Showing papers on "Equiaxed crystals published in 1984"

γ → α transformation in Fe, Fe–Ni, and Fe–Cr alloys

Abstract: Evidence is presented to show that five transformation products are possible in pure iron and dilute substitutional alloys: equiaxed ferrite, massive ferrite, bainitic ferrite, lath martensite (massive martensite), and twinned martensite. The morphology and transformation temperatures of these products are discussed. In conclusion, the chemical driving force for each of the transformations is calculated as a function of composition.

Microstructures of isothermally transformed Fe-Nb-C alloys

Abstract: The microstructures of an Fe-Nb-C alloy, with and without boron, developed by the γ → α transformation during isothermal holding were examined mainly by metallographic techniques. Ferrite formed above about 750°C is equiaxed or polygonal, while Widmanstatten ferrite is developed at lower transformation temperatures increasingly replacing polygonal ferrite, as commonly observed in low alloy steels. The carbide precipitation changes with transformation temperature. Above about 800°C, the structure is not uniform, i.e. almost carbide free ferrite is formed at first and later coarse interphase precipitation takes place. Interphase precipitation occurs readily around 800°C, while uniform precipitation of NbC from supersaturated ferrite is the principal structural change around 730°C. At the lower temperatures, ferrite often contains a high density of dislocations and coarse cementite. The observed structural changes are explained in terms of the γ → α reaction kinetics and the partition of carbon to au...

Microstructural changes produced in a multifilamentary Nb-Ti composite by cold work and heat treatment

Abstract: The microstructural changes produced by cold work and heat treatment of a full-scale filamentary Nb-Ti superconducting composite have been investigated using transmission electron microscopy. Full details of the processing conditions used are given, and both longitudinal and transverse sections of the microstructure have been examined. Heat treatments are found to give a complex precipitate morphology; the precipitates produced by heat treatments at 648 K (375 °C) were found to range from a thin boundary film 3 to 6 nm thick, through small approximately ellipsoidal precipitates (~5 x 10 nm), to larger (~50 to 500 nm) equiaxed precipitates. The larger precipitates could be identified as α-Ti. Cold drawing was found to refine the microstructure, but an even greater degree of refinement was observed when intermediate heat treatments were applied. The observations of this investigation show a strong interdependence between the cold-worked substructure and the precipitate morphology in these heat-treated and heavily cold-worked (e ~ 5 to 9) composites.

The effect of microstructure and deformation behavior on the hot ductility of Ti-6Al-2Nb-1Ta-0. 8Mo

Abstract: Hot ductility of the alloy Ti-6Al-2Nb-lTa-0.8Mo has been correlated with microstructure and fracture behavior. Low hot ductility was found to be associated with strain localization within the grain boundary alpha phase, producing void formation along the prior-beta grain boundaries and inter-granular fracture. Microstructural features that appear to be critical to the strain localization process are beta grain shape and alpha phase morphology. For the case of Widmanstatten + grain boundary alpha phase morphologies, equiaxed prior-beta grains formed by annealing above the beta transus are required to produce significant strain localization. For the beta processed structure with elongated beta grains due to working above the beta transus temperature, the orientation of the grain boundary alpha phase limits strain localization due to low resolved shear stress. The martensitic Widmanst↦ten alpha prime structure formed by quenching from above the beta transus temperature rapidly forms grain boundary alpha upon reheating to temperatures high in the alpha + beta phase field. This results in strain localization in the grain boundary regions in an apparently similar manner to that observed in the Widmanstatten + grain boundary alpha phase morphologies with equiaxed prior-beta grains.

Microstructural Changes during Isothermal Forging of a Co-Cr-Mo Alloy

Abstract: Interest has evolved recently in thermomechanical processing of the cast Co-Cr-Mo surgical implant alloys such as Vitallium and Vinertia. Work has shown that the wrought forms of these alloys exhibit much improved properties over their as-cast counterparts. In this paper, the response of as-cast Vinertia to isothermal forging is examined by means of isothermal and isostrain-rate compression testing. The effects of temperature, strain rate, and strain on the breakdown of the as-cast micro-structure are examined in detail. The effects of prior heat treatment on plastic flow and microstructure achieved are also considered. It is shown that the interaction between the carbide phase and the recrystallization induced during hot working governs the degree of homogeneity that can be achieved in the forged product. Control of carbide volume fraction, size, and distribution by appropriate prior processing can lead to a fine grain equiaxed structure with uniformly distributed carbides. The potential offered by isothermal forging for control of the microstructure in this type of alloy is discussed, as well as the limits imposed on the process by the starting material and by the strain gradients expected during the forging of implants.

Mechanism of Formation of the V-shaped Segregation in the Large Section Continuous Cast Bloom

Abstract: A V-shaped segregation, found in a sulfur print of longitudinal strand section, has been macroscopically investigated. A simple model, using polyethylene particles of 2mm diameter, has been prepared to simulate formation of the V-shaped segregation.The results obtained are summarized as follows:(1) The V-shaped segregation is observed only in an equiaxed crystal zone very periodically along the casting direction.(2) The frequency of the V-shaped segregation becomes smaller, while its density becomes larger with increase in the thickness of the equiaxed zone.(3) The mechanism of formation of the V-shaped segregation may be explained as follows; The enriched liquid between the equiaxed crystals is sucked and flows downwards and is accumulated along the planes which are made by the forcible movement of the equiaxed crystals piling in the end of the solidification regions toward the strand center.(4) The periodicity of the V-shaped segregation is quantitatively explained by the rheological approach assuming the equiaxed crystals piling in the end of the solidification regions as cohesive particles.

Effect of Electromagnetic Stirring at the Final Stage of Solidification of Continuously Cast Strand

Abstract: It is commonly observed that severe V-channel segregates are associated with the equi-axed crystal zone in the central region of a continuously cast strand. This type of segregation may deteriorate the qualities of products especially for sour gas service. The modification of V-segregates is found to be attained by electromagnetic stirring at the final stage of solidification of casts.The laboratory scale stirring experiments have been carried out in order to eliminate V-segregation and the following results have been obtained.(1) V-segregation streaks can be eliminated by stirring at the final stage of solidification with either linear or rotary type stirrer.(2) The upward flow of molten steel at the solidification front induced by the linear type stirrer may prevent the interdendritic liquid flow toward the bottom which leads to the formation of V-segregates.(3) The presence of adequate amount of fine equi-axed crystals at the final stage of solidification is the essential condition for the elimination of V-segregates by the rotary type stirrer. Alternating stirring direction around the cast axis is preferred for the prevention of coagulation of equiaxed crystals and results in the modification of packing of them.(4) In addition to the above results, the timing and intensity of stirring with which the best result can be obtained have been discussed.

Effect of Electromagnetic Stirring at the Final Stage of Solidification of Continuously Cast Strand

Abstract: It is commonly observed that severe V-channel segregates are associated with the equi-axed crystal zone in the central region of a continuously cast strand. This type of segregation may deteriorate the qualities of products especially for sour gas service. The modification of V-segregates is found to be attained by electromagnetic stirring at the final stage of solidification of casts.The laboratory scale stirring experiments have been carried out in order to eliminate V-segregation and the following results have been obtained.(1) V-segregation streaks can be eliminated by stirring at the final stage of solidification with either linear or rotary type stirrer.(2) The upward flow of molten steel at the solidification front induced by the linear type stirrer may prevent the interdendritic liquid flow toward the bottom which leads to the formation of V-segregates.(3) The presence of adequate amount of fine equi-axed crystals at the final stage of solidification is the essential condition for the elimination of V-segregates by the rotary type stirrer. Alternating stirring direction around the cast axis is preferred for the prevention of coagulation of equiaxed crystals and results in the modification of packing of them.(4) In addition to the above results, the timing and intensity of stirring with which the best result can be obtained have been discussed.

Hot Model Experiments on Electromagnetic Stirring at About Crater End of Continuously Cast Bloom

Abstract: For the improvement of centerline segregation and center porosities in continuously cast blooms, the favorable conditions for electromagnetic stirring near the crator end have been determined by hot model experiments in which the influence of rotational stirring on V-shaped streaks formed at the zone with accelerated downward flow of residual melt caused by solidification shrinkage was investigated. It has been found that by rather weak electromagnetic stirring against the melt core kept in a range from 30 to 50mm in diameter, V-shaped segregations are quite improved together with control of solidification structure consisting mainly of non-columnar dendrites. It has been observed also that an excessively strong stirring in the core zone with non-columnar structure brings about rather detrimental influence on V-shaped streaks because of coagulation of a solute enriched residual melt in the slow movement of slurry consisting of equiaxed crystals and the residual melt.The favorable conditions are discussed by taking a solidification sequence into account. They are the stability of non-columnar crystals, the movement of a mushy solid-liquid mixture by electromagnetic stirring, and a change in downward suction flow during solidification.Based on these results obtained by the hot model experiments, test campaigns have been carried out by utilizing a newly designed electromagnetic stirring unit installed to bloom casters in Mizushima Works. Throughout the campaigns, favorable conditions for electromagnetic stirring at two locations, namely, in the spray cooling zone and near the crater end, have been established for the production of sound blooms with less centerline segregation and center porosities.

Formation of equiaxed zone in castings

Abstract: The formation of the equiaxed zone was examined using a transparent succinonitrile–ethyl alcohol solution. The equiaxed crystals were found to be formed by detachment of dendrite arms owing to the convective flow of the bulk liquid. In the early stage of solidification, convection originates from the temperature differences in the liquid, and, in the later stage, from the floating up of alcohol enriched liquid. ejected from inverse V segregate channels in the mushy zone into the bulk liquid~When the superheat of a pouring solution is small, free crystal formation occurs in the liquid adjacent to the mould wall during and immediately after pouring.

An experimental investigation into the formation of an equiaxed zone in ingot casting: Pb-Sb alloy system

Abstract: An experimental investigation was undertaken to obtain a better understanding of the phenomenon of equiaxed zone formation in ingot castings with particular attention to the thermal conditions during dendritic growth and the mechanism of columnar-to-equiaxed transition. Thermal conditions during dendritic growth were measured in the laboratory-scale Pb-Sb castings. Solidification was also directly observed in castings of the cyclohexanol-phenol red system. A close correlation was noted between the general behaviour of the thermal history curves, especially when taken on the centreline of the ingots, and the resulting macrostructure. The dendritic solidification front was found to grow with considerable solute undercooling and also with a slight, but positive, temperature gradient ahead of the front in the bulk liquid. Moreover, the dendritic front temperature was also noted to increase with distance from the mould wall surface, the nominal rate of advance of the front being sensitive to the solute content and the cooling rate. Heterogeneous nucleation ahead of the columnar front was believed to have played a major role in equiaxed zone formation. However, columnar-to-equiaxed transition was observed not to take place immediately when the equiaxed crystals formed, but some time later, equiaxed dendrites growing at a somewhat higher temperature than the columnar dendritic front. Finally, combining the information gained from thermal analysis data with the observations made upon non-metallic castings, a novel picture of how columnar-to-equiaxed transition takes place has been proposed.

Method for producing equiaxed iron or iron alloy particles

Abstract: A method is disclosed for producing iron or iron alloy particles. A metal or metal alloy strip having fine particles of iron or alloy distributed throughout is provided. The metal strip is selectively dissolved without substantial dissolving of the iron or iron alloy particles in order to recover the particles.

Instability of plastic flow and dynamic recrystallization in Cu single crystals

Abstract: The present paper shows the results of high temperature experiments performed on Cu single crystals. The crystals were deformed by rolling at 900°C with the rolling plane either parallel to {123} or 14° off the {111} crystal plane. In the former, slip events were evenly distributed in the samples. No recrystallization was then observed. For the crystals which were 14° off the {111} plane, the deformation was non-uniform and numerous shear bands were formed. It was found that the density of shear bands increased with strain, and recrystallization initiated within the shear bands. A model of dynamic recrystallization in crystals deforming by the shear band mode is discussed in terms of the structural features revealed by optical and electron microscopy.

A single‐crystal silicon thin film formed by secondary recrystallization

Abstract: Thin encapsulated silicon films were zone heated in the solid state just under the melting point with a quartz‐iodine lamp over a range of speeds between 0.005 and 0.05 cm/sec. The recrystallized thin films were examined with electron channeling, transmission electron microscopy, scanning and optical microscopy. The recrystallized 1/2‐μm‐ thick film was essentially a single crystal with large equiaxed subgrains oriented with the (100) nearly normal to the plane of the film and the 〈010〉 direction along the growth direction at growth velocities below 0.01 cm/sec. Misorientations between adjacent large 100‐μm subgrains were of the order of 1°. The driving force of grain growth in the silicon thin film was determined to be the surface energy anisotropy of the silicon oxide interface from observations of grain boundary grooves that were pinned by a few micron‐size included grains. Both the mechanism of recrystallization and the potential for producing silicon‐on‐insulator material by solid‐state recrystalliza...

Grain Growth Processes during Transient Annealing of As-Implanted, Polycrystalline-Silicon Films

Abstract: Polycrystalline silicon films of 300 nm thickness were deposited on oxidized wafer surfaces, implanted with As, and annealed on a Varian IA 200 rapid thermal annealer. Transmission electron microscopy was used to study through-thickness and cross sectional views of grain size and morphology of as-deposited and of transient annealed films. A bimoda] distribution of grain sizes was present in as-deposited polycrystalline silicon films. The first population was due to columnar growth of some grains to a final average diameter of 20 rm. The second population of small equiaxed grains of 5 nm average diameter were formed early in the deposition process. During transient annealing grains in the first population grew rapidly up to 280-nm equiaxed grains. After this the growth rate decreased due to the grain size reaching the thickness of the film. Grains in the second population grew rapidly up to a size of 150 nm, after which the growth rate was lowered due to grains impinging upon one another. The grain growth processes for both populations have been described with a modified model for interfacially driven grain growth. This model accounts for diffusion and grain growth which occur with rapidly rising and falling temperatures during short annealing times characteristic of transient annealing processes.

Method and device for forming equiaxed crystal in continuous casting of casting ingot

Abstract: PURPOSE:To accelerate formation of homogeneous equiaxed crystals by immersing and fixing a refractory heating flask contg. a heating element on the molten metal surface along the inside wall of a casting mold, and sliding or oscillating vertically the casting mold thereby liberating mechanically the crystals forming their nuclei on the casting wall. CONSTITUTION:The molten metal 2 in a tundish 3 is controlled in flow rate by a stopper 5 and is supplied through a nozzle 4 into a water cooled metallic mold 1. A refractory heating flask 6 contg. a heating element is fixed by supporting means 7 and the mold 1 is slided or oscillated vertically to dislodge and liberate equiaxed crystals 8 continuously from the casting wall by the effect of the flask 6, thereby forming a solidified shell 9. The liberated crystals 8 are secondarily cooled by sprays 10 to grow and settle, thereby forming a casting ingot 11 having a large equiaxed crystal band. The ingot is drawn downward by means of pinch rolls 12. The casting ingot consisting of the homogeneous equiaxed structure contg. no abnormal coarse crystals is thus obtd. and the quality of the casting ingot is revolutionarily improved.

Forming and sintering behavior of B- and C-doped alpha- and beta-SiC

Abstract: A study of forming pressure and sintering temperature on the shrinkage, density and phase composition of both alpha- and beta-SiC has determined that the former develops the 4H phase at the expense of the 6H at temperatures above 2150 C, while the latter converted to the 6H phase above 2000 C with intermediate development of the 15R and 4H phases. In beta-SiC, the cubic phase crystals of the starting material progressively transform to an elongated alpha polytype at 2000 C which is less conducive to additional sintering than a more equiaxed structure. 6 references.

The effects of the size of the second phase particles on the mechanical properties of a low alloy steel

Abstract: The effects of the size of second phase particle on the tensile and fracture properties have been studied for a high carbon low alloy steel. The specimens were treated by different heat treatments and four groupings of particle size distribution of nearly equiaxed carbide were obtained. The range of the mean particle spacing, λ, is 0.525-2.17 μm. It is shown that the yield strength, 6y, ultimate strength, 6 U , and fracture strength, Of, are inversely proportional to λ. It is indicated that J 0.05 or δ 0.05 does not vary with the particle size monotonically. An expression which relates tensile properties and a microstructure parameter to fracture property is suggested to describe the present results. Moreover, there is a quite stable relation between J 0.05 and (6 y. δ 0.05 ).

Method and device for continuous casting of casting ingot having equiaxed crystal

Abstract: PURPOSE:To cast continuously a casting ingot consisting of homogenous structure of equiaxed crystal by cooling and solidifying the molten metal in a casting mold while oscillating the metal by means of a heating oscillator and drawing downward the molten metal. CONSTITUTION:A continuous casting device supplies a molten metal 2 from a nozzle 4 of a tundish 3 provided with a stopper 5 into a water-cooled metallic mold 1 and draw out a cast ingot 10 cooled with a cooling spray 9 by means of pinch rolls 11. A heating oscillator 6 consisting of refractories such as alumina or the like contg. a heating element or a corrosion-resistant metal such as Ni- Cr alloy or the like is brought into contact with the surface of the molten metal 2 in the casting mold of such casting device or dipped in said metal and is oscillated to liberate and deposit the equiaxed crystal 7 formed on the casting wall surface and to form the constitution of equiaxed crystal in a surface solidified shell 8 consisting of a stable columnar crystal. Said oscillator 6 is preferably formed into the frame shape along the inside wall of the mold 1 in order to maintain the surface temp. at the solidifying temp. of the metal 2 or above and to prevent the stable growth of the formed equiaxed crystal 7.

Optimization of high-temperature compression of as-cast Al–Cu eutectic alloy

Abstract: The superplastic characteristics achieved after compressive predeformation at high temperature of as-cast Al–Cu eutectic alloy have been studied as function of the predeformation speed Vp. The stress–strain-rate curve was found to be shifted towards higher strain rates as the predeformation speed becomes higher, so that the maximum superplastic strain rate is found to increase with Vp’ with however a tendency for stabilization at high Vp. This phenomenon can be explained by the combined effect of homogeneity of transformation of the initial lamellar structure in the compressed specimen and of the size of the equiaxed structure developed in the transformed regions. The influence of Vp on the subsequent superplastic characteristics of the material allows optimization of that predeformation speed to achieve a given total strain under conditions of minimum energy and/or time of deformation. The effect of the cooling rate of the as-cast material was also examined and is discussed in terms of superplast...