Showing papers on "Equiaxed crystals published in 1987"

Equiaxed dendrite growth in alloys at small supercooling

Abstract: A dimensionless model for growth of equiaxed dendrites in alloys under small Peclet number conditions is presented. The behavior of this model is compared with that of Karma and Langer and to experiments of Chopra. Good agreement between both models and with experiments is found. It is shown that the maximum in growth rate,viz., minimum in tip radius with composition, depends strongly on the ratio of heat-to-solute diffusivity and on the solute distribution coefficient.

The columnar to equiaxed transition in tin-lead alloys

Abstract: Tin-lead alloys were solidified directionally and the position of the columnar to equiaxed transition determined on vertical sections of the ingots. The columnar length was found to increase with decreasing lead concentration and increasing heat transfer coefficient. A mathematical model of the heat flow in the system was used to determine local temperatures, temperature gradients, and velocities in the solidifying alloy. Comparing the position of the columnar to equiaxed transition and local thermal conditions, it was found that the transition occurred when the temperature gradient in the melt at the liquidus temperature was 0.11°C/mm for Sn 10 wt pct Pb, 0.10°C/mm for Sn 5 wt pct Pb, and 0.13°C/mm for Sn 15 wt pct Pb. The position of the transformation was found to be independent of melt superheat for the conditions considered.

Processing properties relationships of Ti-10V-2Fe-3Al

Abstract: Beta alloys, such as Ti-10V-2Fe-3Al, can be processed and heat treated to provide a wide range in mechanical and fracture properties. A systematic study of the effect of processing and heat treatment variations on the properties combinations achievable was undertaken. The ductility and fracture toughness can be very significantly influenced by processing and heat treatment variations. The ductility increases and the toughness decreases as the morphology of the primary alpha changes from acicular to equiaxed. This morphology change was caused by alterations in forging procedures. Reducing the strength increases both fracture toughness and ductility. Microstructure has only a minor influence on fatigue performance in the high strength (about 1310 MPa UTS) condition. The micro-structure does, however, have a significant effect on fatigue behavior in a low strength, about 1034 MPa UTS, condition. More lamellar alpha results in improved notched fatigue performance, while a more equiaxed alpha improves smooth fatigue performance. These changes in properties are rationalized in terms of microstructure.

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Abstract: Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature gradient. The interface morphology, as well as the behavior of the eutectic spacing and undercooling, suggest that the resultant microstructure is governed by two different growth processes. That is, at low growth rates, steady-state columnar eutectic growth is found and obeys the relationship, λ2V = constant, where λ is the eutectic spacing andV is the growth rate. At higher growth rates, the nucleation of equiaxed eutectic grains occurs in the interdendritic liquid. The experimental findings are interpreted in the light of recently developed models for the columnar to equiaxed transition and for irregular eutectic growth.

Microstructural change during superplastic deformation of δ-ferrite/austenite duplex stainless steel

Abstract: Superplasticity of a 25 pct Cr-6.5 pct Ni-3 pct Mo-0.14 pct N δ/γ duplex stainless steel has been studied with particular emphasis on the microstructural change during deformation. Two large superplastic elongations are obtained at temperatures around 1323 K in δ/γ duplex phase region and 1173 K where σ phase particles precipitate dynamically at a strain rate of ~10−3 s−1. During deformation in the higher temperature region, fine Widmanstatten γ particles coarsen and coarse γ grains formed during the prior treatments are broken into spherical particles, resulting in a homogeneous dispersion of γ particles within the σ-ferrite matrix. The dynamic recrystallization of soft σ-ferrite matrix occurs locally in the region where the strain reaches some critical value, and the final microstructure consists of equiaxed σ and γ grains. In the case of lower temperature deformation, a eutectoid decomposition of δ-ferrite into γ and σ phases occurs. The relatively soft γ grains which are severely deformed by hard σ particles recrystallize dynamically, and these processes lead to the γ/σ equiaxed duplex structure. The extremely large superplasticity of this alloy can mainly be explained in terms of the above microstructural change during deformation.

A model of a casting

Abstract: A model of a casting is presented which describes the freezing of a mushy zone, growing with a dynamically calculated undercooling at the dendrite tips. Equiaxed grains are introduced ahead of a columnar front. The model resembles a combination of the Stefan and mushy zone problems.

The Spray Forming of Superalloys

Abstract: Spray forming involves the gas atomization of a stream of molten metal to form droplets, and depositing the droplets on a collector before they have solidified. By manipulating the stream and/or the collector, a variety of useful shapes such as disks, bars, sheet, and plate can be made. As a consequence of rapid solidification, the product is homogeneous and is comprised of small, equiaxed grains, making it well suited to further processing. Discussed are the spray forming of 718 and Rene 95, and the properties after heat treatment, both as-deposited and after forging. Also reviewed are recent developments with respect to the processing of a range of near-net shape products.

Solidification : the separation theory and its practical applications

Abstract: 1 The formation of cast structures.- 1.1 The macro structure of cast metal.- 1.2 Separation theory.- 1.2.1 Principal mechanism of the formation of equiaxed zone.- 1.2.2 Research motives.- 1.2.3 Precipitation of crystals along the mould wall.- 1.2.4 In-situ observations of the solidification phenomenon.- 1.2.5 Formation of the equiaxed zone.- 1.2.6 Separation theory and free chill crystal theory.- 1.2.7 Origin of showering crystals caused by cooling of the molten metal surface.- 1.2.8 Confirmation of the separation theory.- 1.3 Origin of the formation of equiaxed crystals.- 1.4 Formation and separation of equiaxed crystals.- 1.4.1 Growth of crystals on the mould wall.- 1.4.2 Growth of necked-shape crystals.- 1.4.3 Separation and multiplication of crystals.- 2 Controlling the macro structure of cast metals.- 2.1 Equiaxed crystal structure.- 2.1.1 Crystal grain refiners.- 2.1.2 Mould cooling capacity.- 2.1.3 Mould properties.- 2.1.4 Vibration.- 2.1.5 Stirring.- 2.1.6 Pouring temperature.- 2.2 Columnar structure.- 2.2.1 Unidirectional solidification.- 3 Application of the separation theory.- 3.1 Formation of the macro structure of eutectic alloys.- 3.1.1 Primary crystal distribution and equiaxed eutectic grains.- 3.1.2 Eutectic leading phase.- 3.1.3 Refining of equiaxed eutectic grains.- 3.1.4 Separation of eutectic grains from the mould wall.- 3.2 Solidification of cast iron and the creation of an inverse chill structure.- 3.3 Spherical graphite cast iron and the separation theory.- 3.4 Separation theory and macrosegregation.- 3.4.1 Normal segregation.- 3.4.2 Inverse segregation.- 3.4.3 String segregation.- 3.4.4 Band segregation.- 3.4.5 Gravity segregation.- 3.5 Separation theory and the O.C.C. Process.- Conclusion.- References.

Improvements of the magnetic properties of equiaxed Fe-Cr-Co-Mo hard magnets by two-step thermomagnetic treatment

Abstract: Present works describe the developments of the equiaxed Fe-Cr-Co permanent magnets with high coercivity and high energy products. The studied alloy compositions are Fe-(26-32)wt%Cr-15wt%Co-3wt%Mo. The magnetic properties of the alloys have been enhanced by the two-step thermomagnetic treatment (two-step TMT) and by prolonging all of the heat-treatment time. The energy products above 43 kJm-3(5.4MGOe) are obtained for almost all of the studied compositional alloys. Fe- 30%Cr-15%Co-3%Mo alloy gives good magnetic properties as iHc=76.8 kAm-1(960 Oe), (BH)max=49.6 kJm-3(6.2 MGOe). The highest coercivity achieved with Fe-32%Cr- 15%Co-3%Mo alloy is 80.8 kAm-1(1010 Oe).

Growth of YBa2Cu3O9-δ Single Crystals from the High Temperature Solution

Abstract: YBa2Cu3O9-δ single crystals in the shape of a needle and a square plate were grown from a CuO-rich solution at a high temperature. The maximum size of the platy crystals with superconductivity was about 1.5×1×0.2 mm3. The details of the growth procedures of the single crystals are reported here.

Microstructures and superplastic behavior of eutectic Fe-C and Ni-Cr white cast irons produced by rapid solidification

Abstract: Superplastic behavior of two commercial grade white cast irons, eutectic Fe-C and Ni-Cr white cast irons, was investigated at intermediate temperatures (650 to 750 °C). For this purpose, rapidly solidified powders of the cast irons were fully consolidated by compaction and rolling at about 650 °C. The volume fractions of cementite in the eutectic cast iron and in the Ni-Cr cast iron were 64 pct and 51 pct, respectively, and both cast irons consisted of fine equiaxed grains of cementite (1 to 2 μm) and ferrite (0.5 to 2 μm). The cast iron compacts exhibited high strain-rate sensitivity (strain-rate-sensitivity exponent of 0.35 to 0.46) and high tensile ductility (total elongation of 150 pct to 210 pct) at strain rates of 10-4 to 10-3 s-1 and at 650 °C to 750 °C. Microstructure evaluations were made by TEM, SEM, and optical microscopy methods. The equiaxed grains in the as-compacted samples remained unchanged even after large tensile deformation. It is concluded that grain boundary sliding (e.g., along cementite grain boundaries in the case of the eutectic cast iron) is the principal mode of plastic deformation in both cast irons during superplastic testing conditions.

Microstructural development in Co-Cr films for perpendicular recording media

Abstract: A series of Co-Cr films have been examined using transmission electron microscopy (TEM) in bright/dark field imaging, and selected area diffraction (SAD) as well as convergent beam electron diffraction (CBED) Examination of the early stages of deposition by a DC magnetron provides evidence that amorphous Co-Cr films form prior to small randomly oriented equiaxed grains Occasionally, a mixture of equiaxed and elongated grains were observed in the plane-view Furthermore, microvoids are present running parallel to the columnar boundaries in cross-section view and at grain boundaries/edges in plane-view In addition to conical and straight columnar grains, a mixture of small equiaxed and elongated grains were observed in cross-section view

Temperature-orientation dependence of charateristics of short-term strength, Young's modulus, and linear expansion coefficient of single crystals of ZhS6F alloy

Abstract: 1. The short-term mechanical properties, i.e., tensile and yield strength elongation of the single crystals of the creep-resisting ZhS6F alloy are highly anisotropic up to 900°C. The maximum tensile and yield strength values were recorded for the single crystals with orientation, the minimum values for the single crystals with orientation. At 950°C, the properties become isotropic. 2. The single crystals of ZhS6F alloy with [001] orientation are characterized by the minimum values of the static and dynamic Young's moduli (E001= 13.3·104 MPa), and by the maximum values (E111=28.5·104 MPa) for orientation. For all other orientations, Young's modulus has intermediate values. When the temperature is increased to 800°C, the anisotropy of Young's modulus does not disappear. 3. The temperature coefficient of linear expansion for the single crystals of ZhS6F alloy is an isotropic quantity and is similar to the TCLE value of the equiaxed alloy for all the orientations.

Effect of solidification conditions on microstructures of unidirectionally solidified AZ91C alloy.

Abstract: AZ91C magnesium alloys were unidirectionally solidified at various liquid cooling rates (V LC), average cooling rates in solidification range (V SC), temperature gradients (G), moving velocities of solid-liquid interface (R) and G/R ratios. Equiaxed dendrite structure was obtained by the superheating of molten metal. Dendrite cell size decreases exponentially with increasing liquid cooling rate. With increasing average cooling rate and with decreasing temperature gradient, the amount of eutectic compounds increases and it extends at triple junctions of cell boundaries and individual cell boundaies. In AZ91C-T6 alloy, the eutectic compounds dissolve by solution heat treatment. Discontinuous precipitates along cell boundaries were observed by optical microscopy and their width decreases with increasing liquid cooling rate.

Method of forming dense ingots having a fine equiaxed grain structure

Abstract: OF THE DISCLOSURE A method of forming a fine grained equiaxed ingot by melting metal and placing it in a mold having a restriction at the entrance disposed to solidify the metal in the entrance to the mold prior to complete solidification of the metal in the mold such that a shrinkage void is formed below the entrance to the mold. The ingot is then hot isostatically pressed (HIPped) to fully densify the ingot and eliminate the shrinkage void.

Controlling the Macro Structure of Cast Metals

Abstract: So far I have discussed how equiaxed crystals are formed. I have asserted that “The principal mechanism of the formation of equiaxed crystals is the separation of crystals from the mould wall in the initial stage of solidification prior to the formation of a stable solid shell.”

Equiaxed dispersion strengthened copper product and process for making same

Abstract: This invention relates to substantially full-dense, near-net shape bodies made of dispersion strengthened copper (D.S.C.) 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 D.S.C. powder.

Application of the Separation Theory

Abstract: So far I have discussed my motives in beginning research into the solidification of metals, my advocacy of the crystal separation theory, and control of cast structures based on the separation theory. In brief, I have discussed the exceedingly simple principle that in order to obtain equiaxed crystals, one should ascertain where they form and then cause many crystals to form there and separate and precipitate. If one wants columnar crystals, one should ensure that the crystals do not separate from where they have formed.