resistivity (at 0a = 30 MPa) was about 0.005 Qcm at room temperature, which was comparatively lower than most other quasicrystals known to date. This is called a transformation point (critical) for QC phases that nucleated at the grain boundary. At the transformation stress region, the calculated activation energy was about 0.135 eV that characterized dislocation intersection and diffusion mechanism.
The positron annihilation technique was used as a tool in defect spectroscopy. The positron lifetime measurement was used the fast-fast
coincidence technique with a time resolution (FWHM) 273 P s. In each lifetime spectra of a total 1x10 counts, it was accumulated during 3u6 x 103 s and analyzed by POSITRONFIT computer program. The increase in the average lifetime confirmed the deformation of vacancy clusters following the precipitation of Cu-atoms during the increase of the annealing temperatures. This behavior which strongly related to the QC state was produced due to both the increase of the solubility of the elements in the system and the suppression effect of vacancies.
Finally, the mechanical compression creep experiment provided a method to evaluate the deformation characteristics of the tested QC alloy at different temperatures (240 - 270 °C) and pressures up to 50 MPa. The analysis of the data showed that a transition was detected in the primary creep behavior from one controlled process to another one. The calculated activation energies of the transient creep, Q„ were found to be stress independent and had values 0.13 to 0.172 eV at the low temperature range (240 - 257 °C) and high temperature range (257-270 °C) respectively. The critical stresses of the Sn70Sb2oCui0 QC alloy at which the transition from one phase to another was found to be > 30 MPa and changed linearly with the reciprocal of the creep annealing rtemperatures. This transition could be attributed to the change in the stress exponent, nt, from 1.12 to 2.17 at low and high stresses respectively. The transient creep parameters p and n’ increase with working temperature and applied stress and have values changing from 50 x 10”4 to 300 x 10”4 and from 0.46 to 0.95 respectively. The mechanism governing the above creep process might be due to combination of dislocations and the diffusion of solute Cu-atoms in the
&7oSb2oCu1o QC alloy. While, in the quasi-steady state creep process, the values of the activation energy were found to be varied from 0.15 to 0.198 eV at low and high temperatures respectively. The stress exponent values, ns, varied from 1.12 to 2.35 at low and high stresses respectively in a range typically observed for metallic systems. These values were influenced by doping processes on the deformation mechanism. Thus the Sii7oSb2oCu10 QC alloy was accepted to be qualified as a superplastic material where the strain rate sensitivity parameter was higher than 0.3.