Showing papers on "Microstructure published in 2008"

Pillared graphene: a new 3-D network nanostructure for enhanced hydrogen storage.

Abstract: A multiscale theoretical approach was used to investigate hydrogen storage in a novel three-dimensional carbon nanostructure. This novel nanoporous material has by design tunable pore sizes and surface areas. Its interaction with hydrogen was studied thoroughly via ab initio and grand canonical Monte Carlo calculations. Our results show that, if this material is doped with lithium cations, it can store up to 41 g H2/L under ambient conditions, almost reaching the DOE volumetric requirement for mobile applications.

Microstructure and compressive properties of AlCrFeCoNi high entropy alloy

Abstract: An AlCrFeCoNi high entropy alloy was prepared by vacuum arc melting. Only diffraction peak corresponding to a BCC crystal structure is observed for this AlCrFeCoNi high entropy alloy. The microstructure of this AlCrFeCoNi alloy is polygonal grains with intragranular dendritic segregation. Dendritic segregation area is found to be Al, Ni rich and Cr, Fe deplete, while interdendritic segregation area is Cr, Fe rich and Al, Ni deplete. The distribution of Co is essentially identical. The fine microstructure of dendritic segregation area and of interdendritic segregation area is found to be nanoscale spherical precipitates morphology and basket-weave morphology, respectively. Results of EDS attached on high resolution scanning electron microscope (SEM) revealed that these morphological characteristics are also resulted from elements segregation. This AlCrFeCoNi high entropy alloy exhibits excellent compressive properties. The yield stress, compressive strength and plastic strain of the alloy reaches 1250.96, 2004.23 MPa, and 32.7%, respectively. The fracture mechanism of this AlCrFeCoNi high entropy alloy is observed as cleavage fracture and slip separation.

TiC-TiB2 composites : A review of phase relationships, processing and properties

Abstract: Ceramic-matrix composites (CMCs) based on TiC–TiB2 have attracted enormous interest during recent years because, in comparison to single-phase ceramics, they exhibit superior properties including high hardness, good wear resistance and high fracture toughness. This paper begins with a review of the TiC–TiB2 equilibrium system and its possible influence on the processing and properties of the composite. The application of TiC–TiB2 composites has been limited due to the fact that they have been difficult to process. Much of the research effort has therefore focused on the synthesis, processing and fabrication of TiC–TiB2 and is based primarily on self-propagating high-temperature synthesis (SHS) and its derivatives, high-energy milling and sintering. The performance of SHS under the application of pressure has been the subject of particular investigation. These developments are the main subject of this review that also takes into account the resulting effects on the microstructure and the mechanical properties of TiC–TiB2. The influence of the reaction parameters like reactant composition, reactant particle size and green density on the microstructure and properties is also reported.

Modification of eutectic silicon in Al-Si alloys

Abstract: The mechanical properties of Al–Si alloys are strongly related to the size, shape and distribution of eutectic silicon present in the microstructure In order to improve mechanical properties, these alloys are generally subjected to modification melt treatment, which transforms the acicular silicon morphology to fibrous one resulting in a noticeable improvement in elongation and strength. Improper melt treatment procedures, fading and poisoning of modifiers often result in the structure which is far from the desired one. Hence it is essential to assess the effectiveness of melt treatment before pouring. A much investigated reliable thermal analysis technique is generally used for this purpose. The deviation from the standard curve in thermal analysis helps in assessing the level of refinement of the Si structure. In the present review an attempt is made to discuss various aspects of modification, including mechanism, interaction of defects and non-destructive assessment by thermal analysis.

Microstructure and tensile properties of friction stir welded AZ31B magnesium alloy

Abstract: The microstructural change in AZ31B-H24 magnesium (Mg) alloy after friction stir welding (FSW) was examined. The effects of tool rotational speed and welding speed on the microstructure and tensile properties were evaluated. The grain size was observed to increase after FSW, resulting in a drop of microhardness across the welded region from about 70 HV in the base metal to about 50 HV at the center of the stir zone. The obtained Hall–Petch type relationship showed a strong grain size dependence of the hardness. The aspect ratio and fractal dimension of the grains decreased towards the center of the stir zone. The welding speed had a significant effect on the microstructure, with larger grains at a lower welding speed. The yield strength and ultimate tensile strength increased with increasing welding speed due to a lower heat input. A lower rotational speed of 500 rpm led to higher yield strength than a higher rotational speed of 1000 rpm. The friction stir welded joints were observed to fail mostly at the boundary between the weld nugget and thermomechanically affected zone at the advancing side. Fracture surfaces showed a mixture of cleavage-like and dimple-like characteristics.

Structure-property correlations in Al 7050 and Al 7055 high-strength aluminum alloys

Abstract: The 7XXX series age-hardenable high-strength aluminum alloys find useful applications in the field of aerospace engineering. Constant efforts are being made to tailor the mechanical and corrosion properties of these alloys as per requirements for a particular application. These properties are a function of factors like microstructure, chemical composition and processing parameters. An effort has been made to collate the information available from different studies conducted on alloys Al 7050 and Al 7055. Databases were created to consolidate the information about microstructure, mechanical properties and corrosion behavior for the two alloys. Existing models were utilized to predict strength and fracture toughness for these alloys and these models were validated using experimental values and a qualitative evaluation was made for the corrosion behavior of these alloys. Available data were utilized to prepare maps that are intended to serve as guides to design aluminum alloys with desired combination of properties.

Inverse Temperature Dependence of Toughness in an Ultrafine Grain-Structure Steel

Abstract: Materials are typically ductile at higher temperatures and become brittle at lower temperatures. In contrast to the typical ductile-to-brittle transition behavior of body-centered cubic (bcc) steels, we observed an inverse temperature dependence of toughness in an ultrahigh-strength bcc steel with an ultrafine elongated ferrite grain structure that was processed by a thermomechanical treatment without the addition of a large amount of an alloying element. The enhanced toughness is attributed to a delamination that was a result of crack branching on the aligned {100} cleavage planes in the bundles of the ultrafine elongated ferrite grains strengthened by nanometer-sized carbides. In the temperature range from 60° to –60°C, the yield strength was greater, leading to the enhancement of the toughness.

Microstructure, mechanical properties and bio-corrosion properties of Mg–Zn–Mn–Ca alloy for biomedical application

Abstract: Microstructure, mechanical properties and bio-corrosion properties of as-cast Mg–Zn–Mn–Ca alloys were investigated for biomedical application in detail by optical microscopy, scanning electronic microscopy (SEM), mechanical properties testing and electrochemical measurement. SEM and optical microscopy observation indicated that the grain size of the as-cast alloys significantly decreased with the increasing of Ca content up to 0.5 wt.%. Further increasing of Ca content did not refine the grain more. The phase constitute was mainly controlled by the atomic ratio of Zn to Ca. When the ratio was more than 1.0–1.2, the alloy was mainly composed of primary Mg and lamellar eutectic (α-Mg + Ca 2 Mg 6 Zn 3 ), while the alloy was composed of primary Mg and divorced eutectic (α-Mg + Mg 2 Ca + Ca 2 Mg 6 Zn 3 ) when the atomic ratio was less than 1.0–1.2. The yield strength of the as-cast alloy increased but the elongation and the tensile strength increased first and then decreased with the increasing of Ca content. It was thought that Mg 2 Ca phase deteriorated the tensile strength and ductility. Electrochemical measurements indicated that Mg 2 Ca phase improved the corrosion resistance of the as-cast alloy.

Iron–manganese: new class of metallic degradable biomaterials prepared by powder metallurgy

Abstract: An Fe–35 wt-%Mn alloy, aimed to be used as a metallic degradable biomaterial for stent applications, was prepared via a powder metallurgy route. The effects of processing conditions on the microstructure, mechanical properties, magnetic susceptibility and corrosion behaviour were investigated and the results were compared to those of the SS316L alloy, a gold standard for stent applications. The Fe35Mn alloy was found to be essentially austenitic with fine MnO particles aligned along the rolling direction. The alloy is ductile with a strength approaching that of wrought SS316L. It exhibits antiferromagnetic behaviour and its magnetic susceptibility is not altered by plastic deformation, providing an excellent MRI compatibility. Its corrosion rate was evaluated in a modified Hank's solution, and found superior to that of pure iron (slow in vivo degradation rate). In conclusion, the mechanical, magnetic and corrosion characteristics of the Fe35Mn alloy are considered suitable for further development ...

Corrosion of Pure Mg as a Function of Grain Size and Processing Route

Abstract: This study is a discrete effort towards optimization of microstructure for enhanced corrosion resistance by understanding the largely unknown corrosion - grain size relationship for magnesium. This is particularly important for magnesium that commonly displays poor corrosion resistance. A significant variation in corrosion resistance with grain size exists, which is of key significance; however these trends were strongly dependent upon the specific thermo-mechanical processing route used to prepare the specimens.

Microstructural characteristics and mechanical properties of Ti-6Al-4V friction stir welds

Abstract: Friction stir welding (FSW) was applied to 3 mm-thick Ti–6Al–4V plates under different rotational speeds. Defect-free welds were successfully produced at rotational speeds of 400 and 500 rpm. The base material (BM) had a deformed α/β lamellar microstructure. FSW produced a full lamellar structure with refined prior β grains in the SZ, while the HAZ contained a bimodal microstructure consisting of the equiaxed primary α and α/β lamellar structure within the prior β structure. An increase in rotational speed increased the sizes of α colonies and prior β grains. The SZ exhibited higher hardness than the BM, with the lowest hardness found in the HAZ. Results of the transverse tensile test showed that all welds fractured in the HAZ and that they exhibited lower strength and elongation than the BM. The tensile test for only the SZ showed it to be characterized by higher strength and elongation than the BM.

Effects of Mn, Ti and V on the microstructure and properties of AlCrFeCoNiCu high entropy alloy

Abstract: Mn, Ti and V were added in an equal-molar ratio to the AlCrFeCoNiCu alloy and the effects of these added elements on microstructure and properties of AlCrFeCoNiCu high entropy alloy were studied. The results indicate that the AlCrFeCoNiCuMn alloy shows almost the same microstructure as in AlCrFeCoNiCu alloy except for a long-strip Cr-enriched phase. The additions of Ti change the AlCrFeCoNiCu alloy from a dendrite structure to eutectic-cell one. Hence two phases in the eutectic cells are detected as one Al, Ti, Co, Ni enriched phase and another Cr, Fe enriched phase. V added alloy is also a dendrite structure, but V additions change the morphology of dendritic area from modulated plates to ellipsoidal particles. The addition of V into the AlCrFeCoNiCu alloy shows the best strengthening effect and the lowest decrease in the ultimate strain in our experiments.

Fabrication of Ti-6Al-4V Scaffolds by Direct Metal Deposition

Abstract: Direct metal deposition (DMD) is a rapid laser-aided deposition method that can be used to manufacture near-net-shape components from their computer aided design (CAD) files. The method can be used to produce fully dense or porous metallic parts. The Ti-6Al-4V alloy is widely used as an implantable material mainly in the application of orthopedic prostheses because of its high strength, low elastic modulus, excellent corrosion resistance, and good biocompatibility. In the present study, Ti-6Al-4V scaffold has been fabricated by DMD technology for patient specific bone tissue engineering. Good geometry control and surface finish have been achieved. The structure and properties of the scaffolds were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tension test. The microstructures of laser-deposited Ti-6Al-4V scaffolds are fine Widmanstatten in nature. The tensile and yield strengths of the as-deposited Ti-6Al-4V were 1163 ± 22 and 1105 ± 19 MPa, respectively, which are quite higher than the ASTM limits (896 and 827 MPa) for Ti-6Al-4V implants. However, the ductility of the as-deposited sample was very low (∼4 pct), which is well below the ASTM limit (10 pct). After an additional heat treatment (sample annealed at 950 °C followed by furnace cooling), both strength (UTS ∼ 1045 ± 16, and YS ∼ 959 ± 12 MPa) and ductility (∼10.5 ± 1 pct) become higher than ASTM limits for medical implants.

Improvement of corrosion and wear resistances of AISI 420 martensitic stainless steel using plasma nitriding at low temperature

Abstract: The influence of low temperature plasma nitriding on the wear and corrosion resistance of AISI 420 martensitic stainless steel was investigated. Plasma nitriding experiments were carried out with DC-pulsed plasma in 25% N2 + 75% H2 atmosphere at 350 °C, 450 °C and 550 °C for 15 h. The composition, microstructure and hardness of the nitrided samples were examined. The wear resistances of plasma nitrided samples were determined with a ball-on-disc wear tester. The corrosion behaviors of plasma nitrided AISI420 stainless steel were evaluated using anodic polarization tests and salt fog spray tests in the simulated industrial environment. The results show that plasma nitriding produces a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer on the AISI 420 stainless steel surface. Plasma nitriding not only increases the surface hardness but also improves the wear resistance of the martensitic stainless steel. Furthermore, the anti-wear property of the steel nitrided at 350 °C is much more excellent than that at 550 °C. In addition, the corrosion resistance of AISI420 martensitic stainless steel is considerably improved by 350 °C low temperature plasma nitriding. The improved corrosion resistance is considered to be related to the combined effect of the solid solution of Cr and the high chemical stable phases of ɛ-Fe3N and αN formed on the martensitic stainless steel surface during 350 °C low temperature plasma nitriding. However, plasma nitriding carried out at 450 °C or 550 °C reduces the corrosion resistance of samples, because of the formation of CrN and leading to the depletion of Cr in the solid solution phase of the nitrided layer.

Crystal Structure of Hydroxyapatite Nanorods Synthesized by Sonochemical Homogeneous Precipitation

Abstract: Using a homogeneous precipitation method in an ultrasound field, we synthesized nanosized, platelike hydroxyapatite (HAp). The internal structure of these platelike formations consists of specifically oriented and laterally connected nanorods. The synthesized HAp nanorods have a length of about 500 nm and a diameter of about 100 nm. A closer inspection of the microstructure of a single nanorod revealed a highly regular and defect-free lattice with unique crystallographic plane orientations. The obtained structure was related to the influence of the ultrasound on the growth mechanism. The samples were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

The effects of Mn additions on the microstructure and mechanical properties of Al–Si–Cu casting alloys

Abstract: The effects of Mn on the microstructure and mechanical properties of Type 319 aluminum casting alloys have been examined. It is shown that, as the Mn content is increased up to 0.65 wt.pct (corresponding to an Fe/Mn ratio of ∼1.2) in the baseline alloy (Al–7 wt.%Si–3.8 wt.%Cu–0.5 wt.%Fe), the plate-like β intermetallic phase is completely converted to the Chinese script α phase resulting in improved tensile properties. Excess amounts of Mn, however, deteriorate the mechanical properties by increasing the total amount of iron-containing intermetallic phases. The porosity and volume percent of intermetallic phases are correlated with the tensile properties in order to determine the role of Mn on Type 319 casting alloys.

Microstructure and mechanical properties of pure Cu processed by high-pressure torsion

Abstract: Pure Cu was subjected to severe plastic deformation through high-pressure torsion (HPT) using disc and ring samples. Vickers microhardness was measured across the diameter and it was shown that all hardness values fall well on a unique single curve regardless of the types of the HPT samples when they are plotted against the equivalent strain. The hardness increases with an increase in the equivalent strain at an early stage of straining but levels off and enters into a steady-state where the hardness remains unchanged with further straining. It was confirmed that the tensile strength also follows the same single function of the equivalent strain as the hardness. The elongation to failure as well as the uniform elongation also exhibits a single unique function of the equivalent strain. Transmission electron microscopy showed that a subgrain structure develops at an early stage of straining with individual grains containing dislocations. The subgrain size decreases while the misorientation angle increases and more dislocations are formed within the grains with further straining. In the steady-state range, some grains appear which are free from dislocations, suggesting that recrystallization occurs during or after the HPT process. The mechanism for the grain refinement was discussed in terms of dislocation mobility.

Friction stir welding of dissimilar AA2024 and AA7075 aluminum alloys

Abstract: The present study focuses on the microstructure and mechanical properties of dissimilar joints of 2024-T3 Al alloy to 7075-T6 Al alloy produced by friction stir welding. Effects of welding speed and fixed location of base metals on microstructures, hardness distributions, and tensile properties of the welded joints were investigated. SEM-EDS analysis revealed that the stir zone contains a mixed structure and onion ring pattern with a periodic change of grain size as well as a heterogeneous distribution of alloying elements. The maximum tensile strength of 423.0 MPa was achieved for the joint produced at welding speed of 1.67 mm/s when 2024 Al alloy was located on the advancing side.