Showing papers on "Microstructure published in 2009"

Microstructure and mechanical behavior of Ti-6Al-4V produced by rapid-layer manufacturing, for biomedical applications.

Abstract: The microstructure and mechanical behavior of simple product geometries produced by layered manufacturing using the electron beam melting (EBM) process and the selective laser melting (SLM) process are compared with those characteristic of conventional wrought and cast products of Ti-6Al-4V. Microstructures are characterized utilizing optical metallography (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and included alpha (hcp), beta (bcc) and alpha(') (hcp) martensite phase regimes which give rise to hardness variations ranging from HRC 37 to 57 and tensile strengths ranging from 0.9 to 1.45 GPa. The advantages and disadvantages of layered manufacturing utilizing initial powders in custom building of biomedical components by EBM and SLM in contrast to conventional manufacturing from Ti-6Al-4V wrought bar stock are discussed.

Laser aided direct metal deposition of Inconel 625 superalloy: Microstructural evolution and thermal stability

Abstract: Direct metal deposition technology is an emerging laser aided manufacturing technology based on a new additive manufacturing principle, which combines laser cladding with rapid prototyping into a solid freeform fabrication process that can be used to manufacture near net shape components from their CAD files. In the present study, direct metal deposition technology was successfully used to fabricate a series of samples of the Ni-based superalloy Inconel 625. A high power CO2 laser was used to create a molten pool on the Inconel 625 substrate into which an Inconel 625 powder stream was delivered to create a 3D object. The structure and properties of the deposits were investigated using optical and scanning electron microscopy, X-ray diffraction and microhardness test. The microstructure has been found to be columnar dendritic in nature, which grew epitaxially from the substrate. The thermal stability of the dendritic morphology was investigated in the temperature range 800–1200 °C. These studies demonstrate that Inconel 625 is an attractive material for laser deposition as all samples produced in this study are free from relevant defects such as cracks, bonding error and porosity.

Fabrication of artificial Lotus leaves and significance of hierarchical structure for superhydrophobicity and low adhesion

Abstract: The superhydrophobic and self-cleaning leaves of Lotus (Nelumbo nucifera, Gaertn.) have been used as a model for the development of artificial biomimetic surfaces. The hierarchical structure of the Lotus leaf has been recreated to characterize the influence of hierarchical roughness on superhydrophobicity and adhesion. Hierarchical structures were fabricated by a fast and precise molding of the Lotus leaf microstructure, and self-assembly of the natural Lotus wax deposited by thermal evaporation to create the wax tubules nanostructures. Tubule formation was initiated by exposure of the specimens to a solvent vapor phase at a selected temperature. In order to study the influence of structures at different scale sizes on superhydrophobicity, a flat surface, microstructured Lotus leaf replica and a micropatterned Si replica, and a nanostructure were fabricated. Static contact angle, contact angle hysteresis, tilt angle and adhesive forces were measured. The data show that microstructures and nanostructures lead to superhydrophobicity, whereas hierarchical structures further improve this property and show low contact angle hysteresis, superior to that of the natural Lotus leaves.

Densification of ZrB2-based composites and their mechanical and physical properties: A review

Abstract: This study reviews densification behaviour, mechanical properties, thermal, and electrical conductivities of the ZrB2 ceramics and ZrB2-based composites. Hot-pressing is the most commonly used densification method for the ZrB2-based ceramics in historic studies. Recently, pressureless sintering, reactive hot pressing, and spark plasma sintering are being developed. Compositions with added carbides and disilicides displayed significant improvement of densification and made pressureless sintering possible at ≤2000 °C. Reactive hot-pressing allows in situ synthesizing and densifying of ZrB2-based composites. Spark plasma sintering displays a potential and attractive way to densify the ZrB2 ceramics and ZrB2-based composites without any additive. Young's modulus can be described by a mixture rule and it decreased with porosity. Fracture toughness displayed in the ZrB2-based composites is in the range of 2–6 MPa m1/2. Fine-grained ZrB2 ceramics had strengths of a few hundred MPa, which increased with the additions of SiC and MoSi2. The small second phase size and uniform distribution led to higher strengths. The addition of nano-sized SiC particles imparts a better oxidation resistance and improves the strength of post-oxidized ZrB2-based ceramics. In addition, the ZrB2-based composites showed high thermal and electrical conductivities, which decreased with temperature. These conductivities are sensitive to composition, microstructure and intergranular phase. The unique combinations of mechanical and physical properties make the ZrB2-based composites attractive candidates for high-temperature thermomechanical structural applications.

Effect of nanoparticles on the anticorrosion and mechanical properties of epoxy coating

Abstract: Homogeneous epoxy coatings containing nanoparticles of SiO 2 , Zn, Fe 2 O 3 and halloysite clay were successfully synthesized on steel substrates by room-temperature curing of a fully mixed epoxy slurry diluted by acetone. The surface morphology and mechanical properties of these coatings were characterized by scanning electron microscopy and atomic force microscopy, respectively. The effect of incorporating various nanoparticles on the corrosion resistance of epoxy-coated steel was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy. The electrochemical monitoring of the coated steel over 28 days of immersion in both 0.3 wt.% and 3 wt.% NaCl solutions suggested the beneficial role of nanoparticles in significantly improving the corrosion resistance of the coated steel, with the Fe 2 O 3 and halloysite clay nanoparticles being the best. The SiO 2 nanoparticles were found to significantly improve the microstructure of the coating matrix and thus enhanced both the anticorrosive performance and Young's modulus of the epoxy coating. In addition to enhancing the coating barrier performance, at least another mechanism was at work to account for the role of the nanoparticles in improving the anticorrosive performance of these epoxy coatings.

Microstructural effects on charge-storage properties in MnO2-based electrochemical supercapacitors.

Abstract: The charge-storage mechanism in manganese dioxide (MnO2)-based electrochemical supercapacitors was investigated and discussed toward prepared MnO2 microstructures. The preparation of a series of MnO2 allotropic phases was performed by following dedicated synthetic routes. The resulting compounds are classified into three groups depending on their crystal structures based on 1D channels, 2D layers, or 3D interconnected tunnels. The 1D group includes pyrolusite, ramsdellite, cryptomelane, Ni-doped todorokite (Ni-todorokite), and OMS-5. The 2D and 3D groups are composed of birnessite and spinel, respectively. The prepared MnO2 powders were characterized using X-ray diffraction, scanning electron microscopy, the Brunauer−Emmett−Teller technique, cyclic voltammetry (CV), and electrochemical impedance spectroscopy. The influence of the MnO2 microstructure on the electrochemical performance of MnO2-based electrodes is commented on through the specific surface area and the electronic and ionic conductivities. It ...

Microstructure, thermophysical and electrical properties in AlxCoCrFeNi (0 ≤ x ≤2) high-entropy alloys

Abstract: AlxCoCrFeNi (0 ≤ x ≤2) alloys were prepared by an arc remelter and investigated. With increasing x, the AlxCoCrFeNi alloys change from single FCC phase to single BCC phase with a transition duplex FCC/BCC region. The weak X-ray diffraction intensities indicate severe X-ray scattering effect of lattice in these high-entropy alloys. Electrical conductivity and thermal conductivity much smaller than those of pure component metals is ascribed as due to this lattice effect. The behavior of electrical conductivity and thermal conductivity can be divided into three parts according to microstructure. Both values of electrical conductivity and thermal conductivity decrease with increasing x in single-phase regions. Values of electrical conductivity and thermal conductivity are even higher than those in the duplex phase region because of the additional scattering effect of FCC/BCC phase boundaries in the alloys. Relative contribution of electron and phonon to electrical resistivity and thermal conductivity is evaluated in this study. It is shown that both electron and phonon components are comparable in these high-entropy alloys, and their transport properties are similar to that of semi-metal.

Determining exciton bandwidth and film microstructure in polythiophene films using linear absorption spectroscopy

Abstract: We analyze the linear absorption spectrum of regioregular poly(3-hexylthiophene) films spun from a variety of solvents to probe directly the film microstructure and how it depends on processing conditions. We estimate the exciton bandwidth and the percentage of the film composed of aggregates quantitatively using a weakly interacting H-aggregate model. This provides a description of the degree and quality of crystallites within the film and is in turn correlated with thin-film field-effect transistor characteristics.

Impact of Anode Microstructure on Solid Oxide Fuel Cells

Abstract: We report a correlation between the microstructure of the anode electrode of a solid oxide fuel cell (SOFC) and its electrochemical performance for a tubular design. It was shown that the electrochemical performance of the cell was extensively improved when the size of constituent particles was reduced so as to yield a highly porous microstructure. The SOFC had a power density of greater than 1 watt per square centimeter at an operating temperature as low as 600°C with a conventional zirconia-based electrolyte, a nickel cermet anode, and a lanthanum ferrite perovskite cathode material. The effect of the hydrogen fuel flow rate (linear velocity) was also examined for the optimization of operating conditions. Higher linear fuel velocity led to better cell performance for the cell with higher anode porosity. A zirconia-based cell could be used for a low-temperature SOFC system under 600°C just by optimizing the microstructure of the anode electrode and operating conditions.

Strengthening Mechanisms in Polycrystalline Multimodal Nickel-Base Superalloys

Abstract: Polycrystalline γ-γ′ superalloys with varying grain sizes and unimodal, bimodal, or trimodal distributions of precipitates have been studied. To assess the contributions of specific features of the microstructure to the overall strength of the material, a model that considers solid-solution strengthening, Hall–Petch effects, precipitate shearing in the strong and weak pair-coupled modes, and dislocation bowing between precipitates has been developed and assessed. Cross-slip-induced hardening of the Ni3Al phase and precipitate size distributions in multimodal microstructures are also considered. New experimental observations on the contribution of precipitate shearing to the peak in flow stress at elevated temperatures are presented. Various alloys having comparable yield strengths were investigated and were found to derive their strength from different combinations of microconstituents (mechanisms). In all variants of the microstructure, there is a strong effect of antiphase boundary (APB) energy on strength. Materials subjected to heat treatments below the γ′ solvus temperature benefit from a strong Hall–Petch contribution, while supersolvus heat-treated materials gain the majority of their strength from their resistance to precipitate shearing.

Generalized Fabrication of Nanoporous Metals (Au, Pd, Pt, Ag, and Cu) through Chemical Dealloying

Abstract: Nanoporous metal ribbons including Au, Pd, Pt, Ag, and Cu can be fabricated through chemical dealloying of rapidly solidified Al-based alloys under free corrosion conditions. The formation and microstructure of these nanoporous metals have been investigated using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy, and high-resolution transmission electron microscopy. All metal ribbons exhibit an open, three-dimensional bicontinuous interpenetrating ligament-channel structure with nanometer length scales. For a given dealloying solution, the length scale of ligaments/channels in these nanoporous metals is associated with surface diffusion of more noble atoms, and increases with increasing diffusion coefficients in sequence: Pt/Pd < Au < Ag < Cu. In addition, the length scale of ligaments/channels of these nanoporous metals can be modulated by simply changing the dealloying solution. Nanoindentation tests show that Young’s modulus and hardness...

Fabrication and properties of dispersed carbon nanotube–aluminum composites

Abstract: Powder metallurgy techniques have emerged as promising routes for the fabrication of carbon nanotube (CNT) reinforced metal matrix composites. In this work, planetary ball milling was used to disperse 2 wt% MWCNT in aluminum (Al) powder. Despite the success of ball milling in dispersing CNTs in Al powder, it is often accompanied with considerable strain hardening of the Al powder, which may have implications on the final properties of the composite. Both un-annealed and annealed Al–2 wt% CNT composites were investigated. It was found that, ball-milled and extruded (un-annealed) samples of Al–2 wt% CNT demonstrated high notch-sensitivity and consistently fractured outside the gauge length during tensile testing. In contrast, extruded samples annealed at 400 and at 500 °C for 10 h prior to testing, exhibited more ductile behavior and no notch sensitivity. Under the present investigated processing conditions, ball milling for 3 h followed by hot extrusion and annealing at 500 °C resulted in enhancements of around 21% in tensile strength compared with pure aluminum with the same process history. The ball-milling conditions used were found to result in the creation of a nanostructure in all samples produced, as shown by XRD and TEM analysis. Such nanostructure was retained after prolonged exposures to temperatures up to 500 °C. The tensile testing fracture surfaces showed uniform dispersion and alignment of the CNTs in the aluminum matrix but also showed CNTs acting as nucleation sites for void formation during tensile testing. This has contributed to the observation of CNT pull-out due to the poor bond between the CNTs and the matrix.

Studies of Standard Heat Treatment Effects on Microstructure and Mechanical Properties of Laser Net Shape Manufactured INCONEL 718

Abstract: Laser net shape manufacturing (LNSM) is a laser cladding/deposition based technology, which can fabricate and repair near-net-shape high-performance components directly from metal powders. Characterizing mechanical properties of the laser net shape manufactured components is prerequisite to the applications of LNSM in aircraft engine industrial productions. Nickel-based superalloys such as INCONEL 718 are the most commonly used metal materials in aircraft engine high-performance components. In this study, the laser deposition process is optimized through a set of designed experiments to reduce the porosity to less than 0.03 pct. It is found that the use of plasma rotating electrode processed (PREP) powder and a high energy input level greater than 80 J/mm are necessary conditions to minimize the porosity. Material microstructure and tensile properties of laser-deposited INCONEL 718 are studied and compared under heat treatment conditions of as deposited, direct aged, solution treatment and aging (STA), and full homogenization followed by STA. Tensile test results showed that the direct age heat treatment produces the highest tensile strength equivalent to the wrought material, which is followed by the STA-treated and the homogenization-treated tensile strengths, while the ductility exhibits the reverse trend. Finally, failure modes of the tensile specimens were analyzed with fractography.

Development of high-performance A356/nano-Al2O3 composites

Abstract: In this study, a new method was used in stir casting to fabricate nano-Al2O3 particulate reinforced aluminum composites and avoid agglomeration and segregation of particles. Different volume fractions of nano-alumina particles were incorporated into the A356 aluminum alloy by a mechanical stirrer and then cylindrical specimens were cast and tested. The microstructural characterization of the composite samples showed uniform distribution of reinforcement, grain refinement of aluminum matrix, and presence of the minimal porosity. The effects of nano-Al2O3 particle content on the mechanical properties of the composites were investigated. Based on experiments, it was revealed that the presence of nano-Al2O3 reinforcement led to significant improvement in hardness, 0.2% yield strength, UTS and ductility. This combination of enhancement in UTS and ductility exhibited by nano-Al2O3 reinforced aluminum is due to uniform distribution of reinforcement and grain refinement of aluminum matrix.

Suspension and solution thermal spray coatings

Abstract: The emerging methods of coating deposition by suspension and solution thermal spraying are described. The liquid suspensions of fine powders and liquid precursors are injected into flames and/or jets generated in the torches. The formulation and stability of suspensions as well as the methods of fine powders synthesis are briefly described. Typical solutions, being often the liquid organo–metallics are also briefly described. An important problem of injection of liquids into jets and flames is then presented. Two principal modes of injection, used at present, are outlined, i.e.: (i) atomization; and, (ii) injection of a continuous jet. Subsequently, the phenomena occurring in flames and plasma jets are discussed and the major differences to these occurring during conventional spraying are stressed up. The build up of coatings starting from the impact of fine particles on the substrate is described and typical microstructures of suspension and solution sprayed coatings are shown. Some properties of the sprayed coatings, including mechanical, electrical, chemical, and thermophysical ones are collected and presented. Finally, the emerging applications of coatings are shown and the possible future applications are discussed.

Microstructures and mechanical properties of Al/Al2O3 surface nano-composite layer produced by friction stir processing

Abstract: In this study, a new processing technique, friction stir processing (FSP) was attempted to incorporate nano-sized Al 2 O 3 into 6082 aluminum alloy to form particulate composite surface layer. Samples were subjected to various numbers of FSP passes from one to four, with and without Al 2 O 3 powder. Microstructural observations were carried out by employing optical and scanning electron microscopy (SEM) of the cross sections both parallel and perpendicular to the tool traverse direction. Mechanical properties include microhardness and wear resistance, were evaluated in detail. The results show that the increasing in number of FSP passes causes a more uniform in distribution of nano-sized alumina particles. The microhardness of the surface improves by three times as compared to that of the as-received Al alloy. A significant improvement in wear resistance in the nano-composite surfaced Al is observed as compared to the as-received Al.

Nanoporous Copper with Tunable Nanoporosity for SERS Applications

Abstract: Nanostructured materials with designable microstructure and controllable physical and chemical properties are highly desired for practical applications in nanotechnology. In this article, it is reported that nanoporous copper with a tunable nanopore size can be fabricated by controlling the dealloying process. The influence of acid concentration and etching potential on the formation of nanoprosity is systematically investigated. With optimal etching conditions, the nanopore sizes can be tailored from ∼15 to ∼120 nm by controlling the dealloying time. It is found that the tunable nanoporosity leads to significant improvements in surface-enhanced Raman scattering (SERS) of nanoporous copper and peak values of SERS enhancements for both rhodamine 6G and crystal violet 10B molecules are observed at a pore size of ∼30–50 nm. This study underscores the effect of complex three-dimensional nanostructures on physical and chemical properties and is helpful in developing inexpensive SERS substrates for sensitive instrumentations in molecular diagnostics.

Electrical and mechanical properties of carbon nanotube reinforced copper nanocomposites fabricated by electroless deposition process

Abstract: Multiwalled carbon nanotube/copper (CNT/Cu) nanocomposite powders with different CNTs volume fractions were prepared by electroless Cu deposition on the CNTs. The CNTs underwent acid treatment, sensitization and electroless copper deposition on their surface respectively. The microstructure of the prepared CNT/Cu nanocomposites was investigated by SEM and HRTEM as well as by XRD analysis. Copper was deposited in a form of a layer on the CNTs surface. The CNT/Cu nanocomposite powders were sintered by spark plasma sintering. The microstructure of the sintered materials were investigated by SEM indicating that the CNTs were homogenous distributed in the copper matrix with good sinterability and porosity content lower than unity in case of 5 and 10 vol.% of CNT/Cu nanocomposites and 2.9 and 3.5% respectively for 15 and 20 vol.% CNT/Cu nanocomposites. The electrical conductivity, hardness and the tensile properties were measured for evaluating the sintered CNT/Cu nanocomposites. The electrical conductivity decreased by increasing CNTs volume fraction in copper matrix, but the hardness was increased by increasing CNTs volume fraction. The Young's modulus was increased and the elongation was decreased by increasing the volume fraction of CNTs in copper matrix. In addition, the yield strength of the sintered materials was increased by increasing CNTs volume fraction except in case of 20 vol.% CNT/Cu composite the material was fractured before yielding.