Mehmet Kaya | Adıyaman University (original) (raw)

Papers by Mehmet Kaya

This work deals with diffusion bonded joints between Ti–6Al–4V alloy and inter-stitial free (IF) ... more This work deals with diffusion bonded joints between Ti–6Al–4V alloy and inter-stitial free (IF) steel at different temperatures under a pressure of 5 MPa for 30 min. The effect of bonding temperature on the microstructure and mechanical properties of the joint interface was investigated using optic microscopy, a scanning electron microscope (SEM) equipped with X-ray energy dispersive spectrometer (EDS) and shear strength measurements. The intermetallic phases such as FeTi and Fe 2 Ti occurred at the interface of bonded specimens. In addition, it was seen that shear strength of bonded specimens decreased with increasing temperature due to growing intermetallics.

ÖZET Farklı gözenek oranlarına sahip gözenekli NiTi alaşımı, implant malzeme olarak kullanılmak a... more ÖZET Farklı gözenek oranlarına sahip gözenekli NiTi alaşımı, implant malzeme olarak kullanılmak amacıyla, toz metalürji yöntemlerinden kendi-ilerleyen yüksek-sıcaklık sentezi (SHS: self-propagating high-temperature synthesis) ile üretildi. Üretimde, Ni-%49,5Ti atomik oranında Ni ve Ti tozları 24 saat süreyle karıştırılarak farklı basınçlarda soğuk preslendi. Preslenen ham gözenekli numuneler hazırlanan bir fırın içerisinde soygaz ortamında 200 °C'de ön ısıtma yapılarak ve yüksek voltajla tutuşturularak sentezlendi. Sonuç olarak, farklı gözenek şekillerine sahip açık ve kapalı gözeneklerin meydana geldiği görülmüştür. Ayrıca presleme basıncının artması ile gözenek oranın azaldığı ve basma dayanımlarının arttığı belirlendi. ABSTRACT Porous NiTi alloy samples to be used as implant material were synthesized in different porosities through a powder metallurgical method, self-propagating high-temperature (SHS) synthesis. The powder blend composing Ni and at%49.5Ti was mixed 24 hours and cold pressed under different pressures. The green compacts were heated in a furnace up to 200 °C under a high purity argon gas protective atmosphere and then ignited by applying a high electrical voltage. It was found that open and closed cell pores formed in different percentages and shapes. In addition; increasing cold compaction pressure applied reduced the porosity levels and increased the compressive strength of the porous compacts synthesized.

The K ˛,ˇ shell production cross-sections and K ˇ /K ˛ intensity ratios of porous Ni −49 at% Ti s... more The K ˛,ˇ shell production cross-sections and K ˇ /K ˛ intensity ratios of porous Ni −49 at% Ti shape memory alloys were determined using energy dispersive X-ray fluorescence (EDXRF) technique. Also, the alloying effect on the K shell fluorescence parameters was investigated. The samples were excited by 59.5 keV-rays from an 241 Am annular radioactive source. The K X-rays emitted by the samples were counted by an Ultra-LEGe detector with a resolution of 150 eV at 5.9 keV. The structure analyses of the samples were also made using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The deviations between the present results and theoretical values, calculated for pure Ti and Ni, were attributed to charge transfer phenomena and/or rearrangement of valance shell electrons and porosity.

This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright a b s t r a c t In this study, a martensitic stainless steel plate of 10 mm thick was welded by keyhole plasma transfer arc (PTA) welding technique both itself and using austenitic stainless steel plate of 2 mm thick in interface. So, penetration depth, microstructure, mechanical behavior and interface layer effects of keyhole plasma welding were investigated. After keyhole plasma welding, microstructural analysis including metallographic examination, and notch impact strength tests were performed. From the results, it was seen that with this technique can be obtained penetration depth of 8 mm without any pretreatment of welding. Also, austenitic stainless steel interface layer was raised impact strength of welding.

In this study, the Ti–6Al–4V alloy was diffusion bonded to austenitic stainless steel at temperat... more In this study, the Ti–6Al–4V alloy was diffusion bonded to austenitic stainless steel at temperatures of 820, 885, 930 and 980uC, under a pressure of 5 MPa for 30 min. The effect of temperature on interface formations and microstructure was investigated using a scanning electron microscope (SEM), energy dispersive spectrograph (EDS) microanalyses, X-ray diffraction and shear strength of bonded specimens. The results showed that intermetallic phases and s-phase formed in the interface region.

In this study, Ni and Ti with 50.5 at.% Ni powders were blended for 12 h and cold pressed in the ... more In this study, Ni and Ti with 50.5 at.% Ni powders were blended for 12 h and cold pressed in the different pressures (50, 75 and 100 MPa). Then, the porous NiTi alloy compacts obtained were synthesized by SHS (self-propagating high-temperature synthesis) at the different preheating temperatures (200, 250 and 300 • C) and heating rates (30, 60 and 90 • C/min). The effects of the pressure, preheating temperature and heating rate were investigated on the porosity and the microstructure. NiTi was seen as the dominant phase in the microstructure with other secondary intermetallic compounds. The porosity of the synthesized products was in the range of 50.7–59.7 vol.%.

In this study, a new solution treatment " solution treatment under loading " was applied to a por... more In this study, a new solution treatment " solution treatment under loading " was applied to a porous Ni–50 at.%Ti shape memory alloy (SMA), which was fabricated by self-propagating high-temperature synthesis (SHS), to explore the micro-structural improvement regarding single-phase NiTi. The effects of solution treatment under loading and without loading on the phase constituent and thermal conductivity were investigated and discussed. The phase constituent and thermal conductivity of the specimens considerably changed with solution treatment under loading, but they were not affected significantly with solution treatment without loading. Intermetallic phases such as B19 (NiTi), Ti 2 Ni, and Ni 4 Ti 3 disappeared, the density of the B2(NiTi) phase increased with solution treatment under loading, and thus the thermal conductivity was increased. It was also seen that the thermal conductivity of porous NiTi was less than that of solid NiTi.

Porous NiTi shape memory alloy was fabricated by self-propagating high temperature synthesis. Eff... more Porous NiTi shape memory alloy was fabricated by self-propagating high temperature synthesis. Effects of solution treatment under load applied on the microstructure were investigated. The densities of the phases changed insignificantly with solution treatment but the intermetallic phases such as Ti 2 Ni, Ni 4 Ti 3 and Ni 3 Ti 2 disappeared and the density of B2(NiTi) phase increased with the load applied during solution treatment. Consequently, porous NiTi SMA with ideal pore characteristics, high chemical homogeneity and high strength for hard tissue implants was obtained.

Shape memory behavior of porous NiTi alloy is dependent on the phases, and mechanical or thermal ... more Shape memory behavior of porous NiTi alloy is dependent on the phases, and mechanical or thermal background. The phases change with solution heat treatment and aging. Fully reversible shape memory behavior was observed during thermal cycling, and recoverable strains increased with the increasing stress from 2 to 50 MPa. The porous NiTi sample shows recoverable transformation strain response under lower constant load.

—Polycrystalline FeMnAlNi shape memory alloys were recently shown to possess a small temperature ... more —Polycrystalline FeMnAlNi shape memory alloys were recently shown to possess a small temperature dependence of the stress for the onset of martensitic transformation (r SIM). In this work, the superelastic behavior of single crystalline Fe 43.5 Mn 34 Al 15 Ni 7.5 samples oriented along the [1 0 0] direction was investigated under tension and compression after a precipitation heat treatment at 200 °C. In constant strain, multi-temperature experiments, the single crystals showed a r SIM vs. temperature slope of 0.54 MPa °C À1 in tension and 0.41 MPa °C À1 in compression, and supere-lasticity over a wide temperature range from À80 °C to 160 °C. The irrecoverable strains in both tension and compression samples detected during the superelastic experiments were found to be due to retained martensite in detailed transmission electron microscopy investigations. The volume fraction of the retained martensite in the samples tested under tension was considerably larger than those for the samples tested in compression showing that the transformation is less recoverable in tension. The differences in the volume fraction of retained martensite and reversibility in both tension and compression are attributed to high density of dislocations in the tension samples as compared to the compression samples. The differences between the shape of the stress–strain curves under tension and compression are attributed to the lower number of martensite variants activated under tension as compared to compression, which were clearly verified with the transmission electron microscopy examinations.

A B S T R A C T It is known that shape memory and phase transition response of NiTi shape memory ... more A B S T R A C T It is known that shape memory and phase transition response of NiTi shape memory alloy (SMA) is dependent on mechanical or thermal background, and the presence of phases such as B2(NiTi), B19'(NiTi), Ni 4 Ti 3, Ni 3 Ti 2 and NiTi 2. The volume fraction of metastable Ni 4 Ti 3 phase in NiTi matrix can be easily changed by heat treatments. Both Ni 4 Ti 3 phases and Ni ingredient affect the phase transition kinetics and the shape memory response. This work deals with the effect of thermomechanical cycling on the shape memory response of a porous Ni 50.8 Ti 49.2 SMA, which is homogenized at 1050 C and aged at 800 C, afterwards slow cooled. Full recoverable shape memory response and cyclic stability were achieved during thermomechanical cycling under a compressive stress of 5 MPa.

The electronic structures of NieTi shape-memory alloy samples were investigated by X-ray absorpti... more The electronic structures of NieTi shape-memory alloy samples were investigated by X-ray absorption fine structure (XAFS) spectroscopy both experimentally and theoretically. In the experimental section, the samples were measured at low temperature to determine the persistent traces of both preheating process and atomic concentration effects on the crystal and electronic structure by X-ray absorption near-edge structure (XANES) spectroscopy. As a second step, the extended-X-ray absorption fine structure (EXAFS) calculations, which are based on different choices of one electron potentials according to Ti coordinations by using the real space multiple scattering method FEFF 8.2 code, were performed. The crystallographic and electronic structures of the porous NieTi alloys were tested at various temperatures ranging from 5 to 1323 K.

Porous NiTi shape memory alloy (SMA) was fabricated by self-propagating high-temperature synthesi... more Porous NiTi shape memory alloy (SMA) was fabricated by self-propagating high-temperature synthesis (SHS). With this study, a new solution treatment "solution treatment under loading" was applied to porous NiTi SMA fabricated by SHS to determine microstructural improvement regarding single phase NiTi. The effect of solution treatment under load on chemical composition, constituent phases and phase transformation behaviors of the specimens was investigated and discussed. The chemical composition of the specimens considerably changed with solution treatment under loading. Intermetallic phases such as Ti 2 Ni and Ni 4 Ti 3 disappeared, the density of B2(NiTi) phase increased and phase transformation temperatures sharply decreased. Porous single phase B2(NiTi) SMA with high chemical homogeneity could be obtained by the load applied during solution treatment at 1050 • C.

GaAs/Si(001) thin films with six Si interlayers grown by molecular beam epitaxy (MBE) were examin... more GaAs/Si(001) thin films with six Si interlayers grown by molecular beam epitaxy (MBE) were examined by transmission electron microscopy (TEM). The interface structure of the films was investigated by using both plan-view and cross-sectional TEM specimens. It was seen that the films have a smooth interface associating with misfit and threading dislocations between the GaAs epilayer and the Si substrate. It was also observed that the Si interlayers had an undulated shape reducing the threading dislocations in the structure. It was obtained that the strain relaxation at the interface occurs by misfit and threading dislocations. The g · b contrast experiments displayed 60 • -type misfit dislocations with a/2 110 {111} slip system in the structure. Electron diffraction experiments were also performed to find out the reduction of the lattice mismatch and the strain left in the structure using the cross-sectional GaAs/Si(001) samples.

Porous single phase Al12Mg17 alloy was fabricated by cooling in to water after powder metallurgy ... more Porous single phase Al12Mg17 alloy was fabricated by cooling in to water after powder metallurgy sintering for use as hydrogen storage fuel cell. Because Al12Mg17 phase has more theoretical hydrogen storage capacity than the phases occurred such as Al3Mg2 and R(AlMg) in microstructure of the alloy. The pore morphologies, microstructure and phases were investigated. It was seen that phases such as Al12Mg17, Al3Mg2 and R(AlMg) occur in the microstructure of the sample fabricated by powder metallurgy sintering. The formation of Al3Mg2 and R(AlMg) phases was blocked by speed quenched into water after sintering and single phase Al12Mg17 was obtained.

Porous NiTi shape memory alloys were fabricated as hollow and solid cylinders by selfpropagating ... more Porous NiTi shape memory alloys were fabricated as hollow and solid cylinders by selfpropagating high temperature synthesis for use as implant material. The green compacted specimens were synthesised with different ignition technical, high voltage electric arcs. The pore morphologies, microstructures and phase transformation behaviours of the synthesised products at different preheating temperatures were studied. It was seen that the B2(NiTi) and B199(NiTi) phases were predominant phases. It was also seen that porosity decreased with the increase in preheating temperature and the orientation of the combustion channels was affected by the ignition location. Phase transformation temperatures and activation energies were not changed, but the changes at enthalpy and entropy increased with increasing porosity.

Porous NiTi shape memory alloy (SMA) with ideal porosity and high compressive strength as an impl... more Porous NiTi shape memory alloy (SMA) with ideal porosity and high compressive strength as an implant material was fabricated by self-propagating high-temperature synthesis (SHS). In this study, a new ignition technique ''high voltage electric arc" was used to ignite the green specimens and control the orientation of combustion channels which effect compressive strength. It was determined that the compressive strength of specimens was increased when the combustion channels were parallel along the specimen axis, and the compressive strength was decreased when the combustion channels were perpendicular to specimen axis. The desired phases such as B2(NiTi) and B19 0 (NiTi) were dominant while the second phases (Ni 4 Ti 3 and NiTi 2 ) in small amount. The undesired phases (such as pure Ni and Ni 3 Ti) for biocompatibility are not found in the structure. The transformation temperatures were higher for medical applications by heat treatment and partly decreased at every next thermal cycle where the heating rate of the specimen was increased.

In this study, a new solution treatment "solution treatment under loading" was applied to a porou... more In this study, a new solution treatment "solution treatment under loading" was applied to a porous Ni-50 at.%Ti shape memory alloy (SMA), which was fabricated by self-propagating high-temperature synthesis (SHS), to explore the microstructural improvement regarding single-phase NiTi. The effects of solution treatment under loading and without loading on the phase constituent and thermal conductivity were investigated and discussed. The phase constituent and thermal conductivity of the specimens considerably changed with solution treatment under loading, but they were not affected significantly with solution treatment without loading. Intermetallic phases such as B19 (NiTi), Ti 2 Ni, and Ni 4 Ti 3 disappeared, the density of the B2(NiTi) phase increased with solution treatment under loading, and thus the thermal conductivity was increased. It was also seen that the thermal conductivity of porous NiTi was less than that of solid NiTi.

This work deals with diffusion bonded joints between Ti–6Al–4V alloy and inter-stitial free (IF) ... more This work deals with diffusion bonded joints between Ti–6Al–4V alloy and inter-stitial free (IF) steel at different temperatures under a pressure of 5 MPa for 30 min. The effect of bonding temperature on the microstructure and mechanical properties of the joint interface was investigated using optic microscopy, a scanning electron microscope (SEM) equipped with X-ray energy dispersive spectrometer (EDS) and shear strength measurements. The intermetallic phases such as FeTi and Fe 2 Ti occurred at the interface of bonded specimens. In addition, it was seen that shear strength of bonded specimens decreased with increasing temperature due to growing intermetallics.

ÖZET Farklı gözenek oranlarına sahip gözenekli NiTi alaşımı, implant malzeme olarak kullanılmak a... more ÖZET Farklı gözenek oranlarına sahip gözenekli NiTi alaşımı, implant malzeme olarak kullanılmak amacıyla, toz metalürji yöntemlerinden kendi-ilerleyen yüksek-sıcaklık sentezi (SHS: self-propagating high-temperature synthesis) ile üretildi. Üretimde, Ni-%49,5Ti atomik oranında Ni ve Ti tozları 24 saat süreyle karıştırılarak farklı basınçlarda soğuk preslendi. Preslenen ham gözenekli numuneler hazırlanan bir fırın içerisinde soygaz ortamında 200 °C'de ön ısıtma yapılarak ve yüksek voltajla tutuşturularak sentezlendi. Sonuç olarak, farklı gözenek şekillerine sahip açık ve kapalı gözeneklerin meydana geldiği görülmüştür. Ayrıca presleme basıncının artması ile gözenek oranın azaldığı ve basma dayanımlarının arttığı belirlendi. ABSTRACT Porous NiTi alloy samples to be used as implant material were synthesized in different porosities through a powder metallurgical method, self-propagating high-temperature (SHS) synthesis. The powder blend composing Ni and at%49.5Ti was mixed 24 hours and cold pressed under different pressures. The green compacts were heated in a furnace up to 200 °C under a high purity argon gas protective atmosphere and then ignited by applying a high electrical voltage. It was found that open and closed cell pores formed in different percentages and shapes. In addition; increasing cold compaction pressure applied reduced the porosity levels and increased the compressive strength of the porous compacts synthesized.

The K ˛,ˇ shell production cross-sections and K ˇ /K ˛ intensity ratios of porous Ni −49 at% Ti s... more The K ˛,ˇ shell production cross-sections and K ˇ /K ˛ intensity ratios of porous Ni −49 at% Ti shape memory alloys were determined using energy dispersive X-ray fluorescence (EDXRF) technique. Also, the alloying effect on the K shell fluorescence parameters was investigated. The samples were excited by 59.5 keV-rays from an 241 Am annular radioactive source. The K X-rays emitted by the samples were counted by an Ultra-LEGe detector with a resolution of 150 eV at 5.9 keV. The structure analyses of the samples were also made using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The deviations between the present results and theoretical values, calculated for pure Ti and Ni, were attributed to charge transfer phenomena and/or rearrangement of valance shell electrons and porosity.

This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright a b s t r a c t In this study, a martensitic stainless steel plate of 10 mm thick was welded by keyhole plasma transfer arc (PTA) welding technique both itself and using austenitic stainless steel plate of 2 mm thick in interface. So, penetration depth, microstructure, mechanical behavior and interface layer effects of keyhole plasma welding were investigated. After keyhole plasma welding, microstructural analysis including metallographic examination, and notch impact strength tests were performed. From the results, it was seen that with this technique can be obtained penetration depth of 8 mm without any pretreatment of welding. Also, austenitic stainless steel interface layer was raised impact strength of welding.

In this study, the Ti–6Al–4V alloy was diffusion bonded to austenitic stainless steel at temperat... more In this study, the Ti–6Al–4V alloy was diffusion bonded to austenitic stainless steel at temperatures of 820, 885, 930 and 980uC, under a pressure of 5 MPa for 30 min. The effect of temperature on interface formations and microstructure was investigated using a scanning electron microscope (SEM), energy dispersive spectrograph (EDS) microanalyses, X-ray diffraction and shear strength of bonded specimens. The results showed that intermetallic phases and s-phase formed in the interface region.

In this study, Ni and Ti with 50.5 at.% Ni powders were blended for 12 h and cold pressed in the ... more In this study, Ni and Ti with 50.5 at.% Ni powders were blended for 12 h and cold pressed in the different pressures (50, 75 and 100 MPa). Then, the porous NiTi alloy compacts obtained were synthesized by SHS (self-propagating high-temperature synthesis) at the different preheating temperatures (200, 250 and 300 • C) and heating rates (30, 60 and 90 • C/min). The effects of the pressure, preheating temperature and heating rate were investigated on the porosity and the microstructure. NiTi was seen as the dominant phase in the microstructure with other secondary intermetallic compounds. The porosity of the synthesized products was in the range of 50.7–59.7 vol.%.

In this study, a new solution treatment " solution treatment under loading " was applied to a por... more In this study, a new solution treatment " solution treatment under loading " was applied to a porous Ni–50 at.%Ti shape memory alloy (SMA), which was fabricated by self-propagating high-temperature synthesis (SHS), to explore the micro-structural improvement regarding single-phase NiTi. The effects of solution treatment under loading and without loading on the phase constituent and thermal conductivity were investigated and discussed. The phase constituent and thermal conductivity of the specimens considerably changed with solution treatment under loading, but they were not affected significantly with solution treatment without loading. Intermetallic phases such as B19 (NiTi), Ti 2 Ni, and Ni 4 Ti 3 disappeared, the density of the B2(NiTi) phase increased with solution treatment under loading, and thus the thermal conductivity was increased. It was also seen that the thermal conductivity of porous NiTi was less than that of solid NiTi.

Porous NiTi shape memory alloy was fabricated by self-propagating high temperature synthesis. Eff... more Porous NiTi shape memory alloy was fabricated by self-propagating high temperature synthesis. Effects of solution treatment under load applied on the microstructure were investigated. The densities of the phases changed insignificantly with solution treatment but the intermetallic phases such as Ti 2 Ni, Ni 4 Ti 3 and Ni 3 Ti 2 disappeared and the density of B2(NiTi) phase increased with the load applied during solution treatment. Consequently, porous NiTi SMA with ideal pore characteristics, high chemical homogeneity and high strength for hard tissue implants was obtained.

Shape memory behavior of porous NiTi alloy is dependent on the phases, and mechanical or thermal ... more Shape memory behavior of porous NiTi alloy is dependent on the phases, and mechanical or thermal background. The phases change with solution heat treatment and aging. Fully reversible shape memory behavior was observed during thermal cycling, and recoverable strains increased with the increasing stress from 2 to 50 MPa. The porous NiTi sample shows recoverable transformation strain response under lower constant load.

—Polycrystalline FeMnAlNi shape memory alloys were recently shown to possess a small temperature ... more —Polycrystalline FeMnAlNi shape memory alloys were recently shown to possess a small temperature dependence of the stress for the onset of martensitic transformation (r SIM). In this work, the superelastic behavior of single crystalline Fe 43.5 Mn 34 Al 15 Ni 7.5 samples oriented along the [1 0 0] direction was investigated under tension and compression after a precipitation heat treatment at 200 °C. In constant strain, multi-temperature experiments, the single crystals showed a r SIM vs. temperature slope of 0.54 MPa °C À1 in tension and 0.41 MPa °C À1 in compression, and supere-lasticity over a wide temperature range from À80 °C to 160 °C. The irrecoverable strains in both tension and compression samples detected during the superelastic experiments were found to be due to retained martensite in detailed transmission electron microscopy investigations. The volume fraction of the retained martensite in the samples tested under tension was considerably larger than those for the samples tested in compression showing that the transformation is less recoverable in tension. The differences in the volume fraction of retained martensite and reversibility in both tension and compression are attributed to high density of dislocations in the tension samples as compared to the compression samples. The differences between the shape of the stress–strain curves under tension and compression are attributed to the lower number of martensite variants activated under tension as compared to compression, which were clearly verified with the transmission electron microscopy examinations.

A B S T R A C T It is known that shape memory and phase transition response of NiTi shape memory ... more A B S T R A C T It is known that shape memory and phase transition response of NiTi shape memory alloy (SMA) is dependent on mechanical or thermal background, and the presence of phases such as B2(NiTi), B19'(NiTi), Ni 4 Ti 3, Ni 3 Ti 2 and NiTi 2. The volume fraction of metastable Ni 4 Ti 3 phase in NiTi matrix can be easily changed by heat treatments. Both Ni 4 Ti 3 phases and Ni ingredient affect the phase transition kinetics and the shape memory response. This work deals with the effect of thermomechanical cycling on the shape memory response of a porous Ni 50.8 Ti 49.2 SMA, which is homogenized at 1050 C and aged at 800 C, afterwards slow cooled. Full recoverable shape memory response and cyclic stability were achieved during thermomechanical cycling under a compressive stress of 5 MPa.

The electronic structures of NieTi shape-memory alloy samples were investigated by X-ray absorpti... more The electronic structures of NieTi shape-memory alloy samples were investigated by X-ray absorption fine structure (XAFS) spectroscopy both experimentally and theoretically. In the experimental section, the samples were measured at low temperature to determine the persistent traces of both preheating process and atomic concentration effects on the crystal and electronic structure by X-ray absorption near-edge structure (XANES) spectroscopy. As a second step, the extended-X-ray absorption fine structure (EXAFS) calculations, which are based on different choices of one electron potentials according to Ti coordinations by using the real space multiple scattering method FEFF 8.2 code, were performed. The crystallographic and electronic structures of the porous NieTi alloys were tested at various temperatures ranging from 5 to 1323 K.

Porous NiTi shape memory alloy (SMA) was fabricated by self-propagating high-temperature synthesi... more Porous NiTi shape memory alloy (SMA) was fabricated by self-propagating high-temperature synthesis (SHS). With this study, a new solution treatment "solution treatment under loading" was applied to porous NiTi SMA fabricated by SHS to determine microstructural improvement regarding single phase NiTi. The effect of solution treatment under load on chemical composition, constituent phases and phase transformation behaviors of the specimens was investigated and discussed. The chemical composition of the specimens considerably changed with solution treatment under loading. Intermetallic phases such as Ti 2 Ni and Ni 4 Ti 3 disappeared, the density of B2(NiTi) phase increased and phase transformation temperatures sharply decreased. Porous single phase B2(NiTi) SMA with high chemical homogeneity could be obtained by the load applied during solution treatment at 1050 • C.

GaAs/Si(001) thin films with six Si interlayers grown by molecular beam epitaxy (MBE) were examin... more GaAs/Si(001) thin films with six Si interlayers grown by molecular beam epitaxy (MBE) were examined by transmission electron microscopy (TEM). The interface structure of the films was investigated by using both plan-view and cross-sectional TEM specimens. It was seen that the films have a smooth interface associating with misfit and threading dislocations between the GaAs epilayer and the Si substrate. It was also observed that the Si interlayers had an undulated shape reducing the threading dislocations in the structure. It was obtained that the strain relaxation at the interface occurs by misfit and threading dislocations. The g · b contrast experiments displayed 60 • -type misfit dislocations with a/2 110 {111} slip system in the structure. Electron diffraction experiments were also performed to find out the reduction of the lattice mismatch and the strain left in the structure using the cross-sectional GaAs/Si(001) samples.

Porous single phase Al12Mg17 alloy was fabricated by cooling in to water after powder metallurgy ... more Porous single phase Al12Mg17 alloy was fabricated by cooling in to water after powder metallurgy sintering for use as hydrogen storage fuel cell. Because Al12Mg17 phase has more theoretical hydrogen storage capacity than the phases occurred such as Al3Mg2 and R(AlMg) in microstructure of the alloy. The pore morphologies, microstructure and phases were investigated. It was seen that phases such as Al12Mg17, Al3Mg2 and R(AlMg) occur in the microstructure of the sample fabricated by powder metallurgy sintering. The formation of Al3Mg2 and R(AlMg) phases was blocked by speed quenched into water after sintering and single phase Al12Mg17 was obtained.

Porous NiTi shape memory alloys were fabricated as hollow and solid cylinders by selfpropagating ... more Porous NiTi shape memory alloys were fabricated as hollow and solid cylinders by selfpropagating high temperature synthesis for use as implant material. The green compacted specimens were synthesised with different ignition technical, high voltage electric arcs. The pore morphologies, microstructures and phase transformation behaviours of the synthesised products at different preheating temperatures were studied. It was seen that the B2(NiTi) and B199(NiTi) phases were predominant phases. It was also seen that porosity decreased with the increase in preheating temperature and the orientation of the combustion channels was affected by the ignition location. Phase transformation temperatures and activation energies were not changed, but the changes at enthalpy and entropy increased with increasing porosity.

Porous NiTi shape memory alloy (SMA) with ideal porosity and high compressive strength as an impl... more Porous NiTi shape memory alloy (SMA) with ideal porosity and high compressive strength as an implant material was fabricated by self-propagating high-temperature synthesis (SHS). In this study, a new ignition technique ''high voltage electric arc" was used to ignite the green specimens and control the orientation of combustion channels which effect compressive strength. It was determined that the compressive strength of specimens was increased when the combustion channels were parallel along the specimen axis, and the compressive strength was decreased when the combustion channels were perpendicular to specimen axis. The desired phases such as B2(NiTi) and B19 0 (NiTi) were dominant while the second phases (Ni 4 Ti 3 and NiTi 2 ) in small amount. The undesired phases (such as pure Ni and Ni 3 Ti) for biocompatibility are not found in the structure. The transformation temperatures were higher for medical applications by heat treatment and partly decreased at every next thermal cycle where the heating rate of the specimen was increased.

In this study, a new solution treatment "solution treatment under loading" was applied to a porou... more In this study, a new solution treatment "solution treatment under loading" was applied to a porous Ni-50 at.%Ti shape memory alloy (SMA), which was fabricated by self-propagating high-temperature synthesis (SHS), to explore the microstructural improvement regarding single-phase NiTi. The effects of solution treatment under loading and without loading on the phase constituent and thermal conductivity were investigated and discussed. The phase constituent and thermal conductivity of the specimens considerably changed with solution treatment under loading, but they were not affected significantly with solution treatment without loading. Intermetallic phases such as B19 (NiTi), Ti 2 Ni, and Ni 4 Ti 3 disappeared, the density of the B2(NiTi) phase increased with solution treatment under loading, and thus the thermal conductivity was increased. It was also seen that the thermal conductivity of porous NiTi was less than that of solid NiTi.