Showing papers on "Equiaxed crystals published in 1989"

Grain refining of aluminium and its alloys using inoculants

Abstract: Grain structure is an important and readily observable feature in cast aluminium alloys. Three different types of grain morphology are possible, namely, columnar, twinned columnar, and equiaxed. Inoculants in the form of master alloys are used to promote the formation of a fully equiaxed grain structure and this is termed grain refinement. Initially, fundamental aspects of solidification are outlined in order that the principles of grain refining using master alloys can be understood. Techniques for the commercial production and testing of common Al–Ti-based master alloys are then discussed briefly. The exact mechanisms by which grain refinement occurs are not yet fully understood and experimental and theoretical studies on the problem are critically reviewed with particular emphasis on (a) the role of solute titanium, (b) the thermodynamics of Al–Ti-based alloy systems, and (c) the nature of heterogeneous nuclei. Finally, current and future trends in the use of grain refining alloys are summarised.

Modeling of equiaxed microstructure formation in casting

Abstract: A general micro/macroscopic model of solidification for 2-D or 3-D castings, valid for both dendritic and eutectic equiaxed alloys, is presented. At the macroscopic level, the heat diffusion equation is solved with an enthalpy formulation using a standard FEM implicit scheme. However, instead of using a unique relationship between temperature and enthalpy (i.e., a unique solidification path), the specific heat and latent heat contributions, whose sum equals the variation of enthalpy at a given node, are calculated using a microscopic model of solidification. This model takes into account nucleation of new grains within the undercooled melt, the kinetics of the dendrite tips or of the eutectic front, and a solute balance at the scale of the grain in the case of dendritic alloys. The coupling between macroscopic and microscopic aspects is carried out using two time-steps, one at the macroscopic level for the implicit calculation of heat flow, and the other, much finer, for the microscopic calculations of nucleation and growth. This micro/macroscopic approach has been applied to one-dimensional and axisymmetric castings of Al-7 pct Si alloys. The calculated recalescences and grain sizes are compared with values measured for one-dimensional ingots cast under well-controlled conditions. Furthermore, the influence of casting conditions on temperature field, undercooling, grain size, and microstructural spacings is shown to be predicted correctly from axisymmetric calculations with regard to the expected experimental behavior.

Computer simulation of grain growth and macrostructure development during solidification

Abstract: A Monte Carlo computer simulation technique, previously applied to the simulation of a number of solid state processes involving microstructural evolution, has been employed to simulate and represent pictorially grain growth and grain interactions during solidification. By careful control of the number, location, and time of origin of grain nuclei it has been possible to simulate a wide range of features of grain structure in castings. Also, in support of an existing statistical argument, it has been demonstrated that the columnar to equiaxed transition occurs when the volume fraction of equiaxed grains ahead of the columnar interface is 0·50.MST/912

Effects of cooling rate control during the solidification process on the microstructure and mechanical properties of cast Co-Cr-Mo alloy used for surgical implants

Abstract: Both dendritic structure and equiaxed grain structure are produced in cast Co-Cr-Mo alloy by control of the cooling rate of castings during the solidification process, to determine whether a significant improvement in the mechanical properties of the alloy can be obtained. The different structural characteristics of the two kinds of casting are examined by optical microscopy, scanning electron microscopy and transmission electron microscopy. Tensile and fatigue tests as well as hardness measurement are carried out using individually cast test-pieces. Fracture surface appearance characteristics of tensile and fatigue specimens are also studied. It is concluded that the mechanical properties, including both transient and permanent properties, of the equiaxed grain structure castings obtained by fast cooling are superior to those found in the coarse dendritic structure castings.

Microstructure of alumina scales formed on NiCoCrAl alloys with and without yttrium

Abstract: The microstructure of alumina scales formed by oxidation at 1100 °C in air for times ranging from 25 to 500h of undoped and yttrium-doped Ni40Co21Cr23Al15 (at.%) alloys have been thoroughly studied by transmission and scanning electron microscopies. The alloys have been deposited with a dual-target high-rate sputtering triode unit on vacuum-cast Ni40Co21Cr23Al15 substrates. The alumina scales formed on both the doped and the undoped alloys are characterized by a thin outer layer having equiaxed grains, on top of a thicker columnar layer in contact with the metallic alloy. The formation of the oxide on the yttrium-free alloy causes plastic deformation of the subscale zone of the alloy; the oxide scale becomes partially adherent, and subsequently cracking and spallation of the scale occur. Addition of yttrium significantly decreases the growth stresses and enhances the scale adhesion, thereby improving the cyclic oxidation resistance. Detailed transmission electron microscopy investigations show that the average grain size, 〈Φg〉, of the alumina scales regularly increases from the external surface to the internal oxide-metal interface. In the case of the yttrium-containing alloy, it was possible to show that the growth, as a function of time, of alumina grains at a given distance from the gas-oxide interface is very slow, in contrast with the more rapid grain growth at the oxide-metal interface. Possible effects of yttrium are discussed taking into account these observations.

A computer simulation of the influence of processing conditions on as-cast grain structures

Abstract: A computer model for simulating solidification and involving a Monte Carlo procedure has been used to qualitatively predict the influence of solidification conditions on the grain structures of small castings. In agreement with experimental observations, it was found that increasing either superheat or mould temperature results in coarsening of the grain structure. This agreement supports the contention that the principal mechanism of equiaxed grain formation in small castings is heterogeneous nucleation. The model also illustrates the importance of simulating equiaxed grain movement in the bulk liquid in order to obtain realistic predictions of grain structures.

Single-crystal YBa2Cu3O7 particle formation by aerosol decomposition

Abstract: Single‐crystal YBa2Cu3O7 particles have been formed in a carbon‐free gaseous flow system by thermally decomposing droplets containing the nitrate salts of Y, Ba, and Cu in water followed by calcining and annealing in the gas phase. The electron diffraction pattern for particles ranging in size from 0.1 to 1.5 μm corresponded to single crystals of orthorhombic YBa2Cu3O7 while the powder x‐ray diffraction pattern confirmed the presence of single‐phase material. Energy dispersive spectroscopy showed a similar composition for all particles examined. Iodometric titration showed that the particles were fully oxygenated. Two types of single‐crystal particles were observed, equiaxed and elongated. Formation of solid single‐crystal particles is favored by operation near the melting point of the material and by long reactor‐residence times.

Crystallographic and magnetic alignment in die-upset Nd-Fe-B magnets

Abstract: Fully dense and aligned Nd-Fe-B permanent magnets can be prepared from melt-spun amorphous ribbons by a two-step process involving hot compaction and hot deformation. It is the latter processing step that produces alignment. During hot deformation, the randomly oriented and equiaxed Nd2Fe14B grains change to platelet shaped grains such that their crystallographicc axes lie normal to the flat surfaces. The final microstructure of the die-upset magnet, consisting of closely stacked flat Nd2Fe14B grains separated by a Nd-rich boundary phase, is produced by a combination of grain boundary sliding and grain boundary migration. Factors such as temperature, strain rate, grain size, etc. which affect these processes also affect deformation and alignment. Processing optimum die-upset magnets involves optimizing such variables as composition (to obtain maximum percentage of magnetic phase while retaining an appropriate intergranular phase to aid in grain boundary sliding and migration), alignment (to obtain maximum remanence) and microstructure (to provide for domain wall pinning sites).

Articles having coatings of fine-grained and/or equiaxed grain structure

Abstract: Fine-grained and/or equiaxed tungsten and/or rhenium coatings, substantially free from columnar structure, are deposited on substrates (especially graphite) by chemical vapor deposition by directing the flow of reactant gases to the substrate with high velocity and in close proximity thereto, most often at a velocity gradient of at least about 1050 and preferably at least about 2000 cm./cm.-sec. The deposition process is preferably conducted while moving the substrate so as to coat large areas thereof. By this method, tungsten and/or rhenium-coated articles useful as X-ray targets having excellent properties under conditions of rapid temperature cycling may be produced.

Titanium alloy treatment process and resulting article

Abstract: A carefully controlled amount of hydrogen is diffused into titanium or its alloys at an elevated temperature above the transformation temperature. After the elevated temperature is maintained for an approprate duration of time, eutectoid transformation is performed in an inert atmosphere, again for an appropriate period of time, during which or alternatively after which the hydrogen is removed and the metal cooled to room temperature. A sintered titanium alloy component of the type intended for use as a joint replacement subjected to such a treatment displays a fatigue strength which is noticeably improved over a similar article with an equiaxed or lamellar microstructure.

On the stability of the creep substructure in NaCl single crystals

Abstract: Microstructural observations were conducted on NaCl single crystals after creep. The microstructure after a stress increase followed by a stress decrease consisted primarily of cells; no significant number of subgrains were observed although they were present in the microstructures produced during uninterrupted tests. Prestraining in the exponential creep regime produced an uniform distribution of dislocations and a few subboundaries which transformed to equiaxed subgrains when tested in the power law creep region. This substructure was similar to that observed in an as-received specimen deformed to an equivalent strain. Prior creep in the power law creep region produced equiaxed subgrains whose boundaries were found to be mechanically stable when the specimen was retested in the exponential creep region. The role of subboundary migration in the formation of new subgrains is discussed.

Creep of Mullite Ceramics

Abstract: Creep in mullite ceramics containing 71, 74 and 78wt% Al2O3 made from coprecipitated powder has been studied between 1350° to 1500°C. Microstructure of 71wt% Al2O3 mullite was characterized by rod-like and equiaxed grains (average grain size: 2.13μm), that of 74wt% Al2O3 mullite, by equiaxed grains (grain size: 5.44μm), and that of 78wt% Al2O3 mullite, by equiaxed mullite grains (grain size: 2.11μm) and Al2O3 particles of 1.31μm in grain boundaries. Creep rate in 74wt% Al2O3 mullite was the lowest and that of 71wt% Al2O3 was the highest. That of 78wt% Al2O3 mullite located in between. Activation energies for creep were 138, 168 and 119kcal/mol for 71, 74 and 78wt% Al2O3 mullite, respectively. Vertical offsets from grain sliding out of the specimen surface were observed after creep test. The number of grains indicating the vertical offsets increased and cavities evolved with increasing creep time or applied stress. A slight grain growth was observed for 71 and 78wt% Al2O3 mullites after creep tests. On the other hand, grain growth was not recognized for 74wt% Al2O3 mullite. It was proposed that the creep mechanism in mullite ceramics is grain boundary sliding accompanied by diffusion.

Three-dimensional model for twinning in polymer single crystals with inclined chains

Abstract: A model describing the three-dimensional structure of twinned crystals of poly(paraxylylene) (PPX) in the aL form is proposed. PPX crystals were grown at 218°C in 1-methylnaphthalene at a concentration of 0.005% using the self-seeding technique. A large number of twinned crystals are obtained, corresponding to two basic modes: aLta and aLtb, involving respectively {110} and {130} twinning planes. Lattice images of the boundary in αta crystals occasionally show 110 lines. Since chains are inclined to the surface normal in untwinned crystals, the presence of 110 lattice fringes at the boundary is explained by the collapse onto the carbon substrate of a three-dimensional roof-like structure consisting of two lamellar twin components. This ‘nonplanar twin’ model, in a sense that the two lamellar components do not lie in the same plane, is extended to the crystals with aLtb twins and further demonstrated by electron diffraction tilt experiments. Multiple twins as well as coalesced α–β crystals are als...

Equiaxed dispersion strengthened copper product

Abstract: This invention relates to substantially full-dense, near-net shape bodies made of dispersion strengthened copper (DSC) powder and products which are cold formed and/or machined from these bodies, such as resistance welding electrodes The invention includes processes for manufacturing substantially full-dense, near-net shape, substantially equiaxed bodies from DSC powder

Process for producing fine copper powder with enhanced sinterability

Abstract: Copper powder with enhanced sinterability and process for producing same is disclosed which comprises fluid energy milling dendritic copper powder particles having a mean particle size of no greater than about 25 micrometers in diameter to produce a deagglomerated cold-worked powder wherein the particles are essentially equiaxed in shape and are of a mean particle size of no greater than about 7 micrometers in diameter.

Production of ferritic stainless steel sheet

Abstract: PURPOSE: To form a specific part in the structure of a steel sheet into equiaxed crystal structure and to easily produce a ferritic stainless steel sheet excellent in ridging resistance by controlling the difference between the pouring temp. and the solidification temp. of a slab as a stock to a specific value at the time of producing a ferritic stainless steel sheet. CONSTITUTION: At the time of producing a cold-rolled ferritic stainless steel sheet by hot rolling, annealing and cold rolling of an ingot of ferritic stainless steel to cause a desired sheet thickness followed by a final annealing, the difference between the pouring temp. and the solidification temp. at the time of casting a stainless steel slab as a stock is regulated to ≤40°C and the slab is water-cooled after solidification. By using this stock, the central part 2 of the cold-rolled steel sheet 1 can have an equiaxed crystal solidified structure comprising ≥70% of the sheet thickness and, further, the equiaxed crystal part can have a structure with the average grain size of the equiaxed crystal regulated to ≤1.0mm, by which the ferritic stainless steel preventive of the occurrence of ridging can be obtained. COPYRIGHT: (C)1990,JPO&Japio

Microstructural study of bronze-processed commercial multifilamentary superconducting Nb3Sn wires

Abstract: The thickness, morphology, grain size and grain morphology of A-15 diffusion layers in bronze-processed commercial multifilamentary superconducting Nb3Sn wire have been investigated using optical microscopy and transmission electron microscopy The layer growth appears to be controlled by the availability of tin in the matrix, and the remanent tin concentration in the matrix depends on the reaction temperature While there is some elongation of grains in the inner parts of the annular layers grown at lower temperatures, at high temperatures all the grains are equiaxed

Fatigue Crack Growth of Short Cracks in an ALMGSI Alloy

Abstract: The behaviour of short fatigue cracks in the range of 300–1000 μm has been investigated in a precipitation hardened AlMgSi alloy aged to maximum hardness. The grain structure was equiaxed with a diameter of 100 μm.

Production of sheet of intermetallic compound

Abstract: PURPOSE:To allow the superplastic molding of an intermetallic compd. by subjecting a thin slab subjected to the formation of the finer structure to constant-temp. rolling at the temp. and strain speed at which dynamic recrystallization takes place and at a specific draft. CONSTITUTION:The molten metal adjusted to the component compsn. of the prescribed intermetallic compd. is cast at about >=20 deg.C/sec solidifying rate to the sheet. Since the structure is fine, the sheet has high-temp. plastic deformability and has no plasticity to withstand the superplastic moldability; therefore, the thin steel sheet obtd. by the casting is subjected to the constant- temp. rolling at the temp. and strain rate at which the dynamic crystal takes place and at >=20% draft. The accumulation of the strain energy arise at the boundary of the fine structure and the increase in recrystal nucleus forming sites is resulted. The fine structure is changed to the finer equiaxed crystal structure by the static and dynamic recrystallization under the constant-temp. rolling. As a result, the superplastic deformation which is not exhibited by the fine structure after the casting is caused and the superplastic molding is enabled.

Unidirectional solidified ingot and its manufacture and application

Abstract: PURPOSE:To improve the yield and quality of a thin film by making the vicinity of the circumferential face of an ingot in an equiaxed crystal or a columnar crystal in the radial direction and forging or rolling this steel ingot in case of executing the uni-directional solidification of the molten metal of the mold inside toward the upper part from the lower part CONSTITUTION:A small equiaxed crystal or columnar crystal in the radial direction is formed on the side face of the ingot developed to the columnar crystal by adding a Co contg refining agent into a molten metal, impressing a fluctuating magnetic field on the outer peripheral part of a mold or making the mold inside in rugged shape, etc Casting or rolling is then executed by constraining this uni-directional solidified ingot directly or by coating with an elongating material The size of the equiaxed crystal or columnar crystal thus developed is formed in >=5mm, etc, the cracking at working time is prevented and the generation of the re-crystalline nuclei of the working time is facilitated The yield and quality of the thin film forming part of a sputtering target, etc, are improved because forging and rolling can be done on the ingot

Ultrasonic Examination of Cast Stainless Steel Component in Nuclear Power Plant

Abstract: CSS, used extensively for nuclear power plant components, has complex grain structures that compromise the accuracy of ultrasonic examination. Figure 1 shows a pipe-to-elbow joint that is typically used in many pressurized water reactors (PWRs). The joint has a columnar-dendritic grain structure at one end and an equiaxed grain structure at the other. Ultrasonic wave propagation is significantly different in these two structures.

Grain Refinement in Continuously Cast Ingots of Light Metals by Electromagnetic Casting

Abstract: The working principle of a new electromagnetic continuous casting process is described. The main pecularities of this process lie, first, in the presence in the sump of a strong electromagnetically driven forced convection, which promotes the production of a fine equiaxed structure and, secondly, in the fact that the thickness of the segregation zone tends to zero. The evolution of both the grain size and the thickness of the segregated cortical layer with the electric power input is presented.