Feathery Crystals Research Articles

TEM investigations on twin boundary structures of feathery crystals in aluminum alloys during Bridgman solidification

Feathery crystals are an ensemble of twinned dendrites, and are characterized by a unique twin boundary (TB) structure in the solidification pattern of aluminum alloys. In this work, the high-density twinned dendrites of Al-4.5 wt% Cu alloys, produced during the Bridgman solidification, have been studied using electron backscattered diffraction (EBSD) and high-resolution transmission electron microscopy (HRTEM). The experimental results showed that, after systematically decreasing the growth rate from 3000 μm/s to 1 μm/s, the TBs remained stable, while the solute field around the TBs changed significantly. According to the HRTEM results, successive stacking faults were occurred near the TBs at 1 μm/s, while slight distortion was observed around the TBs at 3000 μm/s. The composition analysis revealed an obvious solute enrichment near the TBs. Furthermore, the solute gradient profile within the TBs became smoother with the decrease in the growth speed. This is due to the more sufficient solid-state back diffusion occurring perpendicular to the twin plane after the solidification.

Prediction of Snow Avalanches by Using Surface Layer Heat Fluxes

Snow avalanches are formed in the mountainous snow regions mostly due to the presence of weak layers between the adjacent snow layers. Since the prevailing at mosphere in these regions is stable, weak snow layers are formed due to the presence of surface hoars, which is measured by surface layer heat flu xes. Surface hoar forms when relatively mo ist air over a cold snow surface beco mes over saturated with respect to the snow surface; causing a flu x of water vapor, which condenses on the surface, and creates feathery crystals (the icy layer equivalent to dew). Once buried, the resulting weak layer is a serious consideration for avalanche format ion. Surface hoar tends to form at night when the snow surface generally cools and the adjacent air becomes over saturated. The surface layer heat flu xes in the atmospheric boundary layer over Himalayan regions are calculated here by stability criteria using the bulk aerodynamic method.

Formation of twinned lamellas with the application of static magnetic fields during semi-continuous casting of Al–0.24 wt%Fe alloy

Al–0.24 wt%Fe was semi-continuously cast with the application of static magnetic field of 0.1 and 0.2 T. The magnetic field transfers the microstructure from the columnar grains growing in the 〈1 0 0〉 direction into twinned lamellar crystals growing in the 〈1 1 0〉 direction. The lamellas possess the morphology and the crystallographic growth direction of the “feathery crystals”. However, the “feathers” (secondary arms) that branch off the twinning plane only start to form at 0.2 T. Under the two strengths of the magnetic field applied, no side arms in the twinning planes were observed. The increase of the lamellar spacing with the magnetic field is due to the initiation of the secondary arms.

Will the True Originator of the Storm Glass Please Own Up

The "storm glass" or "chemical weather glass" is a slender glass tube, eight to ten inches long, containing a liquid composed of a mixture of nitre and a compound described variously as sal ammoniac or Roman alum, dissolved in water, and added to camphor dissolved in ethanol. From time to time, feathery or star-like crystals appear. Some people claimed that these phenomena indicated coming weather, and others that they resulted from changes in temperature, light, wind direction, or atmospheric electricity. In the later nineteenth century such storm glasses were the subject of considerable publicity generated in England by Robert Fitzroy, then Director of the Meteorological Office of the Board of Trade, and the London instrument makers Negretti & Zambra. Storm glasses had, however, been known in France since at least the 1780s, possibly as a consequence of experimentation by industrial chemists.

Casting and Extrusion of 7003 Alloy Billets Having Complete Feathery Crystal by Using Unidirectional Solidifying Process

Casting and Extrusion of 7003 Alloy Billets Having Complete Feathery Crystal by Using Unidirectional Solidifying Process

Dendrite growth morphologies in aluminium alloys

Different aluminium alloys, in particular Al–Cu, Al–Mg and Al–Si, have been directionally solidified under well-controlled thermal and convection conditions. For relatively high solidification rates, particular growth morphologies were observed. The most common is linked with the formation of feathery grains: these are constituted by series of twinned lamellae, in which the dendrites have 〈110〉 trunks cut through by {111} twin planes. These grains undergo a selection mechanism which is similar to that occurring for regular 〈100〉 dendritic grains. The dendritic growth along 〈110〉 crystallographic directions is supposed to be due to a change in the anisotropy of certain properties of the alloy, such as the solid/liquid interfacial energy and/or the atom attachment kinetics. When solidification conditions become less favourable to 〈110〉 growth morphologies, a mixed dendritic form containing 〈110〉 trunks and 〈100〉 arms may be obtained. In the case of the 5182 Al–Mg type alloy, 〈110〉 columnar grains which were not twinned could be observed together with feathery crystals. The possibility of such changes in the growth direction of aluminium alloys was further demonstrated by the observation of dendrites of Al–Cu–Mg solidified in a Bridgman device. In this case, 〈112〉 dendrites grow and progressively change their growth direction, thus showing curved shapes.

Investigation of feathery crystals developed in weld metal in aluminium alloys by SAM and TEM

Summary The macroscopic solidification structures developed in the weld metal of aluminium alloys are generally columnar crystals and equiaxed dendrites. The development of these macrostructures depends on the welding conditions. Feathery crystals are sometimes developed during low-speed GTA (TIG) welding. This paper describes an investigation of the characteristics of feathery crystals by scanning acoustic microscopy (SAM) and transmission electron microscopy (TEM). SAM enables structures to be observed without etching. The growth features of feathery crystals are clarified by observations made normal and parallel to the welding direction. Measurements of the velocity of the elastic surface wave of feathery crystals by SAM with an anisotropic acoustic lens show the velocity to be around 100 m/sec faster parallel to the welding direction than normal to it, confirming that the feathery crystals are acoustically anisotropic. Feathery crystals are also observed in electron beam welds by SAM, although it is not generally easy to observe the macrostructures of these welds because of the beam width being narrow. The diffraction patterns and TEM observations of the feathery crystals show them to be twinned crystals with the (111) plane as their twinning plane.

Electron backscattered diffraction investigation of the texture of feathery crystals in aluminum alloys

Scanning electron microscopy (SEM), metallographic observations, and automated electron back-scattered diffraction (EBSD) experiments were carried out on “feathery crystals” of a unidirectionally solidified (1D) Al-Cu alloy and of a direct-chill (DC) cast Al-Mg-Si alloy. The results clearly show that the “feathery grains” are made of twinned lamellae, which are parallel to a (111) twin plane. The contrast seen in the metallographic sections after a Barker etching or observed in an SEM is perfectly corroborated with the EBSD reconstructed microstructure. The lamellae are separated by an alternance of straight and wavy lines. Some equiaxed grains are also observed occasionally in the specimens. From the 〈111〉 pole figures of the various grains, it is concluded that the thermal gradient direction is close to, but not necessarily within, the (111) twin plane: its direction is in between a [011] and a [112] direction. Within a given feathery grain, small variations of the crystallographic orientations (subgrain boundaries) are observed. The lamellae of one grain can sometimes penetrate into another one. Based upon this information, the mechanism of feathery grain growth previously proposed by Eady and Hogan is ruled out. Although no other growth mechanism is proposed yet, it is believed that feathery grains are the result of a competition similar to that occurring in normal dendritic alloys, but with twinned dendrites.

ＳＡＭ及びＴＥＭによるアルミニウム合金の溶接金属中に形成される羽毛状晶の検討

Macrostructures which are usually developed near the center of weld metal of aluminum alloys are columnar crystals and equiaxed dendrites. The development of the macrostructures depends on the welding conditions. A feathery crystal is sometimes developed when welded by GTA in a low welding speed. In this study, we investigate characteristics of the feathery crystal using both a scanning acoustic microscope (SAM) and a transmission electron microscope (TEM). It is possible to observe structures without etching by SAM. By observing normal and parallel section to the welding direction, we made clear the growth feature of the feathery crystal. When we measure the velocity of elastic surface wave of the feathery crystal using SAM with an anisotropic acoustic lens, the velocity is about 100 m/s faster in parallel to the welding direction than in normal direction. That is, the feathery crystal is acoustically anisotropic. We could observe the feathery crystal even in electron beam welds using SAM. It is not generally easy to observe macrostructure of electron beam welds, because the bead width is narrow. Moreover, we confirm that the feathery crystal is a twin by analyzing electron diffraction patterns obtained using TEM.

Visualization and Analysis of Air Currents in the Plant Tissue Culture Vessel

Air currents, and consequent diffusion processes, in the plant tissue culture vessel are slow and limit photosynthesis, transpiration, and uptake of water and nutrients by in vitro plantlets. Air currents in Magenta-type and Mayonnaise bottle-type culture vessels were visualized using fine particles of feathery crystal of metaldehyde [(CH3CHO)4] as tracers. Pictures of visualized air currents were recorded by the video camera. Air current speeds in the culture vessel were determined by analyzing the changes in video pictures with time. Air current speed around the culture vessel was set at 0.1 and 1.0 m–s–1. Shortwave radiation flux density on the shelf was varied between 0 and 34 W–m–2. Shortwave absorptivity of the medium was 44% for agar medium and 97% for agar medium containing activated charcoal. Under a shortwave radiation flux density of 10–34 W–m–2, the upward air currents were observed at the central part of the culture vessel and downward air currents were observed near the inside walls of the culture vessel. The upward air current speed was affected by air current speed around the culture vessel, shortwave radiation flux density, and shortwave absorptivity of the medium. Under shortwave radiation flux density of 34 W–m–2, the upward air current speed in the culture vessel was ≈4 times greater with 1.0 m–s–1 air current speed around the culture vessel than with 0.1 m–s–1.

Ａｌ‐Ｍｇ合金の羽毛状晶における双晶境界の間隔ならびに傾斜角に及ぼす凝固条件の影響

The feathery crystal billets in Al-Mg alloys were solidified by using Graphite Mold Submerging (GMS) process, which is that the graphite mold filled with molten metal is constantly submerged into water directly and the solidifying rate and cooling rate can be controlled by combining descending speed of mold and casting temperature. The changes of the spacing or direction of twin boundaries in each feathery crystal billet were examined microscopically. Features of twin boundaries are as follows: (1) The twin boundary spacing decreases with the increase of solute concentration and solidifying rate and is little affected by the temperature gradient. (2) Immediately after the generation of feathery crystals, the mean spacing of twin boundary λ increases temporarily under any circumstance. (3) In the stable growth state, the angle θ between the twin boundary and the billet axis and λ tends to become lager at the periphery than at the center of the billet.

Podogenic columnar calcite from the Oolite Group (Lower Carboniferous), South Wales

Calcrete palaeosols in the Oolite Group have microspar hardpan layers overlying clay-rich nodular layers which, in turn, overlie the degraded host oolite. Within and immediately below hardpan layers there are extensive sheet-like and fan developments of columnar calcite with individual crystals up to 30 cm long. Crystals range from those with straight extinction and normal growth zonation through feathery crystals to those showing sweeping extinction. The calcretes accumulated within a seasonal climate with hardpan accumulation probably occurring through evaporation during the dry season. The columnar calcite precipitated from more dilute solutions with maximum growth probably occurring during the wet season. Precipitation may have been partly or wholly bacterially mediated, or induced by the reduction in pco2 of fluids as they migrated out of the extremely restricted microporosity of the hardpan. Columnar fabrics reflect low supersaturations relative to those at which the hardpan accreted, inhibition of crystal nucleation and of growth on x-axis parallel faces, and an excess of CO22− availability over Ca2+. Growth and nucleation may have been inhibited by the presence of particular organic compounds or clays. The range in crystal form is related to the degree of poisoning of crystal growth faces by organic compounds.

Formation of Feathery Crystals in Al–Si Alloys

Formation of Feathery Crystals in Al–Si Alloys

羽毛状晶における双晶境界の増殖

Feathery crystals formed in Al-5 mass%Mg alloys unidirectionally solidified were microscopically observed at intervals of about ten micronmeters for examining multiplication of twin boundaries. Two types of site for multiplication of feathery crystals are found. Type I is that a new twin boundary originates at a site separating from preexisting twin boundaries. And type II is that a new twin crystal originates at a site on the subgrain boundary generated in either individual of a preexisting twin crystal. These types of twin multiplication have particular features of the variations of length and spacing of twin boundaries.

羽毛状晶発生時における双晶境界形成過程の組織観察

The repeated micrography on transverse sections in Al-5 mass%Mg alloys unidirectionally solidified was carried out at intervals of about ten micronmeters until the first twin boundary disappeared in order to make clear the generation process of feathery crystals. Microstructures near the chilled end show that a curved grain boundary progressively straightens with growing of crystals, that is, the grain boundary transforms successively into a twin boundary. It is suggested that the transformation starts at the point where two neighboring crystals hold a (111) plane in common on the boundary. Either of crystals associated with the formation of twin boundary may be resulted from anneal-twinning with regard to solidifying conditions for generation of feathery crystals.

６０６３合金全羽毛状晶鋳塊の溶製と押出しへの応用

６０６３合金全羽毛状晶鋳塊の溶製と押出しへの応用

Ａｌ‐Ｍｇ合金の凝固条件による羽毛状晶の生成

The solidifying conditions for formation of feathery crystal in Al-Mg alloys were investigated by using unidirectional solidification and the new solidifying method "Graphite-Mold Submerging (GMS) process". This was able to control solidifying rate R and temperature gradient G respectively and its prominent feature was to submerge melt-filled mold heated at casting temperature into water bath directly. In the R-G diagram, the formation zone of feathery crystal in each composition of Al-Mg alloys was enclosed with suitable R and G respectively and its zone was shown as triangles between columnar and granular crystal zone. In view of relationship between G/R and R on the formation zone of each crystal structure, the feathery crystal zone overlapped columnar's and decreased continuously with increaing R. Therefore it is considered that feathery crystal is the peculiar structure belonging to columnar crystal.

Al-7 mass%Si, Al-4および6 mass%Cu合金における羽毛状晶のミクロ組織観察

Feathery crystals formed in Al-7.0 mass%Si, Al-4.0 and 6.0 mass%Cu alloys which were poured into molds at 1163 K and water-cooled on and off during unidirectional solidification were observed microscopically to examine their morphology and etch pits for estimating crystal orientation. Results obtained were as follows:(1) The feathery dendrites in the 7.0 mass%Si alloy have an appearance of a V-shape in the early stage of growth, but later gradually changed into a Y-shape. Primary stalks of the feathery dendrites become very long in the late stage of growth.(2) The primary stalks of the feathery dendrites in the 4.0 and 6.0 mass%Cu alloys seem to be short in both the early and the late stages of growth, but they are connected slightly with other primary stalks or sidebranches.(3) The primary stalks of the feathery dendrites grow in the direction of 〈110〉, but the secondary branches grow in 〈100〉 or 〈112〉.(4) The secondary branches of the feathery dendrites in the early stage are sometimes observed to be growing long upward as columnar dendrites.(5) In the 4.0 and 6.0 mass%Cu alloys, the primary stalks in the feathery dendrites sometimes grew at angles between 5° and 10° from the surface trace of the growth twin plane, and in other cases the surface trace was parallel to the growth direction of the primary stalk, but deviated from the central line of the stalk.(6) In the feathery dendrites, ternary branches are easily generated from secondary branches, but distinguishing the primary stalks from the ternary ones is extremely difficult. This multiplication mechanism of the feathery dendrites may be a controlling mechanism of the primary arm spacing of the columnar dendrites.

全羽毛状晶からなる６０６３ＤＣビレットの工業的規模押出試験について

The extrusion energy required for solid shape extrusion calculated from average pressure and the extrusion time are reduced in accordance with increase in the area ratio of feathery crystals in the cross section of a billet. Complete feathery crystal billets requires the extrusion time shortened by about 25 to 35% from that usual granular crystal billets do when extruded at 390°C, 0.45m/min and at extrusion ratio 49. Such a fact is not so significant in hollow shape extruding. The extrudability of the complete feathery billets considerably depend on the extruding direction. Extruding in the growth direction ensures further extrudability than extruding in the casting direction regardless of the extruding condition and products shape.

全羽毛状晶からなる６０６３ＤＣビレットの現場鋳造について

6063 DC billets having complete feathery crystals are expected reducing the deformation resistance. Practical casting tests were made to produce such billets using an improved mold, float and water pool. The optimum casting variables to ensure the DC billets 152mm in diameter having complete feathery crystals are; casting rate 115mm/min, casting temperature 730°C and cooling water 120l/min.