Spheroidal Grains Research Articles

Pollen diversity in Brazilian species of Monnina (Polygalaceae)

Monnina is subordinated to the Polygalaceae family, which consists of 19 genera and approximately 1300 species widely distributed throughout the continents. The current study aims to characterize pollen grains in the genera to provide the basis for their taxonomy. Thus, 13 species were analyzed: subg. Monninopsis (M. insignis and M. malmeana), subg. Pterocayra (M. cardiocarpa, M. cuneata, M. dictyocarpa, M. exalata, M. goiana, M. itapoanensis, M. oblongifolia, M. resedoides, M. richardiana, M. stenophylla, M. tristaniana). The shape of the species widely varied: M. insignis and M. malmeana showed prolate pollen grains; M. cardiocarpa, M. resedoides and M. richardiana showed subprolate pollen grains; and M. itapoanensis, M. stenophylla, and M. tristaniana showed oblate spheroidal and prolate spheroidal pollen grains in most of the species. According to data obtained in the current study, it can be concluded that it is possible to differentiate the species based on pollen attributes. Another important feature in the characterization of the species was the number of apertures, which varied from 10 to19 colporate ones.

Enhancement of VO2 thermochromic properties by Si doping

Silicon doped vanadium dioxide films were successfully prepared on indium tin oxide coated glass substrates at 255°C annealing temperature by direct current magnetron sputtering. Maintaining spheroidal grains, the size of VO2 films nanoparticles decreased with silicon doping ratios increasing. Transmittance spectra indicated a 46nm blue-shift of absorption edge and improved the samples' integrated luminous transmittances (from 28.4% at Si/V=0 to 36.1% at Si/V=0.17) with similar thickness of 90nm. The samples presented excellent thermochromic properties that possessed higher than 40% near-infrared switching efficiency at 2000nm and showed at least 9.2% solar modulation efficiency. Moreover, the metal–semiconductor phase transition temperature for heavily doped VO2 (Si/V=0.17) decreased to 46.1°C (22°C lower than bulk). These features suggest the practical application of VO2 to smart windows.

Impact of electromagnetic stirring upon weld quality of Al/Ti dissimilar materials resistance spot welding

Abstract The impact of electromagnetic stirring (EMS) upon the microstructure and mechanical properties of the Al/Ti resistance spot weld was investigated. The microstructures in the Al/Ti resistance spot weld joint were characterized by comparing them to those in an Al/Al resistance spot weld joint, and the effects of the welding current, welding time, and electrode force upon the tensile shear properties were also studied. The microstructure in the traditional Al/Ti joint consisted of a partially melted zone, a columnar grain zone, and a transition structure. Under the action of EMS, a fine spheroidal grain structure formed in the Al/Ti joint and, compared with the traditional Al/Ti resistance spot weld, the weld created under the EMS effect exhibited a larger bonding diameter, and higher tensile shear force and energy absorption. The large bonding diameter and fine spheroidal grain structure should account for the superior mechanical performance of the Al/Ti EMS joint.

Microstructure and mechanical properties of the semisolid slurries and rheoformed component of nano-sized SiC/7075 aluminum matrix composite prepared by ultrasonic-assisted semisolid stirring

An innovative method so-called ultrasonic-assisted semisolid stirring was proposed to fabricate semisolid slurries of the nano-sized SiC/7075 aluminum matrix composite. In this method, ultrasonic treatment and semisolid stirring were combined together to disperse the nano-sized SiC and break the primary dendrites of the matrix. The microstructure and mechanical properties of the semisolid slurries and rheoformed cylinder component were investigated. The results show that ultrasonic treatment dispersed the nano-sized SiC particles well due to the effect of transient cavitation and acoustic streaming. With increase of the stirring time the amount of the spheroidal grains of semisolid slurries increases, indicating achieving a better semisolid slurry. When stirring temperature is 620°C, the 20min stirring time led to a desirable semisolid microstructure consisting of a large number of spheroidal grains surrounded by liquid phase. High-quality semisolid slurries were only achieved at the stirring temperatures of 615°C and 620°C upon the 20min stirring time. Liquid fraction in the regions near to lateral surface is larger than those in the top, bottom and central regions due to main dependence on liquid flow and flow of liquid incorporating solid grains. Yield strength and ultimate tensile strength of rheoformed cylinder components of the nano-sized SiC/7075 aluminum matrix composite increased as compared with those of the 7075 matrix. Yield strength, ultimate tensile strength and elongation of the rheoformed cylinder components of the nano-sized SiC/7075 aluminum matrix composite without T6 are 264MPa, 357MPa and elongation of 7.5%, respectively. T6 heat treatment led to a significant improvement of yield strength, ultimate tensile strength and elongation. The mechanical properties of the rheoformed cylinder components with T6 of the nano-sized SiC/7075 aluminum matrix composite involve yield strength of 381MPa, ultimate tensile strength of 478MPa and elongation of 8.5%.

Microstructure and mechanical properties of the rheoformed cylindrical part of 7075 aluminum matrix composite reinforced with nano-sized SiC particles

Abstract The microstructure and mechanical properties of the rheoformed parts of 7075 aluminum matrix composite were investigated. The results show that more liquid phase and spheroidal solid grains were found in the microstructures in the top and medium regions of the side wall of the rheoformed parts due to dependence of the deformation on liquid flow and flow of liquid incorporating solid grains. Yield strength and ultimate tensile strength of the rheoformed composite parts are enhanced as compared with the as-extruded and the rheoformed parts of 7075 matrix alloy. The yield strength, ultimate tensile strength and elongation of the rheoformed composite parts with T6 reach 399 MPa, 492 MPa and 9.6%, respectively. When stirring time increases from 5 min to 20 min, the mechanical properties increased due to fine-grained strengthening and a further increase more than 20 min led to a slight change. Pressure led to a significant increase of the mechanical properties due to some reduction of defects and dislocations strengthening. An increase of SiC volume fraction from 0.5% to 1% led to an improvement of yield strength and ultimate tensile strength and a further increase led to a decrease.

Source and distribution of naturally occurring arsenic in groundwater from Alberta’s Southern Oil Sands Regions

Arsenic (As) concentrations as high as 179μg/L have been observed in shallow groundwater in the Alberta’s Southern Oil Sand Regions. The geology of this area of Alberta includes a thick cover (up to 200m) of unconsolidated glacial deposits, with a number of regional interglacial sand and gravel aquifers, underlain by marine shale. Arsenic concentrations observed in 216 unconsolidated sediment samples ranged from 1 and 17ppm. A survey of over 800 water wells sampled for As in the area found that 50% of the wells contained As concentrations exceeding drinking water guidelines of 10μg/L. Higher As concentrations in groundwater were associated with reducing conditions. Measurements of As speciation from 175 groundwater samples indicate that As(III) was the dominant species in 74% of the wells. Speciation model calculations showed that the majority of groundwater samples were undersaturated with respect to ferrihydrite, suggesting that reductive dissolution of Fe-oxyhydroxides may be the source of some As in groundwater. Detailed mineralogical characterization of sediment samples collected from two formations revealed the presence of fresh framboidal pyrite in the deeper unoxidized sediments. Electron microprobe analysis employing wavelength dispersive spectrometry indicated that the framboidal pyrite had variable As content with an average As concentration of 530ppm, reaching up to 1840ppm. In contrast, the oxidized sediments did not contain framboidal pyrite, but exhibited spheroidal Fe-oxyhydroxide grains with elevated As concentrations. The habit and composition suggest that these Fe-oxyhydroxide grains in the oxidized sediment were an alteration product of former framboidal pyrite grains. X-ray absorption near edge spectroscopy (XANES) indicated that the oxidized sediments are dominated by As(V) species having spectral features similar to those of goethite or ferrihydrite with adsorbed As, suggesting that Fe-oxyhydroxides are the dominant As carriers. XANES spectra collected on unoxidized sediment samples, in contrast, indicated the presence of a reduced As species (As(−I)) characteristic of arsenopyrite and arsenian pyrite. The results of the mineralogical analyses indicate that the oxidation of framboidal pyrite during weathering may be the source of As released to shallow aquifers in this region.

Effect of accumulative plastic deformation on generation of spheroidal structure, thixoformability and mechanical properties of large-size AM60 magnesium alloy

Accumulative plastic deforming (APD) at multistep temperatures was induced into the recrystallization and partial melting (RAP) route as a novel predeformation method to replace conventional upsetting or extrusion. AM60 magnesium alloy ingots with diameter of 520mm were produced by semi-continuous casting, and semi-solid slugs with diameter of 320mm were prepared by partial remelting of as-cast and APD formed alloys. The microstructure evolution during semi-solid remelting was investigated. The tensile mechanical properties of AM60 alloy at room temperatures were determined. The results show that large semi-solid AM60 alloy slugs with fine and homogeneous microstructures can be produced by employing APD in RAP route, and the microstructure consists of spheroidal solid grains surrounded by intergranular liquid matrix. The as-cast AM60 alloy during partial remelting showed inferior microstructures with coarsened solid grains and high proportion of intragranular liquid droplets. During partial remelting, coalescence and Ostwald ripening are dominant in as-cast alloy and APD formed alloy, respectively. The formation and migration mechanism of intragranular liquid droplets were discussed. Although both of APD formed and as-cast AM60 slugs could fill the die during thixoforming process, the tensile mechanical properties of the components thixoformed from APD formed alloys far exceed those from the as-cast alloys.

Study of Microstructure Evolution during Semi-Solid Processing of an In-Situ Al Alloy Composite

In the present work, effect of pouring temperature (650°C, 655°C, and 660°C) on semi-solid microstructure evolution of in-situ magnesium silicide (Mg2Si) reinforced aluminum (Al) alloy composite has been studied. The shear force exerted by the cooling slope during gravity driven flow of the melt facilitates the formation of near spherical primary Mg2Si and primary Al grains. Shear driven melt flow along the cooling slope and grain fragmentation have been identified as the responsible mechanisms for refinement of primary Mg2Si and Al grains with improved sphericity. Results show that, while flowing down the cooling slope, morphology of primary Mg2Si and primary Al transformed gradually from coarse dendritic to mixture of near spherical particles, rosettes, and degenerated dendrites. In terms of minimum grain size and maximum sphericity, 650°C has been identified as the ideal pouring temperature for the cooling slope semi-solid processing of present Al alloy composite. Formation of spheroidal grains with homogeneous distribution of reinforcing phase (Mg2Si) improves the isotropic property of the said composite, which is desirable in most of the engineering applications.

The Development of Spheroidal Grains and Thixoforming of AZ91D Magnesium Alloy Treated by Different Routes

A new process squeeze casting-solid extrusion was introduced to prepare semi-solid billets for thixoforging together with partial remelting. The microstructure development and tensile properties of AZ91D alloy reheated from states of as-cast and SCSE-formed were researched. During partial remelting, samples from SCSE-formed obtained finer and more uniform microstructure obviously than from as-cast. As the holding time went on, both solid particles from the two states experienced continuous spheroidization, and in the meantime from SCSE-formed were always to coarsen, while from as-cast were refined initially and coarsened then. Microstructure development is determined by the combination effects of various factors, including distortion energy from SCSE deformation, grain coalescence, Ostwald ripening mechanism, etc. During thixoforging, components with good forming quality were prepared successfully. Excellent tensile properties were obtained for the thixoforged alloy prepared by SCSE deformation, mainly with the microstructure refinement and the decrease of defects related. DOI: http://dx.doi.org/10.5755/j01.ms.20.4.6482

Comparison of Microstructure and Mechanical Properties of Semi-Solid Castings Produced Using Billets Made by EMS and SEED

The spheroidal grains in billets used for semi-solid casting are generally manufactured by electromagnetic stirring (EMS) during the casting process. This method however, is not economically applicable for small quantities of the thixo billets. Swirled Enthalpy Equilibration Device (SEED) has been developed as a rheocasting process, and the SEED process is of interest for developing new thixo alloys, as well as for optimizing the thixocasting processes for high quality components. The objective of this paper is to compare the microstructure and mechanical properties of aluminum alloy 319s billets and castings produced using EMS and SEED feed materials. The experimental results show that for as-cast billets made from SEED process, a well-developed spheroidal grain structure is distributed throughout the cross-section of the billet, while for as-cast EMS billets, the grain structure is inhomogeneous, i.e., a dendritic structure was present adjacent to the surface of the billet, while a uniform, spheroidal structure was present at the centre. After the thixocasting process, however, the both SEED and EMS billets have well-developed, spheroidal grain structures. Mechanical properties of thixocast and T61 heat treated components are comparable for the both SEED and EMS billets.

Visualization of solute distributions in dendritic and spheroidized Al grains characterized by both color etching method and electron probe microanalysis

Spheroidization of Al grains was realized by compressing and partially remelting the as-received A356 aluminum alloy billets with a dendritic microstructure. A color etchant named Weck’s reagent was used to characterize the microstructures of both dendritic and spheroidal Al grains. After etching, color differences were observed inside Al grains. In the spheroidal grains, both the original dendritic structure and grain growth during water quenching were revealed by this reagent. By electron probe microanalyses, it was found that such color difference was strongly related to the microsegregation of Ti. Also, both microsegregations of Si and Ti were found in dendritic Al grains, as well as the grain growth of spheroidal Al grains during water quenching. Quantitative analyses indicated that after heating and partial remelting, the concentration of Ti was increased from dendritic grain to spheroidal grain. This phenomenon was discussed and considered to be related to Ostwald ripening and the increase of Ti solubility in Al as the temperature increased.

Study of Soft Impingement of Diffusion Fields and Its Effect on Cast Microstructure

Rigorous Mullins and Sekerka perturbation theory has been applied to two solidification interfaces. The results indicate that as the distance between the two interfaces decreases, the rate of amplitude growth of perturbations on the planar solid–liquid interface becomes negative, which is a further verification of the stability of a planar interface due to soft impingement of diffusion fields during solidification. This work conclusively shows that spheroidal grains form due to grain refinement of Mg alloys by Zr addition because of the high density of nucleation.

Comparisons of microstructure, thixoformability and mechanical properties of high performance wrought magnesium alloys reheated from the as-cast and extruded states

Abstract The solid grain size, degree of spheroidization, contiguity and practical liquid fraction of Mg–Zn–Y–Zr alloys reheated from the as-cast and extruded states are characterized and compared. Thixoforming tests are performed to examine thixoformability of the as-cast and extruded alloys. Tensile mechanical properties for Mg–Zn–Y–Zr alloys thixoformed from the as-cast and extruded alloys are also determined. Reheating from the extruded alloy has been shown to produce ideal, fine semi-solid structures, in which completely spheroidal primary solid grains are essentially free of entrapped liquid. However, the as-cast alloy exhibits non-uniform microstructures with a large amount of entrapped liquid, which are less spheroidal compared with the extruded alloy in the semi-solid state. The grain coarsening rate is relatively low in the extruded alloy than that in the as-cast alloy during reheating. During thixoforming, despite the low liquid fraction, the fine and spheroidal solid grains are weakly connected and slide over each other without any plastic deformation in the extruded alloy. Therefore, the as-cast alloy which has heterogeneous and coarse solid grains is not as thixoformable as the extruded alloy. The tensile mechanical properties for Mg–Zn–Y–Zr alloys thixoformed from the extruded alloy are better than those of Mg–Zn–Y–Zr alloys thixoformed from the as-cast alloy.

Microstructural Evolution and Mechanical Properties of AZ31 Magnesium Alloy Prepared by Casting-solid Extrusion Forging During Partial Remelting

Abstract The casting-solid extrusion forging plus semi-solid partial remelting route is used to improve the properties of AZ31 magnesium alloy products. The effect of remelting temperature and holding time on the microstructure of AZ31 magnesium alloy is studied. Furthermore, the properties of AZ31 magnesium alloy components produced by the casting-solid extrusion forging plus partial remelting route are examined. The results show that the AZ31 components have very good smooth surface and are formed completely. The increases in holding time and remelting temperature result in the formation of spheroidal grains surrounded by liquid phases. The best combination of properties of thixoforged alloy is 290 MPa of tensile strength, 220 MPa of yield strength and 10% of percentage elongation.

Microstructure in the semi-solid state and mechanical properties of AZ80 magnesium alloy reheated from the as-cast and extruded states

The multi-axial forging (MAF) process was introduced into the strain induced metal activation (SIMA) process to replace conventional forging. Microstructure evolution of MAF formed AZ80 magnesium alloy during partial remelting was investigated. Furthermore, the tensile mechanical properties for AZ80 magnesium alloy thixoextruded from the starting materials treated by MAF were determined. For comparison, as-cast AZ80 magnesium alloy was also thixoextruded. The results show that the SIMA route produced ideal, fine semi-solid microstructure, in which almost completely spheroidal primary solid grains had a little amount of entrapped liquid. The microstructure of the as-cast alloy in the semi-solid state is less spheroidized compared with the MAF alloy under the similar isothermal holding conditions. With prolonged holding time, the size of the solid grain increases and the degree of spheroidization is improved in the MAF formed alloys. However, the solid grain size of the as-cast alloys decreases initially, and then increases with further increasing temperature. The tensile mechanical properties for AZ80 magnesium alloy thixoextruded from the starting material produced by MAF are better than those of AZ80 magnesium alloy thixoextruded from the starting material produced by casting. The ultimate tensile strength, yield strength and elongation of the alloy thixoextruded from the starting material produced by MAF are 314 MPa, 238 MPa and 14%, respectively.

Globular structure generation and mechanical properties of thixoformed magnesium alloy

For the alloy to be shaped in the semisolid state, it must have an appreciable melting range; before thixoforming, the microstructures must consist of solid metal spheroids in a liquid matrix. Equal channel angular extrusion (ECAE) was used as a process to predeform as cast AZ80 magnesium alloy before partial remelting. The results show that fine and equiaxed grains are obtained due to dynamic recrystallisation during ECAE. Following partial remelting in each case, the microstructure consisted of globular solid grains with liquid film along the grain boundaries. During partial remelting, solid grain sizes at 560°C were smaller than those at 550°C. Moreover, the degree of spheroidisation at 560°C was greater than that at 550°C. Good mechanical properties are obtained for the thixoformed AZ80 alloy prepared by six-pass ECAE with a yield strength of 210 MPa, an ultimate tensile strength of 320 MPa and an elongation to fracture of 9·2%. The results also show that the process, ECAE plus partial remelting, can produce fine and spheroidal solid grains surrounded by liquid film, which are suitable for thixoforming.

Systematic variations of interstellar linear polarization and growth of dust grains

The observed relation between the interstellar linear polarization curve parameters K and λ max characterizing the width and the wavelength of the polarization maximum, respectively, is interpreted quantitatively. We have considered 57 stars located in four dark clouds with evidence of star formation: in Taurus, Chamaeleon, around the stars ρ Oph and R CrA. In our modeling we have used the spheroidal dust grain model applied previously to simultaneously interpret the interstellar extinction and polarization curves in a wide wavelength range. The observed trend K ≈ 1.7λ max is shown to be most likely related to the growth of dust grains due to coagulation rather than accretion. The relationship of the interstellar polarization curve parameters K and λ max to the mean dust grain size is discussed.

Microstructure evolution and grain coarsening behaviour during partial remelting of cyclic extrusion compression formed AZ61 magnesium alloy

The effects of CEC passes, isothermal holding time and reheating temperature on the microstructure evolution and grain coarsening behaviour of AZ61 magnesium alloy produced by the recrystallisation and partialmelting (RAP) process were investigated. Before partial remelting, as-cast AZ61 alloy was deformed by cyclic extrusion compression (CEC) with one pass and two pass at 330 °C. After CEC, the microstructure consisted of unrecrystallized grains and deformed eutectic compounds. Increasing isothermal holding time resulted in the formation of spheroidal grains surrounded by liquid films. With increasing the isothermal holding time, the solid grain size increased and the degree of spheroidization was improved. With increasing the reheating temperature, namely increasing liquid fraction, the solid grain size obviously decreased during the period from 560 °C to 570 °C and then slightly increased after 570 °C, while the shape factor increased monotonously. During partial remelting, increasing reheating temperature can properly short the isothermal holding time to obtain fine structure. Moreover, increasing the numbers of CEC passes could produce finer semi-solid microstructure. The coarsening behavior of solid grains in the semi-solid state obeys Ostwald ripening and grain coalescence mechanisms. The coarsening rate constant, K, was 80 µm3·s−1 for samples partially remelted at 595 °C. After CEC plus partial remelting, the ideal and fine semi-solid state structure can be obtained, which was suitable for thixoforming.

Pollen morphology of Iranian species of Anemone L. & Pulsatilla Mill. (Ranunculaceae) and its implications

In this study, pollen analyses of Iranian species of Anemone L. and Pulsatilla Mill. were conducted through light microscopy and scanning electron microscopy. Two pollen classes were observed in these species: trizonocolpate and pantoporate. The trizonocolpate class consists of only Anemone caucasica species, which are isopolar and prolate with microechinate ornamentations, and the pantoporate class consists of four other Anemone taxa and Pulsatilla albana, which have isopolar and spheroidal pollen grains, with 8–18 circular pores and microechinate ornamentations. Some characters show variations between/within species and populations and must be regarded in taxonomic studies. It is necessary to write a key for the porate pollen grains of Ranunculaceae, especially Anemone species. Pollen character evolution of these species is not clear.

Microstructure-induced hotspots in the thermal and elastic responses of granular media

This work is a combined analytical and numerical study of the extreme values of the thermal and elastic fields occurring in a propellant composite material, a granular medium containing dense self-assembled spheroidal grains embedded in a matrix. First, a 3D microtomography image of a representative sample is segmented using morphological operators. Second, the local temperature and heat flux in the quasi-static thermal response is computed numerically, making use of the segmented microstructure and of experimental values of the heat conductivity in each phase. Such fields are readily derived by means of a Fast Fourier Transform method. Emphasis is put on the maximum values of the local fields: even at low contrast of properties between the grain and the matrix, the heat flux patterns is made of hot-spot zones located in-between grains that are close to each other with preferential directions. Third, the local extrema of the fields are investigated in the context of linear elasticity. Finally, analytical approximations are examined at low and high contrast of properties, on a simple hard-core model of cylinders, which is tantamount to a 2D granular microstructure.