Showing papers on "Equiaxed crystals published in 1981"

The structure and properties of aluminum alloys produced by mechanical alloying: Powder processing and resultant powder structures

Abstract: Mechanical alloying of two aluminum alloy powders to form composite A1-A12O3 powders has been studied. Changes in powder microstructure with processing are reported and interpreted. Mechanical alloying proceeds by the continual cold welding and fracturing of the constituent powder particles when subjected to the large compressive forces of a high speed mill. A suitable organic surfactant must be added so that a balance between cold welding and fracturing is obtained. The organic surfactant is embedded and finely distributed in the powder particles during mechanical alloying and is converted to discrete A14C3 particles after hot pressing. The establishment of steady state processing conditions, characterized by equiaxed powder particles, a constant particle size distribution and a saturation hardness, is found to depend on the size distribution of the initial powders. The oxide particles formed and distributed during mechanical alloying are equiaxed, small (30 nm) and homogeneously distributed with a volumetric center to center distance of about 60 nm.

Grain refinement in magnetically stirred GTA welds of aluminum alloys

Abstract: The mechanisms of grain refinement have been examined for magnetically stirred gas tungsten arc (GTA) welds completely penetrating thin sheets of several aluminum alloys. Grain refinement in unstirred welds may be brought about by adding sufficient titanium to produce heterogeneous nucleation by Ti-rich particles. In some alloys magnetic stirring is shown to extend the range of welding conditions which produce a partially equiaxed structure, and to widen the equiaxed fraction of partially equiaxed welds. This is attributed to magnetic stirring lowering the temperature gradient, allowing nucleation and growth of Al-rich grains further ahead of the columnar interface growing in from the fusion boundaries. In alloys with low Ti levels, magnetic stirring may cause refinement by sweeping grains from the partially molten zone ahead of the advancing solidification interface. This mechanism requires that the partially molten zone be sufficiently wide, and that the grain size in this zone remain small.

Controlled rolling of steel plate and strip.

Abstract: The major purpose of controlled rolling is to refine grain structure and, thereby, to enhance both the strength and toughness of steel in the as-hot-rol1ed condition. If a survey is made of the development of controlled rolling, it can be seen that controlled rolling consists of three stages: (a) deformation in the recrystallization region at high temperatures; (b) deformation in the non-recrystallization region within a low temperature range above Ar3; and (c) deformation in the austenite–ferrite region. It is stressed that the importance of deformation in the nonrecrystallization region is in dividing an austenite grain into several blocks by the introduction of deformation bands within it. Deformation in the austenite-ferrite region gives a mixed structure consisting of equiaxed grains and subgrains after transformation and, thereby, it increases further the strength and toughness. The fundamental difference between conventionally hot-rolled and controlled -rolled steels lies in the fact that t...

Dispersion strengthened aluminum-4 pct magnesium alloy made by mechanical alloying

Abstract: Mechanical alloying is a unique high energy milling process for producing metal powders with a controlled microstructure When applied to aluminum based alloys, a uniform, equiaxed fine dispersion of oxygen- and carbon-based particles is obtained In addition, a very fine grain structure, pinned by the dispersoid, is generated Relatively low volume loadings of dispersoid may be employed to achieve attractive combinations of properties including ambient temperature tensile strength, corrosion resistance and stress corrosion cracking resistance The development of a dispersion strengthened aluminum-4 pct magnesium alloy is discussed

On the origin of equiaxed austenite grains that result from the hot rolling of steel

Abstract: An investigation has been conducted into the influence of hot rolling on the microstructure of austenite in both a low alloy (transformable) and an austenitic stainless steel. Specimens of each steel were reheated and then given a single 50 pct reduction at various temperatures, after which the specimens were water quenched. The specimens were analyzed using optical, transmission electron and X-ray metallography. The results of these experiments showed that both steels exhibited equiaxed grains after high temperature rolling, elongated grains after low temperature rolling and a mixture of both types of grains after rolling at intermediate temperatures. One of the principal goals of this work was to study the origin of the equiaxed grains that result from high temperature rolling. Information obtained from dislocation observations and texture analysis has led to the conclusion that the mechanism most likely responsible for the equiaxed grains in as-rolled austenite is dynamic recrystallization.

The structure and mechanical properties of an injection-moulded acetal copolymer

Abstract: Processing conditions, microstructure and mechanical property correlations have been explored in injection-moulded plaques of an acetal copolymer. Barrel temperature was varied systematically between 453 and 503 K, with a constant mould temperature of 343 K. The microstructure and texture were determined by polarized light microscopy and X-ray pole-figure analysis, respectively. The overall structure of the mouldings was layered through the thickness and symetrical about the moulding centre line. At all barrel temperatures five layers were present: the outer three layers possessed significant preferred chain-axis orientation in the crystalline phase, while the two layers at the centre of the moulding were equiaxed. The texture and morphology of each layer is described and related to a model of mould filling. Increases in the barrel temperature reduced the extent of the outer oriented layers while increasing the extent of the equiaxed layers. Tensile tests were conducted on samples taken at 0° and 90° to the injection direction. Increases in barrel temperature had no influence on modulus but decreased both (α=0 and 90°) engineering yield stresses. The yield stress could be correlated with the extent of the oriented layers within mouldings. At all barrel temperatures the yield stress was greater when α=90°; this behaviour is explained in terms of a composite model.

Refining of Solidification Structures of Continuously Cast Type 430 Stainless Steel Slabs by Electromagnetic Stirring

Abstract: Refining of the solidification structure of Type 430 stainless steel by the electromagnetic stirring was studied. By the preliminary tests for small ingots, it was made clear that the following conditions must be satisfied for a preferably equiaxed solidification:1) the formation of sufficient fragments of solidified steel which act as nuclei, and2) the stability of nuclei in the molten pool.A pair of linear-motor type stirrers was set in the slab machine and Type 430 steels were treated by the electromagnetic stirring under various conditions of stirring powers, stirring modes and superheats of liquid steels. By controlling these conditions, the formation of equiaxed structure having the thickness more than 50% of slab was realized and the ridging of cold sheet was improved.

Sintering of Acicular Fe2O3 Powder

Abstract: The sintering of acicular Fe2O3 powder has been studied in comparison with an ordinary equiaxed powder. The acicular powder gave a dense material more than 99 % theoretical even from the relatively low green density. Oriented granular structures were observed in the 1200°C compacts. The observed densification behavior has been attributed to the pore configuration in the green compact.

Solidification of Steel Ingots

Abstract: The central aim of this paper is to explain the solidi-fication process and the control of the solidified structure of steel ingots using the newly proposed theory of the formation of an equiaxed zone, the so called "Separation theory".

The Effects of Electromagnetic Stirring on Solidification Structure of Continuously Cast SUS43O Stainless Steel Slabs

Abstract: Effects of electromagnetic stirring on the solidification structure of continuously cast SUS430 slabs were investigated to reduce the ridging of cold-rolled sheets by grain refinement of the solidification structure. The following results were obtained:(1) The area in which equiaxed crystals form increases with increasing stirring intensity and decreasing superheat (ΔT) of molten steel. The most suitable stirring mode is the alternate stirring. The effect of ΔT can be moderated by lowering the position of the stirrer, but the ratio of equiaxed crystals decreases in this case. Appropriate conditions for obtaining a ratio of equiaxed crystals over 60% were determined.(2) Ridging is reduced as the ratio of equiaxed crystals increases. Stable ridging obtained for 50% or more equiaxed crystals is the same as that of steel sheets rolled from ingots.

The Role of the Peritectic Reaction in the Formation of the Equiaxed Zone in Iron Base Alloys

Abstract: It has been shown that the peritectic reaction does not influence the size of the equiaxed zone in iron base alloys cast at low superheats. The equiaxed zone size did increase with alloy content but this could be attributed to the decrease in the liquidus temperature and its influence on the survival of nuclei from the “big-bang” mechanism.At high superheat (-90°C), the larger equiaxed zone size in the peritectic alloys can be partly attributed to the peritectic reaction for its influence on the survival of nuclei from the “big-bang” mechanism and partly to the greater resistance offered to resolution of the nuclei generated by dendrite arm remelting.It is interesting to note that two mechanisms for the formation of equiaxed crystals must operate in order to explain the different behaviour patterns at the two superheats.

On Refining of Solidification Structure of Ferritic Stainless Steel by Vibration Method

Abstract: Refining of the solidification structures of Type 430 stainless steel by the mechanical vibration and the liquid surface vibration was tried. The structures of 15kg test ingots were remarkable improved by these vibration methods. Change of structure into the equiaxed one starts after the superheat of liquid steel disappears.The liquid surface vibration was applied to 6t ingots for practical use and its effect on structure refining was confirmed. By the investigation of size of crystals and negative segregation in the equiaxed zone, it was concluded that the structure refining was caused by the showering of the free crystals.In the case of continuously cast slabs, the liquid surface vibration was effective only under low superheat.

Nickel-base alloy for single crystal casting

Abstract: Nickel alloys for single crystal casting have been produced by varying the proportions of the constituents of a commercial alloy originally developed to give equiaxed polycrystalline castings and optimum compositions suitable for the production of single crystal castings have been obtained. The new alloys have compositions within the range (in weight per cent): Individual alloys have been shown to have enhanced creep rupture strength and high impact resistance compared with commercial alloys where the latter are cast as single crystals.

Structure and morphology of heavily deformed single crystals of a polydiacetylene. I. Electron microscopy and x‐ray diffraction

Abstract: The structure and morphology of heavily deformed single crystals of a diacetylene polymer have been studied using a combination of x-ray diffraction and electron microscopy. Crystals have been deformed by both rolling and hammering. The crystals remain intact during deformation and can be reduced in thickness by a factor of over 5 in directions perpendicular to their chain axes. It is found that the chain orientation is maintained during both hammering and rolling. A greenish-colored surface skin develops during both types of deformation but the structure of the interior of the crystals depends upon the type of deformation employed. The interior of the hammered crystals consists of crystal blocks ca. 50 μm thick formed by cleavage perpendicular to the chain direction whereas the rolled crystals tend to be fibrous with no evidence of molecular fracture. The possible deformation mechanisms which have given rise to the different structures have been discussed.

Inclusion Distribution in Ingots - A Guide to Segragation Mechanism

Abstract: The melting point of non-metallic inclusion, and their nature and distribution can be a guide in deciphering the mode of solidification and segregation in an ingot. A commercial ingot was sectioned, and examined micros-copically. Inclusion identification was carried out by optical method as well as by electron probe micro analy-sis. The exten of segregation was obtained by extensive spectro-chemical analysis. It has been found that the shape and position of bottom cone of negative segregation coincide with that of bottom cone of inclusions. A theory of ingot segregation has been suggested that the bottom cone of oxide inclusions is a result of incorporation of these inclusions inthe dendrites of equiaxed grains. The inclusions drop to the bottom end with the shower of relatively pure equiaxed grains. While the interdendritic fluid is enriched in elements like C, Mn, S, P, it is denuded of oxygen. Since the oxygen solubility is low, oxygen enrichment leads to immediate precipitation of oxides.

Microsegregation of an Aluminum and Magnesium Alloy at High Solidification Rates

Abstract: Results are presented of an on-going study of structure and solute redistribution in ribbon cast A1-Cu and Mg-Zn alloys. Regions found in these ribbons are (1) an apparently chemically homogeneous coarse-grained zone adjacent the chill, (2) a transition cellular region, and (3) a dendritic zone, usually equiaxed. In both alloys, structure in the equiaxed region becomes coarser with increasing distance from the chill. In Mg-Zn alloys, the amount of nonequilibrium second phase increases essentially linearly with distance in the cellular and dendritic regions. Scanning transmission electron microscopy can accurately measure solute distribution across rapidly solidified cells or dendrites, but the possibility of surface compositional changes during specimen preparation must be recognized.

The Effect of The Natural Convection on The Transition from Columnar to Equiaxed Crystals

Abstract: During dendritic solidification of metal alloys there normally occur temperature gradients in the liquid ahead of the solidification front. The gradient causes a natural convection ahead of the front. The natural convection causes a heat transfer from the interior of the liquid, which gives the possibility for crystals to grow ahead of the solidification front. The growth of these crystals is determined by the cooling rate and the kinetics of the solidification process. The paper will deal with the theory behind the growth of free crystals ahead of a solidification front. Some experimental results will be given. The effect of gravity on the growth process will be discussed.

Comparison of the Quench Rates Attained in Gas Atomized Powders and Melt-Spun Ribbons of Co and Ni Base Superalloys : Influence on Resulting Microstructures

Abstract: The cooling rates during solidification of gas atomized Stellite 6 (S6) powders range from 10 3 K/s to 10 6 K/s for 400μm to 10 μm diameter powder particles respectively. The microstructures vary from equiaxed type in the coarser particles to fully dendritic, cellular and microcrystalline in the finer particles. In the case of melt spun ribbons, the variations in freezing rate along a cross section of the ribbon produce a change in the microstructure from microcrystalline at cooling rates greater than 10 7 K/s to cellular and finally dendritic at rates on the order of 10 6 K/s.