nan Abdel-Rahman

Vacancy Migration and Formation in A356.0 Aluminum Casting Alloys by Positron Annihilation Technique

The migration and formation curves for the A365.0 (Al-7% Si-0.4Mg) casting alloys are investigated by positron lifetime (PL) and S-parameter measurements from room Temperature (RT) to 873 K. The observed dependence of PL and S-parameter on quenching temperature (QT) is attributed to vacancy formation. PL results were analyzed applying the two state trapping model and the formation enthalpy H v f 1 was estimated by using different methods. Introduction The phenomenon of positron annihilation has been utilized in solid state physics research to probe a variety of materials. This technique has become established as a useful tool and is successfully applied for the investigation of defect structures in various materials including technologically important materials. The positron annihilation technique (PAT) is shown to be a sensitive nondestructive tool in the study of defects in solids. The advantage of PAT is its ability to distinguish different types of defects such as vacancies, vacancy clusters, dislocations, grain boundaries, voids, etc. Experimental The composition of the casting Al-Si alloy used in the present investigation is given in Table 1. Table 1. The composition of the casting Al-Si alloy used in the present work in wt. % Al-alloy Si Fe Cu Mn Mg Zn A356.0 7.0 0.2 0.2 0.1 0.3 0.1 Preparation, dimensions and polishing of the samples are described in [1,2]. Positron lifetime studies were performed at RT using a fast-fast coincidence system with a time resolution of about 281 ps (FWHM) which is described elsewhere [3,4]. Doppler broadening (D. B.), S-parameters were measured with an Ortec HPGe detector having an energy resolution 1.95 keV for the 1.33 MeV line of 60 Co[5]. Each positron lifetime spectrum was measured for 9×10 3 s and about 1.2×10 3 coincident events at the peak were accumulated. The lifetime spectra were analyzed using the program POSITRONFIT [6]. The spectra were satisfactorily analyzed as a single lifetime component after subtraction of the background and source component. Results and Discussion Migration of Thermal Defects in (A356.0) The lifetime spectra were resolved into exponential terms via least-squares fitting. The long lifetime is due to annihilation in the source while the short lifetime is a mixture of the lifetime components from bulk lattice, vacancy and grain boundaries. Mean lifetime reflects the characteristics of the defect formation and recovery processes and is a function of the quenching temperature (Q.T). All the samples were homogenized for 10 h at 823 K for the lifetime and D.B Materials Science Forum Online: 2004-01-15 ISSN: 1662-9752, Vols. 445-446, pp 45-47 doi:10.4028/www.scientific.net/MSF.445-446.45

Probing the Phase Transition in Nanocrystalline TiO2 Powders by Positron Lifetime (PAL) Technique

Positron annihilation techniques (PAT) have recently been successfully employed for the characterization of phase transitions in metals and compounds. In the present study, positron annihilation lifetime (PAL) measurements have been carried out on a nanocrystalline titania (TiO2) in the form of powders that had been heat-treated at temperatures ranging from 300 to 1273K. The PAL spectra were analyzed into two lifetime components. The shorter lifetime τ1 (185-300 ps) is attributed to positron annihilation in vacancies and the longer lifetime τ1 (400-580 ps) to positrons in microviods at interfaces. The rutile phase of TiO2 powders was utilized as a reference in order to compare their behavior with the commercially supplied and widely available anatase phase (Degussa P25). The influence of the heat-treatment upon the nanostructure during the transition of the anatase to rutile phase were also investigated by X-ray diffraction (XRD), TEM and BET surface area methods. Understanding of this effect is expected to enhance our knowledge of the morphology and nanocrystallite size of TiO2 powders and their T-dependence, and hence their physical properties.

Positron Annihilation & Micro-Hardness Measurement of 6063 and 6066 with Compromise with Ingot Al

The aim of this work was to study the resistance of this type of alloy to quenching. Hardness measurements can be defined as macro-, micro- or nano- scale, according to the forces applied and the displacements obtained. This effect can also be studied using a nuclear, (PALT): positron annihilation lifetime, technique [1]. Microhardness is the hardness of a material, as determined by forcing an indenter such as a Vickers or Knoop indenter into the surface of the material under a 15 to 1000gf load; the indentations are usually so small that they must be measured using a microscope. These samples were quenched at different temperatures ranging from 50 to 500oC. We studied the effect of the quenching temperature upon the hardness measurements. We also studied this variation via the positron annihilation (lifetime) parameter. It is clear from the Vickers hardness that 1050 has the lowest value of Hv, while 6063 is higher and 6066 has the highest values of Hv. Also we could observe ( recognize) that the Hv (number) is reduce as a function of temperature (6066) but for (1050) and (6063) there is no observation of a variation in Hv (number) as a function of quenching temperature. The same observation was also made for 1050, 6063 and 6066 via the lifetime measurements. Here, 6063, 6066 give higher values than 1050. It is clear that the data from both techniques (positron annihilation lifetime and Vickers hardness) for 1050 ingot Al gives lower values of both parameters for Hv and lifetime technique. While Hv for 6066 is higher than the values of 6063 alloy at the same quenching temperature. Using the lifetime technique, one cannot distinguish between the 6063 and 6066 alloys. The applied force has no real effect upon the levels of the hardness values. Also, alloys 6066 and 6063 were defined as heat-treatable alloys but 1050 is not a heat-treatable alloy. The Hv of the 1050 is not affected by the changes in quenching temperature. Alloy 6066 heat-treatable alloy is more affected by the heat treatment than is 6063 alloy, and this is related to the structure of the precipitates in these alloys since 6066 alloy has much more Si and Mg than does the 6063 alloy. The Hv values vary from 14 to 23.9 for 6063 alloy and from 15.7 to 69.8 for 6066 alloy; in comparison with ingot alloy (1050) where it varies from 10.4 to 18.6.

Activation Enthalpy of Dislocation Migration in Aircraft (Aerospace) (2024) Alloy by Positron Spectroscopy

Positron annihilation lifetime is one of the most important nuclear techniques, used to study the isochronal and isothermal annealing in one of the most important engineering aluminum alloys which is 2024 alloy. Samples of 25 % deformation have been used for these studies. Two recovery stages during the isochronal annealing [1] were observed which were ascribed to the recovery of point defects and dislocations introduced by the deformation. The isothermal annealing measurements were performed at 583, 603, 623 and 643 K from which the activation energy obtained was 1.24 ± 0.08 eV.

Testing Natural Aging Effect on Properties of 6066 & 6063 Alloys Using Vickers Hardness and Positron Annihilation Lifetime Techniques

The aim of this work was to produce a high strength 6xxx series Aluminum alloy by adjusting the processing conditions, namely solutionizing and natural aging. It consists of heating the alloy to a temperature at which the soluble constituents will form a homogeneous mass by solid diffusion, holding the mass at that temperature until diffusion takes place, then quenching the alloy rapidly to retain the homogeneous condition. In the quenched condition, heat-treated alloys are supersaturated solid solutions that are comparatively soft and workable, and unstable, depending on composition. At room temperature, the alloying constituents of some alloys tend to precipitate from the solution spontaneously, causing the metal to harden in about four days. This is called natural aging. The mechanical characterization of heat treatable 6xxx (Al-Mg-Si-Cu based) 6066, 6063 wrought aluminum alloys was studied. Their effects were investigated in terms of microstructure using positron annihilation lifetime technique and mechanical properties by hardness measurements. The hardness is the Resistance of material to plastic deformation, which gives it the ability to resist deformed when a load is applied. The greater the hardness of the material, the greater resistance it has to deformation. Hardness measurement can be defined as macro-, micro- or nano- scale according to the forces applied and displacements obtained. Micro hardness is the hardness of a material as determined by forcing an indenter such as a Vickers indenter into the surface of the material under 15 to 1000 gf load; usually, the indentations are so small that they must be measured with a microscope. During this work we are monitoring the effect of natural aging on the properties of positron lifetime and Vickers hardness parameters. The Vickers hardness of 6066 alloy has a maximum value(80) after (10)days of quenching at 530 which is the solution temperature of this alloy .the hardness of 6063 alloy has a maximum value (40) after (14)days of quenching at 520 which is the solution temperature of this alloy. The hardness which is conformed to the references.

Annealing Study of Al-Mg Wrought Alloys Using Two Different Techniques and Estimation of the Activation Enthalpy of Migrating Defects

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.

Correlation Coefficient between Vickers Hardness and Nuclear Technique

The aim of this work is to establish a correlation coefficient between the positron annihilation lifetime technique (PALS) and the Vickers hardness for the heat treatable aluminum alloys (6066, 6063).The potential of positron annihilation spectroscopy in the study of light alloys is illustrated with special regards to age hardening, severe plastic deformation, annealing and quenching in aluminum alloys. Vickers hardness is the standard method for measuring the hardness of metals, particularly those with extremely hard surfaces. Accordingly, a correlation coefficient of 90 % between τ and Hv is obtained. This correlation can help us to explain many behaviors of these alloys under deferent conditions.

Defect and Dislocation Density Parameters of 5251 Al Alloy Using Positron Annihilation Lifetime Technique

The result of positron lifetime measurements of a defected 5251 Al alloy is reported. Positron lifetime is measured as a function of the thickness reduction of the sample which shows a nearly linear increase and then becomes constant; which can be considered to be a reason for the defect movement saturation. The trapping rate, trapping efficiency, trapping cross-section, defect concentration and defect density of positrons are also measured for the sample concerned. The behaviors of these parameters are matched with theoretical calculations. Data are analyzed using the PATFIT88 computer program.

Correlation between Nuclear and Electrical Methods for Estimating the Activation Enthalpy of Defect Formation in 2024 Alloys

The Objective of this Research Was to Investigate the Use of a Sensor System to Monitor Continuously the Defect Formation Response of Heat-Treatable Aluminum Alloys. the System Continuously Monitors a Material Property, Resistivity, which Is Indicative of the Quenching Process. such Studies Will Allow us to Gain Active Control of the Aging Process and Hence the Material Properties of Aluminum Alloys.

Effect of Plastic Deformation on Electrical and Mechanical Properties of some 5xxx Al Wrought Alloys

The Electrical Resistivity and a Mechanical Property (hardness) of some 5xxx Aluminum Alloys Were Investigated. the Samples Were Exposed to a Type of Plastic Deformation (compression) to Nearly40% Deformation; the Resistivity and Vickers Hardness Being Measured for Every Degree of Deformation. the Investigated Alloys Gave a Good Response to the above Two Techniques, and the Results Were Consistent with each other.