Hot shock compaction of nanocrystalline alumina

An experimental investigation of hot shock compaction of a nanocrystalline alumina powder was performed. The effects of variations in shock pressure and compaction temperature on the properties of the compacted materials were studied. It was found that the bulk density and hardness of the compacted material increased with shock pressure. Increasing compaction temperature resulted in increases in compact hardness and bonding, and reductions in cracking within the compacted specimens. The results suggest that dense, well bonded, crack free nanocrystalline ceramics may be fabricated more effectively using hot shock compaction, than by room temperature shock compaction followed by sintering or room temperature static compaction followed by sintering.     

Ceramics superplasticity: Deformation mechanisms and microstructures

The superplasticity shall be generally achieved by grain refinement of various ceramics (ionic polycrystals and covalent polycrystals). This nature can be utilized for novel deformation processing in ceramic industy, for example, superplastic forming and superplastic forging. The superplasticity is a macroscopic phenomena that is very sensitive to slight difference in atomistic structure, and nature and chemical bonding of grain boundary. The mechanism of superplasticity is grain boundary sliding accommodated by matter transport through grain boundary. The models developed for superplasticity are classified by the structure of grain boundary. The experimental results on superplasticity of Zro₂ and Si₃N₄ were reviewed and compared to the predictions from the theories.     

Influence of microstructures on fatigue damage mechanisms in Ti-15-3 alloy

In this study, the fatigue behavior of Ti-15-3 alloy thin plate specimens with two different microstructures was determined. Two kinds of specimens were prepared with different heat treatments: solution treatment (S) and solution treatment followed by aging (S+A). The effects of the microstructures on the fatigue properties and fatigue crack growth behavior were significant in both specimens. The fatigue crack in both specimens propagated in transgranular mode. In the specimen S+A, crack propagation has occurred on non-crystallographic and was closely connected with the configuration of the α-phase platelet, which was caused by the heat treatment. The damage was characterized by dislocation debris clustering ahead of the crack tip.     

Interfacial morphologies and possible mechanisms of copper wafer bonding

The microstructure morphologies of copper bonded wafers were examined by means of transmission electron microscopy (TEM) and atomic force microscope (AFM). Morphologies of non-distinct, zigzag and distinct interfaces in the bonded layer are observed. A strong relationship between the roughness of surfaces and the individual steps in bonding initiation was found. We propose three different mechanisms to explain the observed morphologies. In addition, the role of atomic diffusion and that of annealing effects during bonding is discussed.     

Various forms of drop fractionation in shock waves and their special characteristics

The principal forms of drop fractionation behind shock waves are considered; empirical relationships are established for the most important parameters of the fractionation process.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 27, No. 1, pp. 119–126, July, 1974.     

Virtual texture analysis to investigate the deformation mechanisms in metal microstructures at the atomic scale

Journal of Materials Science - Understanding the deformation behavior of metallic materials at high strain rates requires the characterization of plasticity contributors, such as twins, phase...     

Microtopography of the eye surface of the crab Carcinus maenas: an atomic force microscope study suggesting a possible antifouling potential

Marine biofouling causes problems for technologies based on the sea, including ships, power plants and marine sensors. Several antifouling techniques have been applied to marine sensors, but most of these methodologies are environmentally unfriendly or ineffective. Bioinspiration, seeking guidance from natural solutions, is a promising approach to antifouling. Here, the eye of the green crab Carcinus maenas was regarded as a marine sensor model and its surface characterized by means of atomic force microscopy. Engineered surface micro- and nanotopography is a new mechanism found to limit biofouling, promising an effective solution with much reduced environmental impact. Besides giving a new insight into the morphology of C. maenas eye and its characterization, our study indicates that the eye surface probably has antifouling/fouling-release potential. Furthermore, the topographical features of the surface may influence the wettability properties of the structure and its interaction with organic molecules. Results indicate that the eye surface micro- and nanotopography may lead to bioinspired solutions to antifouling protection.     

Formation mechanisms of processing defects and their relevance to the strength in alumina ceramics made by powder compaction process

With new characterization techniques, the structures of green and sintered bodies were examined in great detail to understand the formation mechanism of potential fracture origins in alumina ceramics made by the powder compaction process. Large pores, which are found at many fracture origins, were formed in green bodies through two mechanisms. One is created from the dimple in the powder granules and the other from inadequate cohesion of the granules. Another potential cause of fracture, the large grains, developed from large particles in the green body. The concentration, especially of large pores was found to be quite high. This is clearly responsible for the moderate strength with high Weibull's modulus of the alumina ceramic examined in this study.     

Reaction mechanisms,resultant microstructures and tensile properties of Al-based composites fabricated in situ from Al-SiO2-Mg system

Reaction mechanisms, microstructures and tensile properties of the aluminum matrix composites made from Al-SiO₂-Mg system were investigated. When the temperature increased from room temperature to around 761 K, Mg dissolved into Al to form Mg-Al alloy. As the temperature increased to about 850 K, the remaining Mg reacted with SiO₂ to form MgO, Mg₂Si and Si as expressed in step reaction I: 6Mg + 2SiO₂ → 4MgO + Mg₂Si + Si. Finally, with a further increase in temperature, the remaining SiO₂ reacted with Al to produce Al₂O₃ and Si, while MgO reacted with Al₂O₃ to form MgAl₂O₄ as expressed in step reaction II: 4Al + 3SiO₂ + 2MgO → 2MgAl₂O₄ + 3Si. The Si also dissolved into matrix Al to form Al-Si alloy. Accordingly, its reaction process consisted of two steps and their apparent activation energies were 218 kJ/mol and 192 kJ/mol, respectively. As compared to the composites prepared by Al-SiO₂ system, its density increased from 2.4 to 2.6 g/cm³, and its tensile strength and elongation increased from 165 MPa and 3.95% to 187 MPa and 7.18%, respectively.     

Synthesis and modification of materials by shock waves: real-time measurements and mechanisms of reaction

The formation of AB-type compounds under isothermal conditions is accompanied by a volume decrease. However, as a result of the heat release, there is a net increase in volume, when the reaction exothermicity is high. Exothermic reactions under high pressure lead to a volume increase of approximately 80% (alkali halogenides) to 20% (other compounds). The kinematic measurements allow the determination of the volume changes for the latter cases if the conversion degree is more than 30% because the experimental error of the volume measurements is usually 3–6%. In this article, a review of the kinematic (shock and particle velocities) shock-wave measurements of reactions in the condensed state is presented. The temperature measurements have an accuracy and time resolution of one order better than the kinematic ones. The mechanism of superfast diffusion based on a difference of the particle velocities of compounds is discussed and it is suggested that the penetration of particles through each other follows their crushing. Taking into consideration that spalling in shock waves leads to domains of 10 nm in size and assuming that chemical reactions occur on surfaces of such domains, a degree of conversion is obtained that correlates well with the experiment.     

Comparison of beginning and ending microstructures in metal shaped charges as a means to explore mechanisms for plastic deformation at high rates

Optical metallography and transmission electron microscopy (TEM) observations were made of a variety of forged or sputtered copper, molybdenum, and tantalum shaped charge components. The beginning shaped charge liner grain sizes and sub-structures were compared with those observed in residual (ending), recovered and corresponding jet fragments and slugs. The wide range of microstructures and evolutionary features of observed microstructures can be characterized by low-energy dislocation structure (LEDS) principles which are altered because the shaped charge deformation corresponds to hot working, and dynamic recovery and recrystallization play a prominent role. There is a prominent relationship between the starting liner grain size, D _o, and the ratio D _o/D _s, where D _s is the ending (slug or jet), steady-state grain size. As a consequence of this relationship, it appears that the volumetric stored energy, which depends upon the grain size and dislocation density (or degree of deformation), is the critical issue in controlling shaped charge jet stability.     

The avalanche ionization rate and energy exchange mechanisms in argon plasma behind the shock wave front

The ionization rate of argon in a relaxation zone behind the shock wave front was measured in stable flow regimes with Mach numbers above 11. Based on an analysis of the electron energy balance equation, it is suggested that high (as compared to the results of calculations) values of the avalanche ionization rate are related to the existence of an additional channel of the energy transfer during the associative ionization of argon atoms.     

Effects of particle size and properties on the microstructures,mechanical properties,and fracture mechanisms of 7075Al hybrid composites prepared by squeeze casting

To investigate the effects of particle size and properties on the mechanical properties of 7075Al matrix composites, hybrid composites reinforced using three different reinforcement combinations, 40 vol. % 7 μm SiC_p with 5 vol. % 7 μm SiC_p, 35 μm SiC_p, and 35 μm Ti, were prepared using squeeze casting. The failure mechanisms and the microstructure–property relationships of hybrid composites were studied using SEM, TEM, and tensile tests. The composite containing Ti particles achieved the highest tensile strength of 626 MPa and an elongation of 1.2 %. Fracture mechanism analyses imply that the reduced strength for the 35 μm SiC_p-containing composite are caused by the inefficient load transfer capability resulting from the preferential breakage of larger-sized SiC_p particles during the deformation process. In contrast, micro-zones formed by Ti particles at the center and matrix alloy with few dislocations around release stress and deform synergistically during deformation, which decreases the breakage of SiC_p and improves the plastic deformation ability of the matrix alloy, resulting in a good combination of strength and ductility.     

An investigation of the possible interaction mechanisms for Si(Li) and Ge detector response functions by Monte Carlo simulation

Simple physical mechanisms implemented by either analog Monte Carlo simulations or analytical models have been used to investigate the Si(Li) and Ge detector response functions for X- and gamma rays, respectively. The mechanisms investigated include the various possible combinations of partial losses of photoelectric and Auger electrons from the detector surfaces and complete losses of the various photons involved such as the Si K X-ray, the 0.511 MeV annihilation photons, and single or multiple Compton scattered photons. The most probable interaction mechanisms for each detector response function feature are identified and the simple analytical functions that have been used in the past are justified.     

On the examination of temperature-dependent possible current-conduction mechanisms of Au/(nanocarbon-PVP)/n-Si Schottky barrier diodes in wide range of voltage

Journal of Materials Science: Materials in Electronics - Au/(nanocarbon-PVP)/n-Si SDs were fabricated and their current-conduction mechanisms (CCMs) have been examined in elaborative by utilizing...     

Photocatalytic properties of BiVO4 prepared by the co-precipitation method: Degradation of rhodamine B and possible reaction mechanisms under visible irradiation

Bismuth vanadate (BiVO₄) was synthesized by the co-precipitation method at 200 °C. The photocatalytic activity of the oxide was tested for the photodegradation of rhodamine B under visible light irradiation. The analysis of the total organic carbon showed that the mineralization of rhodamine B over a BiVO₄ photocatalyst (∼40% after 100 h of irradiation) is feasible. In the same way, a gas chromatography analysis coupled with mass spectroscopy revealed the existence of organic intermediates during the photodegradation process such as ethylbenzene, o-xylene, m-xylene, and phthalic anhydride. The modification of variables such as dispersion pH, amount of dissolved O₂, and irradiation source was studied in order to know the details about the photodegradation mechanism.