THE EFFECT OF ANNEALING AND COLD FORGING ON MICROSTRUCTURE AND HARDNESS PROPERTIES OF Al-SiC COMPOSITE : A PRELIMINARY STUDY

THE EFFECT OF ANNEALING AND COLD FORGING ON MICROSTRUCTURE AND HARDNESS PROPERTIES OF AL-SIC COMPOSITE: A PRELIMINARY STUDY. Aluminium Metal Matrix Composites (AMMCs) are one of the exciting materials that have an extensive function in various applications. By utilizing reinforcement in the fabrication process, Al composites can produce superior properties such as high strength, good fracture resistance, and of course, lightweight. Therefore, many studies are interested in revealing the characteristics of Al composite materials through various methods and variations of reinforcement. This research is a preliminary study with a scope of work, including observing the effects of annealing and cold forging processes on the microstructure morphology and hardness properties of SiC nano-ceramic reinforced Al composites. The aluminium used in this study is a 7xxx series aluminium alloy. The fabrication process was carried out by stir-squeeze casting method. Microstructure analysis was conducted by optical microscopy and Scanning Electron Microscopy (SEM) equipped with Emission Dispersion Spectroscopy (EDS). The hardness properties of the Al-SiC composite were examined by micro Vickers hardness testing. This research reported that the annealing process influences the grain refinement and hardness properties of the Al-SiC composite. The sample experienced to cold forging has to improve the hardness value. Increasing hardness by forging after anneal may introduce due to the grain compression effect of the dislocation mechanism. Comprehensive research is required to find out other potentials of Al-SiC composite materials. Keywords: Al-SiC composite, annealing temperature, cold forging, hardness, microstructure.

Kinetics and mechanism of linear crystal growth in As2Se3 glass

Ранее исследовано влияние малых добавок олова и свинца на кинетические параметры изотермической объемной кристаллизации стекол на основе As2Se3. Кинетика преимущественно поверхностной кристаллизация стекла As2Se3, используемого в электронной технике и волоконной ИК оптике, изучалась методами дифференциальной сканирующей калориметрии и микроскопии. Влияние величины переохлаждения ΔТ на механизм и кинетические параметры кристаллизации стекла As2Se3 изучено недостаточно. Автором статьи выполнен теоретический анализ экспериментальных зависимостей скорости линейного роста кристаллов As2Se3 от температуры и вязкости стекла As2Se3. На основе анализа существующих данных и приводимой работы показано, что в стекле As2Se3 при переохлаждениях ΔT = 30–135 вероятен дислокационный рост кристаллов в форме сферолитов. Для индивидуальных пластинчатых кристаллов As2Se3 с бездислокационными гранями внутри сферолитов при ΔT = 75–135° вероятен механизм роста с поверхностной двумерной нуклеацией, или 2Dsg–model. Скорость линейного роста кристаллов в стекле As2Se3 при 240−345 °C теоретически рассчитывается, если в формулу Тернбала–Коэна ввести вероятностный фактор f (ТΔT) роста на ступенях винтовых дислокаций, а свободную энергию активации роста кристаллов ΔGa'' (T) принять равной кинетическому барьеру при вязком течении стекла ΔGη # (T) в уравнении Эйринга. The influence of small additions of tin and lead on the kinetic parameters of isothermal bulk crystallization of glasses based on As2Se3. The kinetics of mainly surface crystallization of As2Se3 glass used in electronic engineering and fiber IR optics has been studied by differential scanning calorimetry and microscopy. The influence of the supercooling value ΔT on the mechanism and kinetic parameters of crystallization of As2Se3 glass is not sufficiently studied, The theoretical analysis of the experimental dependences of the linear growth rate of As2Se3 crystals on the temperature and viscosity of the As2Se3 glass has been performed. In the temperature range 240–345 °С (overcooling 135°–30°), a dislocation mechanism has appeared to be probable for the linear growth of spherulites and the surface crystallized layer in the bulk As2Se3 glass. For individual crystals with dislocation-free faces in As2Se3 spherulites, the lamellar (plate-like) growth mechanism with surface two dimensional nucleation (2Dsg model) appeared to be probable. The linear growth rate in glass As2Se3 at 240–345°C can be theoretically calculated, if we introduce the probability factor f (ТΔT) to the Turnball–Cohen formula of the growth on the steps of screw dislocations and assume the free activation energy for the crystal growth ΔGa'' (T) to be equal to the kinetic barrier at the viscous flow of the glass ΔGη # (T) in the Heiring equation.

Thermally activated dislocation mechanism in Mo studied by indentation, compression and impact testing

Body-centered cubic metals like molybdenum and tungsten are interesting structural materials for high-temperature applications. These metals, are however, brittle at low homologous temperature, caused by the limited mobility of screw dislocations. In this study, the thermally activated deformation mechanisms in bcc Mo have been investigated using strain rate jump nanoindentation and compression tests as well as Charpy V-notch impact testing. The material shows a significant softening with increasing temperature and a maximum in strain rate sensitivity is found at the critical temperature, before decreasing again in the ductile regime. The activation volume, however, showed a distinct increase from about 5 b3 at the onset of the brittle to ductile transition temperature. Here we propose to use temperature-dependent nanoindentation strain rate jump testing and the activation volume as a complementary approach to provide some indication of the brittle to ductile transition temperature of bcc metals. Graphic Abstract

Hot and Cold Deformation Behaviors and Microstructure Evolutions of Mn18Cr18N Austenitic Stainless Steel with High-Nitrogen

Hot and cold deformation behavious and microstructure evolutions of Mn18Cr18N were investigated by thermo-mechanical modeling experiments and microstructure analysis. The results show that hot deformation flow stress curves characterized by the same work hardening and subsequent stress softening varied with temperatures and strain rates for both of as-cast and as-forged samples. And flow stresses are sensitive to strain rate. At strain rates lower than 0.01s-1, the flow stresses are lower, and microstructure evolution controlled by dislocation mechanism dynamic recrystallization; At strain rates higher than 0.1s-1, the flow stresses are higher, and microstructure evolution controlled by twinning mechanism dynamic recrystallization. But the dynamic recrystallzed fraction of the as-cast sample was much less than that of the as-forged sample. For cold deformation, the simple uniaxial tensile sample shows that the monotone increasing flow stress curve and monotone decreasing work-hanrdening rate. However, for the uniaxial and biaxial compression-tensile samples with different previous compression, the subsequent tensile yield stress, the maximum tensile stress, the reduction of cross sectional area and the elongation have extremums respectively at the previous compressive deformation of about 25%-30%. Microstructure evolution mechanisms during cold deformation were planar slipping and twinning.

Nano- and micromorphological evidences of the colloidal structure of ring silicate crystal inclusions

The substance released naturally during mechanical opening of growth inclusions of aquamarine, beryl and tourmaline crystals (Transbaikalia, Russia) has been studied by scanning atomic force microscopy (AFM) and electron microscopy (SEM). The liquid part of the substance of the inclusions is a sufficiently concentrated colloidal sol-gel-silicate system. The diffused silicate colloid forms a film, poorly removable from the chip surface, which is reliably detected by AFM and transformed under prolonged exposure to an electron beam into a foam-glass. The latter fact can be used to create sub-micron textures that are stable under normal conditions by electron-beam lithography. The study of quartz and phenacite (Ural, Russia) did not reveal such films. In contrast to aquamarine, the inner walls of cavities of quartz and phenacite inclusions, according to AFM data, are covered with growth hills, grown by the dislocation mechanism. This indicates a relatively low degree of polymerization of the silicate component in the initial hydrothermal mother liquor for quartz and phenacite compared to the degree of polymerization in the mineral-forming media of ring silicate crystals.

Alloying effects on the microstructure and mechanical properties of nanocrystalline Ti-based alloyed thin films

The plastic deformation of nanocrystalline Ti alloyed films is mainly mediated by the partial dislocation mechanism emanating from grain boundaries.

Morphology and genealogy of structural defects in vacuum ion-plasma coatings

Introduction. The main work objective is systematization and analysis of structural defects of vacuum ion-plasma coatings; on this basis, their classification principles are developed and given in the paper. Another important part of the work is the experimental study on one of the specific defects of coatings, which the authors propose to call “defect of substructural origin”.Materials and Methods. PVD coatings of various nitride and metal systems 1.5-9.0 μm thick were used as an object of the research. Coatings were applied in vacuum installations using arc and magnetron evaporators. The research results were obtained by high resolution electron microscopy, energy dispersive analysis and indentation.Results. Various types of defects in ion-vacuum coatings are presented as the research results. They include discontinuities, deformation of crystallites, and structural inhomogeneity. The principles of their systematization are validated. It is proposed to classify defects into droplet, substructural, and growth defects (depending on the causes of their nucleation), as well as regular and stochastic ones (depending on their distribution in the coating volume). The study of “substructural defects”, classified by the authors as stochastic, is given special consideration. These micrometric defects are shaped like a cylinder with a conical “head”. Their main axle is oriented perpendicular to the surface of the coating. They can be “extruded” (tore away) by the coating. The paper validates the dislocation mechanism of their nucleation and the helicoid growth principle. Conclusions. The inference is summarized that the proposed systematization of defects in ion-plasma coatings has the character of an intermediate result of research in this scientific area. At this, the “substructural defects” do not have a fatal effect on the structure and properties of the coating due to a small size.