Synthesis of hydroxyapatite (HAP) nano strips was carried out by chemical precipitation method fo... more Synthesis of hydroxyapatite (HAP) nano strips was carried out by chemical precipitation method followed by microwave irradiation. The microwave assisted reactions proceed at fast rates. It is found that the presence of the complex reagent EDTA plays an important role in the morphological changes of nanostructure hydroxyapatite. EDTA acts as a hexadentate unit by wrapping itself around the Ca2+ metal ion with, four oxygen and two nitrogen atoms and forms several five member chelate rings. The relative specific surface energies associated with the facets of the crystal determines the shape of the crystal. Scanning electron microscopy revealed the presence of hydroxyapatite nano strips with the range 50‐100 nm in EDTA influenced HAP powders. Fourier transform‐infrared spectroscopy (FT‐IR) result combined with the X‐ray diffraction (XRD) indicates the presence of amorphous hydroxyapatite (HAP) in the as‐prepared material. X‐ray patterns collected on the powder after heat‐treatment at 11...
In this study, hydroxylapatite (HAp) reinforced polymer composites were produced and evaluated to... more In this study, hydroxylapatite (HAp) reinforced polymer composites were produced and evaluated to be used in bone implants. The aim was to produce a material that has similar modulus of elasticity (E) and other mechanical properties to those of bone in order to achieve mechanical compatibility in the body. Hydroxylapatite was mixed with high density polyethylene (HDPE) and polypropylene (PP) in different amounts. The resulting composite materials were injection molded into standard tensile and impact test specimens and their mechanical properties were examined. Stiffness and hardness of the composites were increased toughness, percent elongation and impact energy were decreased with increasing HAp content. No significant change was observed in ultimate tensile strength while an increase was significant in the yield strength related to HAp content. PP composites were found to have better impact properties in spite of more ductile behavior of HDPE composites under tension. Although the problem of mechanical compatibility in bone implants has not totally been solved, hydroxylapatite reinforced polymer composites are suggested as promising alternatives to traditional implant materials.
Proceedings of the 2nd World Congress on Recent Advances in Nanotechnology, Apr 1, 2017
In tissue engineering, natural/synthetic polymer based fibrous composite scaffolds obtained via e... more In tissue engineering, natural/synthetic polymer based fibrous composite scaffolds obtained via electrospinning method were shown to support the cell adhesion and tissue regeneration. However, electrospinning of natural polymers requires the use of toxic solvents that are negatively affecting the cell proliferation and biocompatibility of the produced scaffolds in addition to the usage of acidic solvents which will result in massive biodegradation inside the body[1] .Here, a method was proposed that is higher safety for the patient and even for the experimentalists who are using harmful and highly volatile solvents. Two types of polymers were used in the synthesis of the scaffolds by mimicking the key features of the tissue extracellular matrix which contains gelatin and coaxially organized nanofibers. Coaxial electrospinning technique was used to obtain core(PVA)-shell (gelatin) nanofibers. While material in the shell provides recognition sites for the tissue cells, core material provides mechanical endurance. Different from conventional methods, proposed work aims to lower the steps of application of the scaffold to the harmed tissue by using only deionized water as solvent. Instead of dissolving PVA and gelatin in toxic and acidic solvents, they were dissolved in the deionized water above the gelation temperature. Later, coaxial electrospinning generated increased cell spread and mechanical stiffness. The samples were characterized by scanning and transmission electron microscopy. Based on the experimental results it is concluded that electrospun fibers obtained from the 8% concentrated gelatin solution had a beaded structure, whereas the coaxially fabricated PVA and gelatin from the same concentration solutions did not show any beaded morphology. Also core-shell fibers have diameters down to 180 nm. This result showed that PVA aids to the uniform gelatin fiber formation which, may give higher mechanical stability. The electron microscopy analysis leading to these results has received support by the
The deformation behavior of an idealized crystal made by stacking of parallel slip planes is stud... more The deformation behavior of an idealized crystal made by stacking of parallel slip planes is studied. Each slip plane is assumed to contain active sources of dislocations leading to a constant density of non-interacting dislocations in the plane which glide through randomly distributed localized point obstacles, representing small precipitates. The dislocation is assumed to have a constant line tension and the dislocation-obstacle interaction is taken to have a simple step form. Using results of computer simulation of thermally activated glide through random arrays of point obstacles the deformation is modeled as a function of temperature and applied stress, determining the strain rate and the morphological characteristics of slip. Stress-strain rate and flow stress-temperature relations are discussed. * Let the dislocation under the applied stress T encounter a configuration of point obstacles denoted by i (Figure 1). Between two adjacent
The plastic deformation of crystals is usually accomplished through the motion of dislocations. T... more The plastic deformation of crystals is usually accomplished through the motion of dislocations. The glide of a dislocation is impelled by the applied stress and opposed by microstructural defects such as point defects, voids, pre cipitates and other dislocations. The planar glide of a dislocation through randomly distributed obstacles is considered. The objective of the present research work is to calculate the critical resolved shear stress (CRSS) for athermal glide and the velocity of the disloca tion at finite tempereature as a function of the applied stress and the nature and strength of the obstacles. Dislocation glide through mixtures of obstacles has been studied analyti cally and by computer simulation. Arrays containing two kinds of obstacles as well as square distribution of obstacle strengths are considered. The critical resolved shear stress for an array containing obstacles with a given distribution -ivof strengths is calculated using the sum of the quadratic mean of the stresses for the individual obstacles and is found to be in good agreement with the com puter simulation data. Computer simulation of dislocation glide through randomly distributed obstacles containing up to 10 6 obstacles show that the CRSS decreases as the size of the array increases and approaches a limiting value. Histograms of forces and of segment lengths are obtained and compared with theoretical predictions. Effects of array shape and boundary conditions on the dislocation glide are also studied. Analytical and computer simulation results are compared with experimen tal results obtained on precipitation-, irradiation-, forest-, and impurity clusterhardening systems and are found to be in good agreement.
We consider the glide of a single dislocation through randomly distributed obstacles of given pro... more We consider the glide of a single dislocation through randomly distributed obstacles of given properties. Using a model we studied the critical resolved shear stress for athermal glide and the velocity of thermally activated glide at finite temperature as functions of the nature and strength of the obstacles. Results obtained from computer simulation are compared with the available experimental data and good agreement is found. Zusammenfassung-Die Gleitung einer einzigen Versetzung durch eine zuf5llige Anordnung von Hindernissen mit vorgegebenen Eigenschaften wird behandelt. Anhand eines Modelles wird die kritische FlieBspannung fiir athermische Gleitung und die Geschwindigkeit der thermisch aktivierten Gleitung bei endlicher Temperatur in Abhlngigkeit von Natur und StLke der Hindernisse untersucht. Die aus der Computersimulation erhaltenen Ergebnisse werden mit den verfiigbaren experimentallen Daten verglichen; es ergibt sich gute iibereinstimmung.
... 1300 2.48 ± 0.08 65.01 ± 41.57 130.68 ±17.90 Fig.3 ESEM micrograph of HAp with PVA binder by ... more ... 1300 2.48 ± 0.08 65.01 ± 41.57 130.68 ±17.90 Fig.3 ESEM micrograph of HAp with PVA binder by 1%, sintered at 1200 o C, 4h. ... [4] AC Tas, F. Korkusuz, M. Timucin and N. Akkas: Journal of Materials Science: Materials in Medicine, Vol. 8 (1997), p. 91. ...
In composite production, the shortest route is via an in situ composite in which a melt dissociat... more In composite production, the shortest route is via an in situ composite in which a melt dissociates simultaneously into two rather different solid phases. The monotectic alloys can be included in this group. The present work was aimed at extending our recent squeeze casting experience on the Zn-Bi monotectic alloy in order to increase its cast quality and mechanical properties. A squeeze casting unit was built, and its die and punch were machined. The molten monotectic alloy was squeezed in this unit under pressures up to 120 MPa in its freezing range until it solidified completely. It was found that an increase in squeeze casting pressure provided increases in density, tensile strength, and Vickers hardness, which resulted in decreases in chip length and electrical resistivity. Before the squeeze casting practice, the freezing characteristics of this monotectic were estimated using basic solidification principles.
The application and effects of squeeze casting were investigated on the density, Brinell hardness... more The application and effects of squeeze casting were investigated on the density, Brinell hardness, tensile strength and fracture elongation values of the wide freezing range Zn-27%A1, A1-20%Si, A1-7%Si, AI-10%Mg, AI-5%Mg, AI-4.5%Cu-0.5%Bi and Fe-3%C-2%Si alloys. It was found that these alloys as a group were suitable for the squeeze casting practice. Sharp increases could be achieved in all mechanical properties due to the reductions in porosity levels. In general, higher squeezing pressures provided better mechanical properties. The trend in the A1-20%Si alloy however, was sporadic and cracking occurred in the vicinity of silicon rich primary particles above 100 MPa pressure. An optimum melt, and die and punch temperatures were found as 1500 °C and 1150 °C, respectively to suppress white cast iron formation in the Fe-3%C-2%Si alloy. The AI-10%Mg and A1-4.5%Cu-0.5%Bi alloys suffered microsegregation in addition to microporosity. Their squeeze cast samples showed higher hardness and tensile properties after annealing them. The machinabilities of the two alloys however, were degraded due to squeeze casting.
Journal of the mechanical behavior of materials, Mar 1, 1993
In this study, the effects of temperature and strain rate on the deformation behavior of aluminiu... more In this study, the effects of temperature and strain rate on the deformation behavior of aluminium 1100 and aluminium 2024 are investigated. Specimens are tested at different temperatures and strain rates under the effect of torsional loading. The data are evaluated using critical radius and effective length concepts. For both materials, as a result of the interaction between strain hardening and thermal softening mechanisms, flow curves pass through a peak value after which aluminium alloy tends to fracture while aluminium 1100 approaches to a steady state deformation. The strain rate sensitivity of materials obtained from torsion testing increases with temperature. The length of aluminium specimens subjected to free-end hot torsion is observed to change with both temperature and strain rate.
The structural strength of materials generally depends on the materials properties, the shape and... more The structural strength of materials generally depends on the materials properties, the shape and size of defects as well as the orientation of such flaws in the medium. In the application of fracture mechanics other kinds of imperfections due to manufacturing are particularly important such as flat cavities which develop during casting, small cracks resulting from residual stresses in welded materials, and fatigue cracks under fluctuating external loads. Thus in a design dealing with such materials, it is necessary to have a good estimate of the stress state disturbed by the existence of these flaws.
Three different models for a pinned dislocation under an applied stress are compared in this pape... more Three different models for a pinned dislocation under an applied stress are compared in this paper: (i) the continuous string model of Koehler, Granato and Liicke; (ii) a twodimensional lattice model which represents the dislocation in its slip plane; (iii) a discrete string model which is obtained as an analytical approximation to model (ii). Good agreement is obtained for the quantities investigated for the three models. Comparison of models (i) and (iii) therefore provides an atomistic interpretation for the parameters appearing in the defining equation of the continuous string model. One result of interest is that these parameters cannot be expressed solely in terms of the characteristics of the interatomic law for atoms in the vicinity of their equilibrium sites for the perfect crystal but must also depend on aspects of the interatomic force law for atoms in configurations such as those which occur near the dislocation.
Recently, application of ceramic coatings to space vehicles to generate thermal barriers upon re-... more Recently, application of ceramic coatings to space vehicles to generate thermal barriers upon re-entry is being considered as a challenge for research. Conventionally, thermal barrier coatings (TBC) are utilized to increase the gas turbine engine components' thermal and oxidation resistance values in order to get higher power and efficiency values. In this study, various superalloys and stainless steels were thermal barrier coated using the atmospheric plasma spraying method. One goal is to determine the effect of the substrate on the performance of the coating. Coating morphology was determined using microscopy analysis. Various heat exposure procedures were applied to the specimens and the changes in the mechanical properties of the top coat were analyzed using indentation techniques. Durability of the coatings after various periods at 1100degC was examined. Microstructures, coatings and reaction products were characterized by optical microscopy and image analysis. Effects of TBC thickness variations were also analyzed in order to achieve a model for determining the optimum coating thickness.
The properties examined as a function of microstructural modification were ultimate tensile stren... more The properties examined as a function of microstructural modification were ultimate tensile strength, fracture elongation, Vickers hardness and wear resistance. The microstructural modification was achieved by rapid cooling and additions of small amounts of strontium and lithium master alloys into the eutectic melt. In all experiments the commercial ETIAL 140 alloy was cast instead of a high-purity aluminium-silicon eutectic. This allowed determination of the effect of modification treatment, both on silicon and intermetallic phases. It was found that the slowly cooled and unalloyed castings which contained coarse silicon flakes showed highest wear resistance and lowest ultimate tensile strength, fracture elongation and Vickers hardness values. Rapid cooling and also additions of strontium and lithium master alloys reduced the eutectic interphase spacing and refined the silicon phase. This usually corresponded to a significant increase in all properties except the wear resistance. It was noted, however, that the size of the intermetallic phase particles increased abruptly above 0.04% Sr content which resulted in a sharp reduction in all mechanical properties. Unlike the strontium effect, the lithium addition did not influence the intermetallic size significantly and, therefore, the mechanical properties were not impaired. In addition, the wear resistance also remained relatively unaffected because lithium solid solution hardened the primary aluminium dendrites appeared in the modified alloys.
The plastic deformation of crystals is usually accomplished through the motion of dislocations. T... more The plastic deformation of crystals is usually accomplished through the motion of dislocations. The glide of a dislocation is impelled by the applied stress and opposed by microstructural defects such as point defects, voids, pre cipitates and other dislocations.
The zinc-aluminium alloys containing 8, 1 2, and 27% aluminium are finding increasing application... more The zinc-aluminium alloys containing 8, 1 2, and 27% aluminium are finding increasing applications in the casting industry. These alloys are stronger than most aluminium alloys. In addition, they possess high wear resistance and bearing properties. However, surface sinks and shrinkage defects are observed on the bottom faces of such castings, contrary to general foundry practice. In the present investigation, this problem observed in the Zn-8%AI, Zn-12%AI, Zn-27%AI alloys was tackled by controlling various casting parameters and also by additions of the master alloys of strontium and lithium into the molten alloys. It was found that the underside shrinkage problem was influenced by the aluminium content of the alloy, melt superheat, casting size and cooling conditions'. The strontium and lithium additions were found to be beneficial in reducing the underside shrinkage problem. The ultimate tensile strength, fracture elongation and Vickers hardness were all increased with aluminium concentration and lithium addition. It was found also that the most problematical Zn-27%AI alloy, which provided the highest mechanical properties, was very suitable for the squeezecasting technique. The mechanical properties were increased sharply in these squeeze-cast bars.
Synthesis of hydroxyapatite (HAP) nano strips was carried out by chemical precipitation method fo... more Synthesis of hydroxyapatite (HAP) nano strips was carried out by chemical precipitation method followed by microwave irradiation. The microwave assisted reactions proceed at fast rates. It is found that the presence of the complex reagent EDTA plays an important role in the morphological changes of nanostructure hydroxyapatite. EDTA acts as a hexadentate unit by wrapping itself around the Ca2+ metal ion with, four oxygen and two nitrogen atoms and forms several five member chelate rings. The relative specific surface energies associated with the facets of the crystal determines the shape of the crystal. Scanning electron microscopy revealed the presence of hydroxyapatite nano strips with the range 50‐100 nm in EDTA influenced HAP powders. Fourier transform‐infrared spectroscopy (FT‐IR) result combined with the X‐ray diffraction (XRD) indicates the presence of amorphous hydroxyapatite (HAP) in the as‐prepared material. X‐ray patterns collected on the powder after heat‐treatment at 11...
In this study, hydroxylapatite (HAp) reinforced polymer composites were produced and evaluated to... more In this study, hydroxylapatite (HAp) reinforced polymer composites were produced and evaluated to be used in bone implants. The aim was to produce a material that has similar modulus of elasticity (E) and other mechanical properties to those of bone in order to achieve mechanical compatibility in the body. Hydroxylapatite was mixed with high density polyethylene (HDPE) and polypropylene (PP) in different amounts. The resulting composite materials were injection molded into standard tensile and impact test specimens and their mechanical properties were examined. Stiffness and hardness of the composites were increased toughness, percent elongation and impact energy were decreased with increasing HAp content. No significant change was observed in ultimate tensile strength while an increase was significant in the yield strength related to HAp content. PP composites were found to have better impact properties in spite of more ductile behavior of HDPE composites under tension. Although the problem of mechanical compatibility in bone implants has not totally been solved, hydroxylapatite reinforced polymer composites are suggested as promising alternatives to traditional implant materials.
Proceedings of the 2nd World Congress on Recent Advances in Nanotechnology, Apr 1, 2017
In tissue engineering, natural/synthetic polymer based fibrous composite scaffolds obtained via e... more In tissue engineering, natural/synthetic polymer based fibrous composite scaffolds obtained via electrospinning method were shown to support the cell adhesion and tissue regeneration. However, electrospinning of natural polymers requires the use of toxic solvents that are negatively affecting the cell proliferation and biocompatibility of the produced scaffolds in addition to the usage of acidic solvents which will result in massive biodegradation inside the body[1] .Here, a method was proposed that is higher safety for the patient and even for the experimentalists who are using harmful and highly volatile solvents. Two types of polymers were used in the synthesis of the scaffolds by mimicking the key features of the tissue extracellular matrix which contains gelatin and coaxially organized nanofibers. Coaxial electrospinning technique was used to obtain core(PVA)-shell (gelatin) nanofibers. While material in the shell provides recognition sites for the tissue cells, core material provides mechanical endurance. Different from conventional methods, proposed work aims to lower the steps of application of the scaffold to the harmed tissue by using only deionized water as solvent. Instead of dissolving PVA and gelatin in toxic and acidic solvents, they were dissolved in the deionized water above the gelation temperature. Later, coaxial electrospinning generated increased cell spread and mechanical stiffness. The samples were characterized by scanning and transmission electron microscopy. Based on the experimental results it is concluded that electrospun fibers obtained from the 8% concentrated gelatin solution had a beaded structure, whereas the coaxially fabricated PVA and gelatin from the same concentration solutions did not show any beaded morphology. Also core-shell fibers have diameters down to 180 nm. This result showed that PVA aids to the uniform gelatin fiber formation which, may give higher mechanical stability. The electron microscopy analysis leading to these results has received support by the
The deformation behavior of an idealized crystal made by stacking of parallel slip planes is stud... more The deformation behavior of an idealized crystal made by stacking of parallel slip planes is studied. Each slip plane is assumed to contain active sources of dislocations leading to a constant density of non-interacting dislocations in the plane which glide through randomly distributed localized point obstacles, representing small precipitates. The dislocation is assumed to have a constant line tension and the dislocation-obstacle interaction is taken to have a simple step form. Using results of computer simulation of thermally activated glide through random arrays of point obstacles the deformation is modeled as a function of temperature and applied stress, determining the strain rate and the morphological characteristics of slip. Stress-strain rate and flow stress-temperature relations are discussed. * Let the dislocation under the applied stress T encounter a configuration of point obstacles denoted by i (Figure 1). Between two adjacent
The plastic deformation of crystals is usually accomplished through the motion of dislocations. T... more The plastic deformation of crystals is usually accomplished through the motion of dislocations. The glide of a dislocation is impelled by the applied stress and opposed by microstructural defects such as point defects, voids, pre cipitates and other dislocations. The planar glide of a dislocation through randomly distributed obstacles is considered. The objective of the present research work is to calculate the critical resolved shear stress (CRSS) for athermal glide and the velocity of the disloca tion at finite tempereature as a function of the applied stress and the nature and strength of the obstacles. Dislocation glide through mixtures of obstacles has been studied analyti cally and by computer simulation. Arrays containing two kinds of obstacles as well as square distribution of obstacle strengths are considered. The critical resolved shear stress for an array containing obstacles with a given distribution -ivof strengths is calculated using the sum of the quadratic mean of the stresses for the individual obstacles and is found to be in good agreement with the com puter simulation data. Computer simulation of dislocation glide through randomly distributed obstacles containing up to 10 6 obstacles show that the CRSS decreases as the size of the array increases and approaches a limiting value. Histograms of forces and of segment lengths are obtained and compared with theoretical predictions. Effects of array shape and boundary conditions on the dislocation glide are also studied. Analytical and computer simulation results are compared with experimen tal results obtained on precipitation-, irradiation-, forest-, and impurity clusterhardening systems and are found to be in good agreement.
We consider the glide of a single dislocation through randomly distributed obstacles of given pro... more We consider the glide of a single dislocation through randomly distributed obstacles of given properties. Using a model we studied the critical resolved shear stress for athermal glide and the velocity of thermally activated glide at finite temperature as functions of the nature and strength of the obstacles. Results obtained from computer simulation are compared with the available experimental data and good agreement is found. Zusammenfassung-Die Gleitung einer einzigen Versetzung durch eine zuf5llige Anordnung von Hindernissen mit vorgegebenen Eigenschaften wird behandelt. Anhand eines Modelles wird die kritische FlieBspannung fiir athermische Gleitung und die Geschwindigkeit der thermisch aktivierten Gleitung bei endlicher Temperatur in Abhlngigkeit von Natur und StLke der Hindernisse untersucht. Die aus der Computersimulation erhaltenen Ergebnisse werden mit den verfiigbaren experimentallen Daten verglichen; es ergibt sich gute iibereinstimmung.
... 1300 2.48 ± 0.08 65.01 ± 41.57 130.68 ±17.90 Fig.3 ESEM micrograph of HAp with PVA binder by ... more ... 1300 2.48 ± 0.08 65.01 ± 41.57 130.68 ±17.90 Fig.3 ESEM micrograph of HAp with PVA binder by 1%, sintered at 1200 o C, 4h. ... [4] AC Tas, F. Korkusuz, M. Timucin and N. Akkas: Journal of Materials Science: Materials in Medicine, Vol. 8 (1997), p. 91. ...
In composite production, the shortest route is via an in situ composite in which a melt dissociat... more In composite production, the shortest route is via an in situ composite in which a melt dissociates simultaneously into two rather different solid phases. The monotectic alloys can be included in this group. The present work was aimed at extending our recent squeeze casting experience on the Zn-Bi monotectic alloy in order to increase its cast quality and mechanical properties. A squeeze casting unit was built, and its die and punch were machined. The molten monotectic alloy was squeezed in this unit under pressures up to 120 MPa in its freezing range until it solidified completely. It was found that an increase in squeeze casting pressure provided increases in density, tensile strength, and Vickers hardness, which resulted in decreases in chip length and electrical resistivity. Before the squeeze casting practice, the freezing characteristics of this monotectic were estimated using basic solidification principles.
The application and effects of squeeze casting were investigated on the density, Brinell hardness... more The application and effects of squeeze casting were investigated on the density, Brinell hardness, tensile strength and fracture elongation values of the wide freezing range Zn-27%A1, A1-20%Si, A1-7%Si, AI-10%Mg, AI-5%Mg, AI-4.5%Cu-0.5%Bi and Fe-3%C-2%Si alloys. It was found that these alloys as a group were suitable for the squeeze casting practice. Sharp increases could be achieved in all mechanical properties due to the reductions in porosity levels. In general, higher squeezing pressures provided better mechanical properties. The trend in the A1-20%Si alloy however, was sporadic and cracking occurred in the vicinity of silicon rich primary particles above 100 MPa pressure. An optimum melt, and die and punch temperatures were found as 1500 °C and 1150 °C, respectively to suppress white cast iron formation in the Fe-3%C-2%Si alloy. The AI-10%Mg and A1-4.5%Cu-0.5%Bi alloys suffered microsegregation in addition to microporosity. Their squeeze cast samples showed higher hardness and tensile properties after annealing them. The machinabilities of the two alloys however, were degraded due to squeeze casting.
Journal of the mechanical behavior of materials, Mar 1, 1993
In this study, the effects of temperature and strain rate on the deformation behavior of aluminiu... more In this study, the effects of temperature and strain rate on the deformation behavior of aluminium 1100 and aluminium 2024 are investigated. Specimens are tested at different temperatures and strain rates under the effect of torsional loading. The data are evaluated using critical radius and effective length concepts. For both materials, as a result of the interaction between strain hardening and thermal softening mechanisms, flow curves pass through a peak value after which aluminium alloy tends to fracture while aluminium 1100 approaches to a steady state deformation. The strain rate sensitivity of materials obtained from torsion testing increases with temperature. The length of aluminium specimens subjected to free-end hot torsion is observed to change with both temperature and strain rate.
The structural strength of materials generally depends on the materials properties, the shape and... more The structural strength of materials generally depends on the materials properties, the shape and size of defects as well as the orientation of such flaws in the medium. In the application of fracture mechanics other kinds of imperfections due to manufacturing are particularly important such as flat cavities which develop during casting, small cracks resulting from residual stresses in welded materials, and fatigue cracks under fluctuating external loads. Thus in a design dealing with such materials, it is necessary to have a good estimate of the stress state disturbed by the existence of these flaws.
Three different models for a pinned dislocation under an applied stress are compared in this pape... more Three different models for a pinned dislocation under an applied stress are compared in this paper: (i) the continuous string model of Koehler, Granato and Liicke; (ii) a twodimensional lattice model which represents the dislocation in its slip plane; (iii) a discrete string model which is obtained as an analytical approximation to model (ii). Good agreement is obtained for the quantities investigated for the three models. Comparison of models (i) and (iii) therefore provides an atomistic interpretation for the parameters appearing in the defining equation of the continuous string model. One result of interest is that these parameters cannot be expressed solely in terms of the characteristics of the interatomic law for atoms in the vicinity of their equilibrium sites for the perfect crystal but must also depend on aspects of the interatomic force law for atoms in configurations such as those which occur near the dislocation.
Recently, application of ceramic coatings to space vehicles to generate thermal barriers upon re-... more Recently, application of ceramic coatings to space vehicles to generate thermal barriers upon re-entry is being considered as a challenge for research. Conventionally, thermal barrier coatings (TBC) are utilized to increase the gas turbine engine components' thermal and oxidation resistance values in order to get higher power and efficiency values. In this study, various superalloys and stainless steels were thermal barrier coated using the atmospheric plasma spraying method. One goal is to determine the effect of the substrate on the performance of the coating. Coating morphology was determined using microscopy analysis. Various heat exposure procedures were applied to the specimens and the changes in the mechanical properties of the top coat were analyzed using indentation techniques. Durability of the coatings after various periods at 1100degC was examined. Microstructures, coatings and reaction products were characterized by optical microscopy and image analysis. Effects of TBC thickness variations were also analyzed in order to achieve a model for determining the optimum coating thickness.
The properties examined as a function of microstructural modification were ultimate tensile stren... more The properties examined as a function of microstructural modification were ultimate tensile strength, fracture elongation, Vickers hardness and wear resistance. The microstructural modification was achieved by rapid cooling and additions of small amounts of strontium and lithium master alloys into the eutectic melt. In all experiments the commercial ETIAL 140 alloy was cast instead of a high-purity aluminium-silicon eutectic. This allowed determination of the effect of modification treatment, both on silicon and intermetallic phases. It was found that the slowly cooled and unalloyed castings which contained coarse silicon flakes showed highest wear resistance and lowest ultimate tensile strength, fracture elongation and Vickers hardness values. Rapid cooling and also additions of strontium and lithium master alloys reduced the eutectic interphase spacing and refined the silicon phase. This usually corresponded to a significant increase in all properties except the wear resistance. It was noted, however, that the size of the intermetallic phase particles increased abruptly above 0.04% Sr content which resulted in a sharp reduction in all mechanical properties. Unlike the strontium effect, the lithium addition did not influence the intermetallic size significantly and, therefore, the mechanical properties were not impaired. In addition, the wear resistance also remained relatively unaffected because lithium solid solution hardened the primary aluminium dendrites appeared in the modified alloys.
The plastic deformation of crystals is usually accomplished through the motion of dislocations. T... more The plastic deformation of crystals is usually accomplished through the motion of dislocations. The glide of a dislocation is impelled by the applied stress and opposed by microstructural defects such as point defects, voids, pre cipitates and other dislocations.
The zinc-aluminium alloys containing 8, 1 2, and 27% aluminium are finding increasing application... more The zinc-aluminium alloys containing 8, 1 2, and 27% aluminium are finding increasing applications in the casting industry. These alloys are stronger than most aluminium alloys. In addition, they possess high wear resistance and bearing properties. However, surface sinks and shrinkage defects are observed on the bottom faces of such castings, contrary to general foundry practice. In the present investigation, this problem observed in the Zn-8%AI, Zn-12%AI, Zn-27%AI alloys was tackled by controlling various casting parameters and also by additions of the master alloys of strontium and lithium into the molten alloys. It was found that the underside shrinkage problem was influenced by the aluminium content of the alloy, melt superheat, casting size and cooling conditions'. The strontium and lithium additions were found to be beneficial in reducing the underside shrinkage problem. The ultimate tensile strength, fracture elongation and Vickers hardness were all increased with aluminium concentration and lithium addition. It was found also that the most problematical Zn-27%AI alloy, which provided the highest mechanical properties, was very suitable for the squeezecasting technique. The mechanical properties were increased sharply in these squeeze-cast bars.
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