Showing papers on "Microstructure published in 2011"

As-Fabricated and Heat-Treated Microstructures of the Ti-6Al-4V Alloy Processed by Selective Laser Melting

Abstract: Selective laser melting (SLM) is a rapid manufacturing process that enables the buildup of very complex parts in short delays directly from powder beds. Due to the high laser beam energy during very short interaction times and the high solidification rates of the melting pool, the resulting microstructure is out-of-equilibrium and particularly textured. This type of as-fabricated microstructure may not satisfy the aeronautical criterion and requires post heat treatments. Optimized heat treatments are developed, in one side, to homogenize and form the stable phases α and β while preventing exaggerated grain growth. In the other side, heat treatment is investigated to relieve the thermal stresses appearing during cooling. This study is aimed at presenting the various types of microstructure of the Ti-6Al-4V alloy after postfabrication heat treatments below or above the β transus. Tensile tests are then carried out at room temperature in order to assess the effect of the microstructures on the mechanical properties. The fine as-fabricated microstructure presents high yield and ultimate strengths, whereas the ductility is well below the standard. A strong anisotropy of fracture between the two loading directions is noted, which is attributed to the manufacturing defects. Conventional and optimized heat treatments exhibit high yield and ultimate strengths while the ductility is significantly improved. This is due to a new optimization of the process parameters allowing drastic reduction of the number of defects. These two heat treatments enable now a choice of the morphology of the grains between columnar or equiaxial as a function of the type of loading.

Influence of blend microstructure on bulk heterojunction organic photovoltaic performance

Abstract: The performance of organic photovoltaic devices based upon bulk heterojunction blends of donor and acceptor materials has been shown to be highly dependent on the thin film microstructure. In this tutorial review, we discuss the factors responsible for influencing blend microstructure and how these affect device performance. In particular we discuss how various molecular design approaches can affect the thin film morphology of both the donor and acceptor components, as well as their blend microstructure. We further examine the influence of polymer molecular weight and blend composition upon device performance, and discuss how a variety of processing techniques can be used to control the blend microstructure, leading to improvements in solar cell efficiencies.

Conversion Reaction Mechanisms in Lithium Ion Batteries: Study of the Binary Metal Fluoride Electrodes

Abstract: Materials that undergo a conversion reaction with lithium (e.g., metal fluorides MF2: M = Fe, Cu, ...) often accommodate more than one Li atom per transition-metal cation, and are promising candidates for high-capacity cathodes for lithium ion batteries. However, little is known about the mechanisms involved in the conversion process, the origins of the large polarization during electrochemical cycling, and why some materials are reversible (e.g., FeF2) while others are not (e.g., CuF2). In this study, we investigated the conversion reaction of binary metal fluorides, FeF2 and CuF2, using a series of local and bulk probes to better understand the mechanisms underlying their contrasting electrochemical behavior. X-ray pair-distribution-function and magnetization measurements were used to determine changes in short-range ordering, particle size and microstructure, while high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) were used to measure the atomic-level s...

Graphene–aluminum nanocomposites

Abstract: Composites of graphene platelets and powdered aluminum were made using ball milling, hot isostatic pressing and extrusion. The mechanical properties and microstructure were studied using hardness and tensile tests, as well as electron microscopy, X-ray diffraction and differential scanning calorimetry. Compared to the pure aluminum and multi-walled carbon nanotube composites, the graphene–aluminum composite showed decreased strength and hardness. This is explained in the context of enhanced aluminum carbide formation with the graphene filler.

Microstructure and mechanical properties of Al-Al2O3 micro and nano composites fabricated by stir casting

Abstract: Aluminum matrix composites (AMCs) reinforced with micro and nano-sized Al2O3 particles are widely used for high performance applications such as automotive, military, aerospace and electricity industries because of their improved physical and mechanical properties. In this study, in order to improve the wettability and distribution of reinforcement particles within the matrix, a novel three step mixing method was used. The process included heat treatment of micro and nano Al2O3 particles, injection of heat-treated particles within the molten A356 aluminum alloy by inert argon gas and stirring the melt at different speeds. The influence of various processing parameters such as heat treatment of particles, injection process, stirring speed, reinforcement particle size and weight percentage of reinforcement particles on the microstructure and mechanical properties of composites was investigated. The matrix grain size, morphology and distribution of Al2O3 nanoparticles were recognized by scanning electron microscopy (SEM), optical microscope (OM) equipped with image analyzer, energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Also, the hardness and compression strength of samples was investigated. The results showed the poor incorporation of nano particles in the aluminum melt prepared by the common condition. However, the use of heat-treated particles, injection of particles and the stirring system improved the wettability and distribution of the nano particles within the aluminum melt. In addition, it was revealed that the amount of hardness, compressive strength and porosity increased as weight percentage of nano Al2O3 particles increased.

Effect of friction stir welding parameters on the microstructure and mechanical properties of the dissimilar Al-Cu joints

Abstract: Butt joints of 1060 aluminum alloy and commercially pure copper were produced by friction stir welding (FSW) and the effect of welding parameters on surface morphology, interface microstructure and mechanical properties was investigated. The experimental results revealed that sound defect-free joints could be obtained under larger pin offsets when the hard Cu plate was fixed at the advancing side. Good tensile properties were achieved at higher rotation rates and proper pin offsets of 2 and 2.5 mm; further, the joint produced at 600 rpm with a pin offset of 2 mm could be bended to 180 degrees without fracture. The mechanical properties of the FSW Al-Cu joints were related closely to the interface microstructure between the Al matrix and Cu bulk. A thin, uniform and continuous intermetallic compound (IMC) layer at the Al-Cu butted interface was necessary for achieving sound FSW Al-Cu joints. Stacking layered structure developed at the Al-Cu interface under higher rotation rates, and crack initiated easily in this case, resulting in the poor mechanical properties. (C) 2011 Elsevier B.V. All rights reserved.

Austenite Stability Effects on Tensile Behavior of Manganese-Enriched-Austenite Transformation-Induced Plasticity Steel

Abstract: Manganese enrichment of austenite during prolonged intercritical annealing was used to produce a family of transformation-induced plasticity (TRIP) steels with varying retained austenite contents. Cold-rolled 0.1C-7.1Mn steel was annealed at incremental temperatures between 848 K and 948 K (575 °C and 675 °C) for 1 week to enrich austenite in manganese. The resulting microstructures are comprised of varying fractions of intercritical ferrite, martensite, and retained austenite. Tensile behavior is dependent on annealing temperature and ranged from a low strain-hardening “flat” curve to high strength and ductility conditions that display positive strain hardening over a range of strain levels. The mechanical stability of austenite was measured using in-situ neutron diffraction and was shown to depend significantly on annealing temperature. Variations in austenite stability between annealing conditions help explain the observed strain hardening behaviors.

Amorphous Carbon Coated High Grain Boundary Density Dual Phase Li4Ti5O12‐TiO2: A Nanocomposite Anode Material for Li‐Ion Batteries

Abstract: This work introduces an effective, inexpensive, and large-scale production approach to the synthesis of a carbon coated, high grain boundary density, dual phase Li4Ti5O12-TiO2 nanocomposite anode material for use in rechargeable lithium-ion batteries. The microstructure and morphology of the Li4Ti5O12-TiO2-C product were characterized systematically. The Li4Ti5O12-TiO2-C nanocomposite electrode yielded good electrochemical performance in terms of high capacity (166 mAh g−1 at a current density of 0.5 C), good cycling stability, and excellent rate capability (110 mAh g−1 at a current density of 10 C up to 100 cycles). The likely contributing factors to the excellent electrochemical performance of the Li4Ti5O12-TiO2-C nanocomposite could be related to the improved morphology, including the presence of high grain boundary density among the nanoparticles, carbon layering on each nanocrystal, and grain boundary interface areas embedded in a carbon matrix, where electronic transport properties were tuned by interfacial design and by varying the spacing of interfaces down to the nanoscale regime, in which the grain boundary interface embedded carbon matrix can store electrolyte and allows more channels for the Li+ ion insertion/extraction reaction. This research suggests that carbon-coated dual phase Li4Ti5O12-TiO2 nanocomposites could be suitable for use as a high rate performance anode material for lithium-ion batteries.

The effects of Na2O/SiO2 molar ratio, curing temperature and age on compressive strength, morphology and microstructure of alkali-activated fly ash-based geopolymers

Abstract: Fly ashes (FA) are byproducts of electricity production from mineral coal in thermoelectric power plants. The pozzolanic properties of FA have been utilized in various applications, including structural concrete, yet the large part of FA is still discarded into the environment. To promote greater FA usage, this study aims to produce a dense matrix, with mechanical properties satisfactory for civil engineering projects, from alkali-activated fly ash-based geopolymers. Three variables were studied: the Na2O/SiO2 molar ratio (N/S 0.20, N/S 0.30 and N/S 0.40); curing temperature in the first 24 h (50, 65 and 80 °C); and age (1, 7, 28, 91 and 180 days). For this study, alkali-activated fly ash pastes and mortars were prepared. In pastes, morphology was studied using scanning electron microscopy (SEM/EDS) and microstructural properties with X-ray Diffraction (XRD) analysis. Mortars were evaluated according to their mechanical performance measured using compression strength tests. Compression strength results were analysed using ANOVA. The results show that the N/S molar ratio plays an important role in the mechanical and morphological characteristics of geopolymers. The mortars prepared with a N/S 0.40 molar ratio had the greatest compression strength. The analysis of paste morphology revealed that N/S 0.40 pastes had a denser appearance, which is in agreement with results of compression strength tests.

Morphology of asphalts, asphalt fractions and model wax-doped asphalts studied by atomic force microscopy

Abstract: Asphalts used in the construction of pavements exhibit unique properties at the micron and nanometre scale. Atomic force microscopy (AFM) images of asphalts and asphalt chromatographic fractions prepared as thin films clearly show that a variety of ‘microstructures’ can develop on the surface of these types of materials. Structure develops to different degrees and in different forms depending on the residua crude source from which the asphalt or asphalt fraction is derived, the thermal history of the sample and the sample thickness. Based on our current best interpretation of a very large number of AFM images obtained over several years, we hypothesise that the interaction between crystallising paraffin waxes and the remaining asphalt fractions is responsible for much of the structuring, including the well-known bee structures (Loeber et al. 1996, Journal of Microscopy, 182 (1), 32–39), which has been observed with asphalt materials.