Defect and Diffusion Forum Vols. 337-338

Abstract: Electrical techniques have been used to study the thermal vacancies in 401.0 casting aluminum alloy. Quenching experiments were usually performed on thin specimens to ensure a uniform quenching rate throughout the specimen. After grinding, polishing and etching, samples of 401.0 were homogenized for 12h at 673K and annealed for 90min., before quenched in water (277K) and then subjected to electrical measurements. From such measurements it is possible to deduce the vacancy formation enthalpy, which in combination with the results of self-diffusion measurements, give a value for the migration enthalpy of the vacancy.

Abstract: Electrical and mechanical techniques have been performed in order to study the effect of natural aging for up to more than 650 days on wrought (2024, 7075) and cast (AlSi_11.35Mg_0.23, AlSi_10.9Mg_0.17Sr_0.06) aluminum alloys, after having been deformed at room temperature to 25% deformation. The results revealed that, unlike dislocations, point defects were removed from the samples under investigation except for 7075 alloy and this can be attributed to no point defect introduced in this particular sample during the deformation.

Abstract: Certain radioactive isotopes decay by emitting positrons, and the positrons can be used to probe the structure of solid materials. For example, one can investigate the grain size due to deformation in a metal. XRD is also a powerful technique for detection the defects introduced into a metal and for investigating the microstructure of a deformed metal. In this paper we shall investigate the plastic deformation effect on the grain size of 5251 Al alloys using two different techniques, PALT and XRD. The study shows a significant effect of the plastic deformation on the grain size which decreases as the deformation increases.

Abstract: Positron annihilation lifetime (PAL) , electrical and mechanical techniques have been used to study the thermal vacancies in one of the most important engineering aluminum alloys which is 2024 alloy. Quenching experiments were usually performed on thin specimens to ensure a uniform quenching rate throughout the specimen. The specimens were prepared with dimensions of 0.15 × 1.5 × 1.5cm³. After grinding, polishing and etching, samples of 2024 were homogenized for 12 h at 673 K and annealed for 90-min., before quenched in water (277 K) and then subject to positron lifetime, electrical and mechanical for measurements. From such measurements it is possible to deduce the vacancy formation enthalpy, which in combination with the results of self-diffusion measurements, give a value for migration enthalpy of the vacancy. These are very important quantities in the study of the annealing of irradiation induced defects.

Abstract: The electrical resistivity and the corresponding mechanical properties (hardness) of some 5xxx Al-Mg alloy processed by plastic deformation were investigated. Plastic deformation was performed at room temperature. Isothermal annealing produced a significant change in both the electrical and mechanical properties of the samples. As the annealing time was increased, the resistivity and hardness up to full recovery. The activation enthalpy of migration of defects was determined, was found to be 0.234±0.06 eV, 0.218±0.049 eV, 0.316±0.016 eV 0.232±0.012 eV for 5005, 5251, 5052 and 5754 alloys, respectively.

Abstract: Metal matrix composites have grown rapidly with their usefulness in many applications for industries. The present research aims to study the formation of Fe₂SiO₄ and pearlite phases, the reaction product of iron-silica sand nanoparticles composites. In this study iron based silica sand nanoparticles composite with 5, 10, 15 and 20wt.% of silica sand nanoparticles were developed using powder metallurgy technique being sintered at 1100°C. It was observed during the X-Ray Diffraction (XRD) and XPS analysis that the reaction between iron and silica sand nanoparticles forms the Fe₂SiO₄ phase. Field Emission Scanning Electron Microscopy (FESEM) analysis at higher magnification also reveals the formation of pearlite phase. The presence of liquid phase sintering is also observed with frozen liquid spots at microstructure of iron-silica sand nanoparticles reaction.

Abstract: The Knight shifts and hyperfine structure constants of Tl₂Ba₂CuO_6+y are theoretically studied from the high order perturbation formulas of these quantities for a tetragonally elongated octahedral 3d⁹ cluster. The calculation results reveal good agreement with the observed values. The obvious anisotropies of the Knight shifts can be ascribed to the local tetragonal elongation of the Cu²⁺ site. The results and the local structure of the system are discussed.