Applied Sciences

The mechanical response of a semisolid body to an applied, uniaxial strain rate has been expressed as a function of strain by modifying an existing analysis based on an idealized representation of the microstructure. An existing mechanical criterion for hot tearing of the semisolid body has been adapted to the deformation mechanisms. The resulting hot tearing model shows that the strength of the body depends on the strain, the viscosity of the intergranular fluid, the solid fraction, the isothermal compressibility of the fluid, the surface tension of the liquid, the limiting liquid-film thickness for viscous flow and a parameter m, which describes microstructure. The effect of each parameter on the mechanical response and the onset of hot tearing has been examined for ranges of values relevant to aluminum alloys and the direct-chill (DC) casting process. The parameter testing has shown that the mechanical response predicted by the model agrees well with some experimental data for both the mechanisms of fracture and the parameters that govern the process. An adjustment of unknown model parameters to experimental data would permit use of the model as a constitutive law and a fracture criterion for numerical modeling of hot tearing during the solidification of Al alloys by DC casting.

Hydrogen solubility in molten aluminium at different temperatures from 973-1123 K, has been measured using Sieverts' method. Inert gas (helium or argon) was used as a reference gas to calibrate the measurement system of the Sieverts' apparatus. The measured hydrogen solubility was found to vary with the reference gases. Helium was detected to be soluble in liquid aluminium. When helium is used as the reference gas, its solubility resulted in lower measured hydrogen solubility than that when argon was used to calibrate the measurement system of the apparatus. Argon gas was therefore considered as an appropriate reference gas when Sieverts' method is used to measure the hydrogen solubility in liquid aluminium. The hydrogen solubility, S, in liquid aluminium as a function of melt temperature, T, determined in the present investigation is expressed as log S = (-2980/T) + 3.07.

Spatially resolved element distributions generated by in situ X-ray fluorescence (XRF) line and area scans are shown to provide information about works of art which may not be obtainable from single spot spectra. In addition to generating visually powerful element maps and line profiles, this method also generates a spectrum at each image point, and this large data set is available for additional analysis. When generating line and area scans in the study of works of art, the collection parameters-including X-ray tube choice, spot size, step size, and scan time-must be optimized not only to produce the best signal, but also to perform the analysis within constraints imposed to ensure the security or safety of the object. Examples of the application of this method to several classes of works of art are presented, including illuminated manuscripts, paintings, bronze sculpture, and glazed ceramics.

The main objective of this study is to examine the two dimensional surface crack problems in a system with an interface between two elastic-plastic solids of different yield strength subjected to mode I mechanical loading. The surface cracks growth is considered to occure along the interface direction of bimaterials which is perfectly bonded to each others. A two dimensional finite elementmethod is used to solve the structural problem. Solid 183-node elements are utilized to simulate the strain singularity around the crack front. The crack surface is subjected to a compressive load by three point bending. The stress intesity factors are computed by using the displacement correlation technique. The primary goal is to develop a model crack tip stresses and strains in a manner that is useful for crack growth initiation and propagation in a FGM.

It is difficult or impossible to eliminate completely the porosity during fabrication of nanocomposite materials. This paper was aimed at predicting the yield strength of particulate-reinforced metal matrix nanocomposites by incorporating the effect of porosity based on Zhang and Chen's model. The influence of porosity has been taken into account along with the underlying strengthening mechanisms including Orowan, CTE (coefficient of thermal expansion) mismatch, and modified load-bearing effects. The model was validated using the experimental data of magnesium-based nanocomposites containing different types of nano-sized particles (namely Al 2 O 3 , Y 2 O 3 , MgO and SiC). The predicted yield strength of nanocomposites increased with increasing volume fraction of nanoparticles and decreasing volume fraction of porosity. The predicted results were in good agreement with the experimental data and more reasonable than those from the earlier models.

The aim of this study was to identify the influence of rare-earth (RE) elements on the strain hardening behavior in an extruded Mg–10Gd–3Y–0.5Zr magnesium alloy via compression in the extrusion direction at room temperature. The plastic deformation behavior of this RE-containing alloy was characterized by a rapidly decreasing strain hardening rate up to a strain level of about 4% (stage A), followed by a fairly flat linear strain hardening rate over an extended strain range from ∼4% to ∼18% (stage B). Stage C was represented by a decreasing strain hardening rate just before failure. The extent of twinning in this alloy was observed to be considerably less extensive than that in the RE-free extruded Mg alloys. The weaker crystallographic texture, refined grain size, and second-phase particles arising from the addition of RE elements were responsible for the much higher strain hardening rate in stage A due to the increased difficulty on the formation of twins and the slip of dislocations at lower strains, and for the occurrence of quite flat linear strain hardening in stage B at higher strains which was likely related to the dislocation debris and twin debris (or residual twins) stemming from dislocation–twin interactions as well as the interactions between dislocations/twins and second-phase particles and grain boundaries.► The alloy studied has threefold higher compressive yield strength than AM30 alloy. ► Formation of twins is less extensive than that in the RE-free extruded Mg alloys. ► Deformation of the RE-containing Mg alloy is characterized by three distinct stages. ► Rare earth elements effectively increase the strain hardening rate in stage A. ► Fairly flat and linear strain hardening occurs in stage B over an extended range.

The application of ultra-lightweight magnesium alloys inevitably involves fatigue resistance under cyclic loading. The present study was aimed at evaluating strain-controlled cyclic deformation behavior and the relevant effect of microstructure in a rare-earth (RE) element containing extruded Mg-10Gd-3Y-0.5Zr (GW103K) alloy. The microstructure of this alloy consisted of fine equiaxed grains with an average grain size of about 12 μm and a large number of RE-containing precipitates. Unlike the RE-free extruded magnesium alloys, this alloy exhibited essentially cyclic stabilization and symmetrical hysteresis loops without tension-compression asymmetry due to the presence of the relatively weaker texture and the suppression of twinning activities arising from the fine grain size and especially RE-containing precipitates. A detailed analysis for understanding the obstructive role of the precipitate to twinning has been presented. While this alloy had a lower cyclic strain hardening exponent than the RE-free extruded magnesium alloys, it had a longer fatigue life which can also be described by the Coffin-Manson law and Basquin's equation. Fatigue crack was observed to initiate from the specimen surface with some cleavage-like facets at the initiation site. Crack propagation was basically characterized by fatigue striations in conjunction with secondary cracks.

Lightweighting in ground vehicles is today considered as one of the most effective strategies to improve fuel economy and reduce anthropogenic environment-damaging and climate-changing emissions. Magnesium (Mg) alloy, as a strategic ultra-lightweight metallic material, has recently drawn a considerable interest in the transportation industry to reduce the weight of vehicles due to their high strength-to-weight ratio, dimensional stability, good machinability and recyclability. However, the hexagonal close-packed crystal structure of Mg alloys gives only limited slip systems and develops sharp deformation textures associated with strong mechanical anisotropy and tension-compression yield asymmetry. For the vehicle components subjected to dynamic loading, such asymmetry could exert an unfavorable influence on the material performance. This problem could be conquered through weakening the texture via addition of rare-earth (RE) elements. Thus, a number of RE-containing Mg alloys have recently been developed. To guarantee the structural integrity, durability, and safety of highly loaded structural components, understanding the characteristics and mechanisms of cyclic deformation and fatigue fracture of such RE-Mg alloys is of vital importance. In this review, the available fatigue properties including stress-controlled fatigue strength, strain-controlled cyclic deformation characteristics, and fatigue crack propagation behavior are summarized, along with the microstructural change and crystallographic texture weakening in the RE-containing Mg alloys in different forms (cast, extruded, and heat-treated states), in comparison with those of RE-free Mg alloys.

The present study was aimed at evaluating strain-controlled cyclic deformation behavior of a rareearth (RE) element containing Mg-10Gd-3Y-0.5Zr (GW103K) alloy in different states (as-extruded, peak-aged (T5), and solution-treated and peak-aged (T6)). The addition of RE elements led to an effective grain refinement and weak texture in the as-extruded alloy. While heat treatment resulted in a grain growth modestly in the T5 state and significantly in the T6 state, a high density of nano-sized and bamboo-leaf/plate-shaped b¢ (Mg 7 (Gd,Y)) precipitates was observed to distribute uniformly in the a-Mg matrix. The yield strength and ultimate tensile strength, as well as the maximum and minimum peak stresses during cyclic deformation in the T5 and T6 states were significantly higher than those in the as-extruded state. Unlike RE-free extruded Mg alloys, symmetrical hysteresis loops in tension and compression and cyclic stabilization were present in the GW103K alloy in different states. The fatigue life of this alloy in the three conditions, which could be well described by the Coffin-Manson law and Basquin's equation, was equivalent within the experimental scatter and was longer than that of RE-free extruded Mg alloys. This was predominantly attributed to the presence of the relatively weak texture and the suppression of twinning activities stemming from the fine grain sizes and especially RE-containing b¢ precipitates. Fatigue crack was observed to initiate from the specimen surface in all the three alloy states and the initiation site contained some cleavage-like facets after T6 heat treatment. Crack propagation was characterized mainly by the characteristic fatigue striations.

Micro injection molding is in great demand due to its efficiency and applicability for industry. Polymer surfaces having micro-nanostructures can be produced using injection molding. However, it is not as straightforward as scaling-up of conventional injection molding. The paper is organized based on three main technical areas: mold inserts, processing parameters, and demolding. An accurate set of processing parameters is required to achieve precise micro injection molding. This review provides a comparative description of the influence of processing parameters on the quality of final parts and the precision of final product dimensions in both thermoplastic polymers and rubber materials. It also highlights the key parameters to attain a high quality micro-nanostructured polymer and addresses the contradictory effects of these parameters on the final result. Moreover, since the produced part should be properly demolded to possess a high quality textured polymer, various demolding techniques are assessed in this review as well.

We describe a simple method for fabricating superhydrophobic high temperature vulcanized (HTV) silicone rubber surfaces by direct replication using a compression molding system. The resulting rubber samples possessed micro-nanostructures on the surface. This micro- and nano-scale roughness produced a water contact angle of >160º and a contact angle hysteresis of <3º. The roughness patterns on chemically etched aluminum surfaces, which served as templates, were successfully replicated on the rubber surfaces. An antistiction coating applied to the template surface ensured that the rubber was completely removed during demolding and that the replicated micro-nanostructures on the silicone surface were preserved. Surface roughness of the aluminum templates was optimized at HCl concentrations of 15 wt. %, with a lower roughness value observed at acid concentrations above and below this value. The developed HTV silicone rubber surfaces also demonstrated a freezing delay and a self-cleaning capacity.

Ultra-water-repellent silicone-based surfaces were produced to study their self-cleaning properties. First, we investigated the consistency of the micro-nano air pockets that are present between the surface asperities responsible for the formation of the Cassie-Baxter regime. We then performed a comprehensive series of self-cleaning experiments involving both suspended and non-suspended contaminants using various materials (e.g., kaolin, carbon black, silica, etc.) and contaminant-applying methods (e.g., dropwise, spraying, wet or dry contaminants). In this paper, the self-cleaning tests were arranged from the less severe, i.e., non-suspended contamination tests, to more severe, i.e., wet suspended contamination test, and ending with the most severe, i.e., dry suspended contamination test. Due to the ultra-low contact angle hysteresis, the produced surfaces showed favorable self-cleaning properties against the various types of contaminants and the different means of contaminant application. The produced surfaces retained their water repellency properties following application of the contaminants and after the cleaning of the surfaces, thus verifying the self-cleaning performance and resistance of the fabricated superhydrophobic silicone surfaces.

Herein, polymer processing systems are used to fabricate superhydrophobic high-temperature vulcanized (HTV) silicone rubber surfaces by direct replication. HTV silicone rubber is one of the main polymeric housing materials used in high-voltage insulators. The selected polymer processing techniques are compression molding and injection molding. The direct replication approach requires that a template or insert having the desired surface patterns be replicated onto a target polymer surface via a polymer processing. The appropriate micronanostructures, required for achieving ultra-water-repellency, were created on the insert materials (an aluminum alloy) using a wet-chemical etching method. As a flawless demolding is essential to acquire desirable replication quality, an antistiction coating was applied to the insert surfaces prior to the molding process to ensure the thorough removal of the silicone rubber during the demolding. The resulting silicone rubber surfaces possessed micro-na...