Showing papers on "Equiaxed crystals published in 1986"

Modification of alpha morphology in Ti-6Al-4V by thermomechanical processing

Abstract: The modification of lamellar alpha phase in Ti-6A1-4V by hot working was investigated with the aim of controlling morphology (aspect ratio) and final grain size. The effect of strain was studied using forging at 955 °C (1750 °F), followed by annealing at 925 °C (1700 °F) to allow the alpha morphology to adjust. Increasing the deformation from 6.5 pct to 80 pct reduction caused the lamellar alpha morphology to become progressively more equiaxed upon annealing. TEM observations showed that annealing of material deformed to 6.5 pct resulted in recovery of the alpha, without a noticeable change in the morphology, while higher deformation resulted in plate shearing and beta cusp formation. It was found that material with an initial thin alpha plate structure (thickness — 3.4 ώm) breaks up at a lower critical strain than a material with a thicker plate morphology (thickness ≃ 6 μm). The material with thin alpha plates more rapidly forms equiaxed alpha grains separated by beta phase, while the material with a thicker plate structure exhibits more alpha/alpha boundaries after deformation and annealing. The morphology change from alpha lamellae into lower aspect ratio grains was identified to be by a break-up of the alpha lamellae, essentially by a two-step process: a formation of low and high angle alpha/alpha boundaries or shear bands across the alpha plates followed by penetration of beta phase to complete the separation. This break-up takes place during hot deformation and subsequent annealing.

Solute diffusion model for equiaxed dendritic growth

Abstract: Abstract A new approach to the modeling of the equiaxed solidification of dendritic alloys is proposed. It is assumed that, in metallic alloys, microstructure formation is primarily controlled by solute diffusion (i.e. there is complete “thermal mixing” at the scale of one grain), and that the dendrite interface is an iso-concentrate at all times. The evolution of one dendritic grain is therefore modelled as follows: (i) complete mixing of solute within the interdendritic liquid; (ii) no back-diffusion in the solid; (iii) spherical solute diffusion in the liquid around the grain envelope; (iv) overall solute balance; (v) overall thermal balance; (vi) growth velocity, υg, of dendrite tips governed by the kinetic equation derived for the isolated dendrite case. By using an explicit finite difference scheme to solve these coupled equations, the concentration profiles, cooling curve, fraction of solid and evolution of dendritic grain envelope can be calculated. The intitial conditions used to start the calculation are provided by two parameters related to nucleation: the initial undercooling and the density of gains. The effect of nucleation and thermal conditions on equiaxed growth are studied. The theoretical predictions of recalescence and of the distribution of an interdendritic eutectic phase are in good agreement with experimental observations.

Development of Al-Ti-C grain refiners containing TiC

Abstract: Cast Al-Ti-C grain refiners were synthesized by reacting up to 2 pct graphite particles of 20 micron average size with stirred Al-(5 to 10) pct Ti alloy melts, which generated submicron-sized TiC particles within the melts, and their solidified structures showed preferential segregation of the carbide phase in the grain or cell boundary regions and occasional presence of free carbon whose amount exceeded equilibrium values. At the usual melt temperatures of below 1273 K, though, TiC formed first, but was subsequently found to react with the melt forming a sheathing of A14C3 and Ti3AlC which resulted into poisoning of the TiC particles. However, it was possible to reverse these reactions in order to regain the virgin TiC particles by superheating the melts in the temperature region where TiC particles are thermodynamically stable. Grain refining tests using the TiC master alloys produced fine equiaxed grains of cast aluminum whose sizes were comparable to that obtainable with the standard TiB2 commercial grain refiner. TiC particles introducedvia the master alloys were found to occur in the grain centers, thereby confirming that they nucleated aluminum crystals.

The structure and properties of a nickel-base superalloy produced by osprey atomization-deposition

Abstract: The production of a nickel-base superalloy, Rene* 80, by the Osprey atomization-deposition process has been investigated. Dense (>99 pct) material with a fine-grained equiaxed microstructure was deposited using either argon or nitrogen as the atomizing gas. Defects present in the material included a chill region at the collector plate interface, entrapped recirculated particles, porosity, and ceramic particles from the melting and dispensing system. In contrast to other rapid solidification techniques, low oxygen pick-ups are noted in the current technique. Tensile strengths above those displayed by castings are found in both nitrogen and argon atomized material, and in both the as-deposited and heat treated conditions. In addition, no profound mid-temperature ductility loss is displayed by this low oxygen material, in contrast to results on other rapidly solidified material with high oxygen contents. These results are explained in terms of oxygen embrittlement. In view of the excellent properties measured, the attractive economics of the process, and the fact that fine control of the gas/metal flow ratio is shown to be unnecessary, it is concluded that atomization-deposition presents an attractive potential production route for advanced alloys.

Microstructure of a quenched and tempered Cu-bearing high-strength low-alloy steel

Abstract: The grain structure, inclusion content, and precipitate types were characterized for six heat treatments of a copper-bearing HSLA steel A higher austenitizing temperature combined with water-quenching resulted in an acicular ferrite microstructure, while lower temperatures produced equiaxed ferrite or ferrite-pearlite structures Refinement of equiaxed ferrite was observed in material austenitized at a high enough temperature to dissolve a portion of the carbonitrides, allowing reprecipitation during the austenite-ferrite transformation Age-hardening precipitated body-centered cubic copper clusters; face-centered cubic copper precipitates were observed in overaged material

Mechanism of transition from columnar to equiaxed zone in ingots

Abstract: A new theory for the transition from columnar to equiaxed crystals has been developed. It is proposed that free crystals are formed in the liquid ahead of the growing front and that these crystals grow in the liquid as a result of natural convection. When the free crystals are sufficiently large or sufficiently numerous they physically block the growth of the columnar crystals by adhering to the solidification front. The growth rate of the free crystals is determined by the cooling rate of the mould and the kinetics of the solidification process. The theory indicates that the higher the cooling rate and the slower the solidification process the easier the transition. The calculations are compared with experimental observations on a large steel ingot.MST/272

A method of forming a fine-grained equiaxed casting

Abstract: A method of forming a fine grained equiaxed casting by melting metal and removing most of the superheat of the molten metal. The molten metal is placed in a mold and optionally subjected to turbulence whereupon it solidifies to form the casting of the desired microstructure.

Factors affecting the final grain size of decarburized lamination steels

Abstract: A study was made of the effects of carbon content, strain, initial grain size, carbide morphology, annealing time, and temperature on the final grain size produced after the decarburization anneal. The results show that columnar grains form only in higher carbon (0.05/0.06 Pct C) steels with low initial grain sizes (<20 µm) when decarburized at 1450 °F (788 °C). Equiaxed grains, however, are obtained in low carbon (0.02/0.03 Pct C) lamination steels for all processing conditions. It was found that the formation of both equiaxed and columnar final grain size (GS f ) was dependent upon percent strain (e), initial grain size (GS i ), and carbide distribution (D c ) according to the following regression equations (all grain size measurements in µm):G5 f (columnar grain) = 204 + 0.9GSi - 12.3e + 37.8D C logGS f (equiaxed grain) = 2.151 + 0.022GSi - 0.038e - 0.0005GS i · e- 0.0002(GS i )2 + 0.079D c

Superplastic deformation of Al-Li-Cu-Mg alloy sheet

Abstract: Al-2.5 Li-1.2 Cu-0.6 Mg-0.12 Zr (wt%) alloy sheet was cold-rolled, solution heat-treated for 20 min at 510° C, prestrained by 3% and superplastically deformed at 450 to 540° C at strain rates between 1×10−4 and 2.8×10−1 sec−1. The maximum elongation obtained was 300%. Significant cavitation occurred above about 0.5 strain at a rate (void volume/unit strain) of 4% at 540° C and 6% at 500° C. The onset of cavitation coincided with a reduction in the room-temperature tensile properties after reheat-treatment. During annealing at 500 to 540° C, grain coarsening near the sheet surface was associated with magnesium and lithium depletion. Superplastic deformation produced a fine equiaxed microstructure by dynamic recrystallization.

Microstructure and stability of melt spun INCONEL 713 LC

Abstract: The alloy IN-713LC* was used in an investigation of the effect of process parameters on the microstructure of a rapidly solidified melt-spun material. The resultant ribbon microstructure consisted of several distinct regions, each of which corresponds to a different thermal history during processing. A chill zone of equiaxed randomly-oriented grains exists in a region of the foil which was in contact with the wheel during casting. This zone develops into a dendritic growth morphology with distance away from the lower ribbon surface. Dendrites inclined in the direction of wheel rotation result from growth into a flowing stream. TEM studies showed that a cell structure formed, the cell size decreasing with increasing wheel speed. Aging studies indicated that the cell structure plays an important role in γ’ precipitation. Results relating to heat treatments (as would be encountered in compaction and use) and the stability of the melt-spun structure are considered.

Tensile properties of directionally solidified Al-4 wt pct Cu alloys with columnar and equiaxed grains

Abstract: The tensile properties of directionally solidified Al-4 wt pct Cu-015-02 wt pct Ti alloys with equiaxed grains were determined and compared with the properties of directionally solidified Al-4 wt pct Cu columnar structures The tensile properties of the equiaxed structure were isotropic, but varied with the distance from the chill face The mechanical properties of the equiaxed structure were generally between those of the longitudinal and transverse columnar structures The 02 pct offset yield stress(σy, MPa) is represented as a function of the grain size,d (mm), the average concentration, Co (wt pct), and the local concentration, C (wt pct), by σy = [(157 + 226 Co) + (124 + 104 Co)d-1/2] + [157 △C], where △C = C - Co The equiaxed structure exhibits inverse segregation similar to that in the columnar structure

High-temperature creep and dislocation structure of MgO single crystals at low stresses

Abstract: MgO single crystals oriented toward (100) have been compressively deformed to strains in the range of 0.04 to 0.09 at temperatures between 1948 and 2023 K using stresses under 6 MPa. Microstructural developments in the crept samples were monitored by etch pitting, SEM and HVEM. Optical microscopy revealed an equiaxed network of subgrains with 〈110〉 forming the framework for the formation of subgrains. Measurement of the dislocation density not associated with cells reveals that the stress dependence of the steady-state values of dislocation density can be described by the relationϱ ∫σ2.15. HVEM observations show that cell boundaries are formed by the process of knitting. Drastic unloading results in a fraction of the sub-boundaries straightening, but the majority of the boundaries are destroyed. It is concluded that creep of MgO at low stresses and high temperature is similar to those of fcc metals.

Production of black plate for tin plate having excellent workability

Abstract: PURPOSE:To produce a black plate for a tin plate for a DI can having stable equiaxed crystal structure by subjecting a continuously cast slab formed by incorporating a specific ratio each of C, Mn and Al into to hot rolling then to coiling and cold rolling at a specific temp. and recrystallization annealing followed by annealing under specific conditions. CONSTITUTION:The continuously cast slab consisting of 0.02-0.10wt% C, 0.10-0.50wt% Mn, 0.01-0.08wt% Al and the balance iron and inevitable impuri ties is hot rolled and is coiled at <=650 deg.C. The coiled plate is then subjected to cold rolling by the conventional practice and is then subjected to the recrystallization annealing by a continuous annealing method, by which the equiaxed recrystallized grains are obtd. The plate is further heated to the temp. exceeding 500 deg.C by a box annealing method and is cooled in the furnace to obtain the substantially soft black plate for the tin plate which has the excellent workability, is decrease in the inclusion level and has the excellent inside surface characteristic of the can is thus obtd.

The effect of post-sintering heat treatments on the tensile properties of Ti-6A1-4V alloy.

Abstract: Previous studies have shown that the application of a porous coating to a solid substrate offers several advantanges over current methods of implant fixation. However, the heat treatments required to sinter porous metal coatings have also been shown to cause significant decreases in the mechanical properties of the substrate. With Ti-6A1–4V alloy, sintering above the material beta transus results in a transformation from the as-received, equiaxed microstructure, recommended for surgical implants, to a lamellar alpha-beta microstructure. This lamellar structure has been shown to have inferior mechanical properties.In the present study, microstructural analysis and mechanical testing were performed on Ti-6A1–4V alloy subjected to various post-sintering heat treatments in an attempt to improve the mechanical properties. The microstructures examined were a fine and a coarse acicular alpha in a retained beta matrix. Tensile tests were performed on specimens containing these structures and results were compared...

Application of computer simulation to correlate structures with solidification parameters

Abstract: A model for heat flow during end-chill solidification of lead ingots is presented. The model treats the thermophysical properties of the metal as a function of temperature, and models the evolution of latent heat. The model is solved numerically by a finite difference technique for one and two-dimensional heat flow in end-chilled cylindrical ingots. The model is evaluated using experimentally obtained cooling curves. Results are presented in terms of cooling curves, temperature distribution along the ingot axis, and local solidification time (ts). Growth rates, temperature gradients and cooling rates are determined as functions of distance from the chill. The resulting structures are related to the different growth parameters. The conditions for formation of columnar and equiaxed grains are discussed. Experimentally measured grain size (D) was found to follow the relationship:

Effects of process parameters on centreline solidification in eb welds

Abstract: Beam deflection techniques and controlled variations in welding conditions were used to produce a range of melt runs in five steel types to investigate effects of process parameters on centreline solidification. A variety of solidification modes was observed and it proved difficult to obtain a favourable equiaxed structure solely by control of welding conditions. Raft-type structures giving poor properties can possibly be avoided by using beam deflection to disrupt the back edge of the weldpool.

The microstructure of Nb3Sn in modified jelly roll superconducting composites

Abstract: Transmission electron microscopy (TEM) has been used to characterize the microstructure of the Nb3Sn layers developed during heat treatment of two superconducting wires, with and without 0.8 wt% titanium addition to the niobium, manufactured by the modified jelly roll (MJR) process. The composites in the as-received state are shown to contain pre-reacted layers formed during fabrication anneals, while heat treatments over the range 650 to 750° C yield a two-fold layer structure of columnar and equiaxed grains. Examples of both transverse and longitudinal TEM micrographs are given. The addition of 0.8 wt% titanium to the niobium before fabrication leads to coarsening of the equiaxed grains after identical reaction times. The results are discussed in terms of a recently proposed model for the development of microstructure in A15 multifilamentary composites.

Analytical ultrasonics for structural materials

Abstract: The application of ultrasonic velocity and attenuation measurements to characterize the microstructure of structural materials is discussed. Velocity measurements in cast stainless steel are correlated with microstructural variations ranging from equiaxed (elastically isotropic) to columnar (elastically anisotropic) grain structure. The effect of the anisotropic grain structure on the deviation of ultrasonic waves in cast stainless steel is also reported. Field-implementable techniques for distinguishing equiaxed from columnar grain structures in cast strainless steel structural members are presented. The application of ultrasonic velocity measurements to characterize structural ceramics in the green state is also discussed.