Showing papers on "Equiaxed crystals published in 1993"

Probabilistic modelling of microstructure formation in solidification processes

Abstract: A new approach to the modelling of grain structure formation in solidification processes is proposed. Based upon a two-dimensional cellular automata technique, the model includes the mechanisms of heterogeneous nucleation and of grain growth. Nucleation occurring at the mould wall as well as in the liquid metal are treated by using two distributions of nucleation sites. The location and the crystallographic orientation of the grains are chosen randomly among a large number of cells and a certain number of orientation classes, respectively. The growth kinetics of the dendrite tip and the preferential 〈100〉 growth directions of cubic metals are taken into account. The model is then applied to small specimens of uniform temperature. The columnar-to-equiaxed transition, the selection and extension of columnar grains which occur in the columnar zone and the impingement of equiaxed grains are clearly shown by this technique. The calculated effect of the alloy concentration and cooling rate upon the resultant microstructure agree with experimental observations.

A Multiphase Solute Diffusion Model for Dendritic Alloy Solidification

Abstract: A solute diffusion model, aimed at predicting microstructure formation in metal castings, is proposed for dendritic solidification of alloys. The model accounts for the different length scales existing in a dendritic structure. This is accomplished by utilizing a multiphase approach, in which not only the various physical phases but also phases associated with different length scales are considered separately. The macroscopic conservation equations are derived for each phase using the volume averaging technique, with constitutive relations developed for the interfacial transfer terms. It is shown that the multiphase model can rigorously incorporate the growth of dendrite tips and coarsening of dendrite arms. In addition, the distinction of different length scales enables the inclusion of realistic descriptions of the dendrite topology and relations to key metallurgical parameters. Another novel aspect of the model is that a single set of conservation equations for solute diffusion is developed for both equiaxed and columnar dendritic solidification. Finally, illustrative calculations for equiaxed, columnar, and mixed columnar-equiaxed solidification are carried out to provide quantitative comparisons with previous studies, and a variety of fundamental phenomena such as recalescence, dendrite tip undercooling, and columnar-to-equiaxed transition (CET) are predicted.

Controlled nucleation and growth of micrometre-size copper particles prepared by the polyol process

Abstract: Fine, equiaxed copper particles have been obtained by reduction of CuO in ethylene glycol. Cu2O always exists as an intermediate solid phase. Copper particles with a narrow size distribution within the micrometee range can be obtained if the nucleation and growth steps are completely separated and if agglomeration is avoided. Addition of D-sorbitol, which acts as a protective agent, prevents particle sintering. Addition of a strong base, e.g. NaOH, enhances the solubility of the precursor CuO and of the intermediate Cu2O. Under these conditions the overall reaction appears to be controlled by the nucleation and growth steps of the metallic particles. Their mean size can be largely controlled by varying the NaOH concentration. The synthesis of copper particles in liquid polyols, which act as both solvent and reducing agent, is a simple method for producing highly pure, equiaxed, non-agglomerated monodisperse particles.

A unified solute diffusion model for columnar and equiaxed dendritic alloy solidification

Abstract: A unified solute diffusion model is proposed for columnar and equiaxed dendritic alloy solidification, in which nucleation, growth kinetics and dendrite morphology are taken into account. Various applications to a uniformly solidified system are demonstrated, with emphasis on three special cases: complete solute mixing in the liquid, columnar growth with significant dendrite tip undercooling, and equiaxed dendritic growth. Theoretical predictions of microsegregation, eutectic fractions and cooling curves are compared with a number of previous theoretical and experimental results, and good agreement is found.

An analytical model for solute redistribution during solidification of planar, columnar, or equiaxed morphology

Abstract: Existing models for solute redistribution (microsegregation) during solidification were reviewed. There are no analytical models that take into account limited diffusion in both the liquid and the solid phases. A new analytical mathematical model for solute redistribution was developed. Diffusion in liquid and in solid was considered. This model does not require a prescribed movement of the interface. It can be used for one-dimensional (1-D) (plate), two-dimensional (cylinder), or three-dimensional (3-D) (sphere) calculations. Thus, it is possible to calculate microsegregation at the level of primary or secondary arm spacing for columnar dendrites or for equiaxed dendrites. The solution was compared with calculations based on existing models, as well as with some available experimental data for the segregation of base elements in as cast Al-4. 9 wt pct Cu, INCONEL 718, 625, and plain carbon (0. 13 wt pct C) steel.

General aspects of the thermomechanical

Abstract: This article represents a survey on the thermomechanical treatment (TMT) of two-phase intermetallic TiAl compounds. Nearly all of the important parameters, such as temperature, cooling rate, forging ratio, and direction, of the TMT steps were examined. The aim of this investigation was to understand the effects of the different parameters and thus to provide the knowledge of how to achieve all desired types of microstructures depending on the application purpose. Along the way, it was necessary to overcome the brittleness of TiAl compounds in order to permit further working and deformation processes, such as near net shape forming and sheet rolling. We obtained the best ductility with homogeneous equiaxed microstructures with true plastic X tensile strains at room temperature of more than 6 pct for Ti-48Al-2Cr (atomic percent). In the case of equiaxed microstructures, the Hall-Petch relationship could be confirmed for both the yield and the fracture stress. Therefore, the microstructures causing highest strength and high ductility and the way to achieve these properties were elaborated and are discussed in detail here.

Spray deposition of a Sn-40 Wt Pct Pb alloy with uniform droplets

Abstract: An apparatus capable of producing and depositing uniform droplets (100 to 200 μm in diameter) was developed and used to study the relationship between spray deposition parameters and the microstructures of Sn-40 wt pct Pb alloy spray deposits. The sprays used in the study consisted of uniform droplets, either 103 or 178 μm in diameter, that were in identical thermal and solidification states as they impacted the substrate. The thermal and solidification states of the uniform droplets were determined as a function of the flight distance (the distance from the metal pouring orifice) by model calculations and calorimetric measurements assuming equilibrium solidification. Although a fair agreement was noted between the model and the calorimetric measurements at small flight distances, corresponding to large liquid fractions, the calorimetric measurements indicated 10 to 20 pct higher liquid fractions at larger flight distances. The resultant microstructures comprised either a mixture of the Pb-rich and Sn-rich phases, both in an equiaxed morphology, or a lamellar eutectic structure with a small amount of the Pb-rich primary phase in a coarse spherical morphology. The mostly lamellar eutectic structure resulted from an excessive enthalpy flux and/or slow heat extraction from the deposit. Fine, equiaxed, two-phase microstructures and high deposit density resulted from optimal combinations of droplet enthalpy, deposition rate, droplet size, and deposit cooling rate which gave short local solidification times.

Grain control in mechanically alloyed oxide dispersion strengthened MA 957 steel

Abstract: Mechanically alloyed oxide dispersion strengthened stainless steels tend to recrystallise into columnar grains, a microstructure ideal for certain creep applications. In other circumstances, equiaxed grain structures are desired. In this paper, two methods are described which have been developed to ensure the reproducible development of equiaxed or refined grain microstructures in an alloy, MA957, which has previously not been amenable to control. Grain refinement has been achieved by controlling the stored energy, so that grain boundary velocities are reduced to a level which allows nucleation to develop at many sites, and by inducing a phase transformation from ferrite to austenite.MST/1779

3D probabilistic modelling of equiaxed eutectic solidification

Abstract: The equiaxed solidification of eutectic alloys is modelled by a probablistic method. The volume of the specimen is divided into a regular network of cubic cells and the temperature is assumed to be uniform. The temperature of the specimen is calculated in a time-stepping scheme from a simple heat balance and a knowledge of the heat flux leaving the metal. However, unlike the classical deterministic models describing equiaxed solidification, the evolution of the volume fraction of solid associated with the latent heat release is directly obtained from the cells of the network which have already been solidified. The liquid-to-solid transition of the cells is calculated by considering the mechanisms of heterogeneous nucleation and grain growth, but the grain impingement is already accounted for by this probabilistic method.

TiAl intermetallic articles

Abstract: A TiAl alloy base melt including at least one of Cr, C, Ga, Mo, Mn, Nb, Ni Si, Ta, V and W and at least about 0.5 volume % boride dispersoids is investment cast to form a crack-free, net or near-net shape article having a gamma TiAl intermetallic-containing matrix with a grain size of about 10 to about 250 microns as a result of the presence of the boride dispersoids in the melt. As hot isostatically pressed and heat treated to provide an equiaxed grain structure, the article exhibits improved strength.

Structure and mechanical properties of high-temperature titanium alloys after rapid heat treatment

Abstract: In this study a new approach to optimizing the mechanical properties of high-temperature titanium alloys was developed It is based on using rapid heating of equiaxed structures into the β-field to achieve a fine β grain size (less than or equal to 100 μm), transforming on subsequent cooling into a fully lamellar structure This fine β grain size is an order of magnitude smaller than the grain sizes achieved by conventional furnace β-treatment Structures and mechanical properties (tensile, fatigue and creep) of high temperature alloys after rapid and conventional furnace heat treatments were compared The results are discussed in terms of structure-property relationships

Microstructural characterization of CO2 laser welds in the Al-Li based alloy 8090

Abstract: The microstructural development of the Al-Li-Cu-Mg-Zr alloy 8090 has been studied after autogenous CO2 laser welding. Sheets ranging in thickness from 1–4 mm were welded at speeds of between 20–120 mm s−1 and powers from 1.5–3.8 kW. Optical microscopy, scanning and transmission electron microscopy were used to study the as-received base metal, the heat-affected zone and the solidified fusion zone. The base metal was supplied in a superplastically formable condition and thus had an unrecrystallized grain structure containing 1–2 μm sized sub-grains with sub-micrometre δ′ and β′ precipitates in the matrix. In the fusion zone, the as-solidified grain structure was columnar at the interface with the base metal but became equiaxed in the central region of the weld pool. The weld depth and top bead width both increased with decreasing welding speed and increasing beam power within the limits investigated. The fusion zone microstructure was cellular-dendritic. Intermetallic precipitates, which are rich in copper, magnesium, silicon (and presumably lithium), formed in the cell/dendrite boundaries. Very fine-scale δ′ precipitates were present in the as-solidified α-Al matrix but there was no evidence for the β′, S′ and T1 phases. The heat-affected zone was only 100 μm wide and was characterized by regions of partial melting. Radiographs of welds reveal that porosity occurred predominantly along the weld centre-line. In partial penetration welds, two types of pores were observed: near spherical and irregular. However, in fully penetrating welds, only the spherical type of porosity was present. Overall volume fractions of porosity were measured from metallographic sections and were found to vary with welding speed and weld type, i.e. partial or full penetration.

Isothermal transformation behavior of near-gamma TiAl alloys

Abstract: During the last few years near-gamma TiAl alloys have been considered as viable engineering materials for aerospace applications. A substantial amount of research effort has been directed towards the understanding of the microstructure-property relationship of these alloys. Similarly, fundamental studies have been conducted to understand the phase transformation mechanisms of single and two-phase titanium aluminides. The results of these studies have clearly established four types of microstructures: fully-lamellar, massive, equiaxed and duplex (lamellar + equiaxed). The two former types are developed through heating and cooling from the [alpha] phase region. Slow cooling produces the fully-lamellar structure and fast cooling leads to the massive microstructure. The two other types result from heating to and cooling from the ([alpha] + [gamma]) intercritical region. The development of the duplex or equiaxed structure depends on both the reheating temperature and holding time. The fully lamellar microstructure exhibits the best combination of mechanical properties at both room and high temperatures when compared to the other microstructures.

Effects of expulsion in spot welding of cold rolled sheet steels

Abstract: The effects of expulsion on microstructure and tensile shear strength of spot welds have been investigated for a high-strength cold rolled sheet steel. Spot welds with expulsion are characteristic for a “double im-age” near the fusion line, an equiaxed dendritic grain zone along the faying surface, and deep indenta-tions on the outer surfaces. Microstructural examinations and tensile shear tests of spot welding specimens were conducted to determine the correlation between microstructure and strength. It was found that the equiaxed dendritic structure was associated with significant solidification cracking. Frac-tography on tensile specimens reveals that brittle failure is associated with an equiaxed microstructure. Furthermore, the surface indentation will change the stress at the nugget edge, and deep surface inden-tations are expected to promote premature failure.

Liquid dynamic compaction of aluminum alloy 7150

Abstract: Two spray deposited, high strength 7150 aluminum alloys, one with increased nominal levels of iron and silicon and the other of standard composition, were prepared by rapid solidification, liquid dynamic compaction (LDC). Spray deposition for this study was done with a circular ultrasonic gas atomization nozzle (USGA). The as-deposited materials showed high densities (96 ± 3%) and fine (< 20mm) equiaxed microstructures with very low alloy segregation and a very fine, uniform distribution of excess phases, the predominant one being metastable Al 6 Fe. No elemental silicon, Mg 2 Si or any other excess phases were detected in the as-deposited alloys

Properties of electron-beam-welded and laser-welded austenitic Fe-28Mn-5Al-1C alloy

Abstract: Austenitic Fe-28Mn-5Al-1C alloy was welded by electron-beam and CW CO2 laser techniques. Tensile tests, impact tests, potentiodynamic and cyclic polarization measurements were used to evaluate the mechanical properties and corrosion behaviour of the weld materials, Metallographic examination showed that the microstructure of the electron-beam-welded and laser-welded metals consisted mainly of the columnar and equiaxed austenitic structures. Grain growth in the heat-affected zone (HAZ) was minimal for welding with these two techniques. The tensile and impact tests indicated that the weld materials exhibited lower tensile strength, percentage elongation, percentage reduction in area and impact energy than those of the base alloy. The polarization measurements revealed that the anodic polarization behaviour of the HAZs of the electron-beam-welded and laser-welded materials was identical to that of the base alloy when exposed in 1M Na2SO4 solution. However, the electron-beam-welded and laser-welded metals exhibited a higher current density in the passive region than that of the base alloy when exposed to 1N H2SO4 acid solution.

gamma and beta dual phase TiAl based intermetallic compound alloy having superplasticity

Abstract: This invention relates to TiAl based intermetallic compound alloy and process for producing; the object of this invention is to improve high temperature deformability. The alloy comprises basic components: TiyAlCrx, wherein 1% /=100%, and comprises a fine-grain structure with a beta phase precipitated on a grain boundary of equiaxed gamma grain having grain size of less than 30 mu m, and possessing a superplasticity such that the strain rate sensitivity factors (m value) is 0.40 or more and tensile elongation is 400% or more tested at 1200 DEG C. and a strain rate of 5x10-4S-1.

Evolution of microstructure of AISI 347 stainless steel during heat treatment

Abstract: The microstructural changes caused by various thermomechanical treatments were investigated for AISI 347 stainless steel. The high dislocation density which results from cold rolling is not annealed out even after long holding times at 600°C. When heating at higher temperatures, e.g. 750°C, precipitation of very fine NbC particles is observed, together with onset of recrystallisation. Complete recrystallisation resulting in equiaxed dislocation free grains occurs only when annealing severely cold worked specimens in the high temperature range of this investigation.MST/1708

Semi-solid deformation in multi-component nickel aluminide: Part I Equiaxed alloys

Abstract: A systematic study was carried out to determine the solidification and tensile behaviour of semi-solid multi-component nickel aluminide. Controlled equiaxed-solidified samples were tested at various temperatures in the mushy (semi-solid) region. A special Gleeble testing procedure was developed where the samples were quickly raised to a predetermined temperature in the semi-solid zone and fractured. The fracture stress was noted to decrease monotonically with temperature. The strain to fracture exhibited a ductility minimum at an intermediate temperature in the semi-solid zone. For the equiaxed-solidified samples, the fracture stress was found to decrease with increasing cooling rate at any given temperature

Investment casting of NiAl single-crystal alloys

Abstract: Significant progress has been made in the understanding of solidification conditions, microstructure evolution, and defect formation during investment casting of NiAl single crystals. The high liquidus temperatures of NiAl alloys result in a larger dendrite arm spacing than is found in superalloy Rene N5. Because of their higher thermal conductivities, NiAl alloys have higher cooling rates and lower temperature gradients during solidification than Rene N5. These differences give NiAl alloys a lower tendency to form freckles and a higher tendency to form equiaxed grains. However, with the aid of process modeling, single crystals of various shapes of NiAl alloys have been produced.

Morphological effect on the mechanical

Abstract: A study has been made on the deformation and fracture behavior of a two-phase (α2 + β) Ti3Al-Nb alloy, with particular emphasis on the effect of morphologies. The coarse colony structure obtained by continuously annealing in the two-phase region directly from the β region results in poor elongation ductility. The fine colony structure obtained by two-phase annealing the β-quenched specimen also results in poor elongation ductility. However, upon obtaining fine equiaxed or basket-weave structures by following different transformation paths, improvements occur in elongation ductility without any sacrifice in strength. For the fine equiaxed structure, maximum elongation is obtained at the intermediate volume fraction of a2, while for the basket-weave structure, an increase in the volume fraction of α2 increases the elongation. Detailed fracture analysis shows that different fracture mechanisms operate for different morphologies, and the critical microstructural unit for arresting fracture also varies, depending on the morphologies.

Surface hardening of Ti-6AI-4V alloy by electrochemical hydrogenation

Abstract: Surface hardening of Ti-6A1-4V alloy can be performed by electrolytic charging in acid and basic solutions, with or without subsequent solution treatment, followed by dehydrogenation to obtain equiaxed α grains in a transformed β matrix. Surface hardnesses of the processed specimens are better than that of the mill-annealed specimen. The depth of the hardened layer depends on the processing parameters.

Interdiffusion-induced devitrification of porcelain enamel on steel

Abstract: Devitrification which occurred in the ground coat (fired at 830°C for 0–30 min) due to interdiffusion near the steel-enamel interface was characterized by electron microscopy (SEM and TEM) coupled with energy-dispersive X-ray analysis. Spinel oxide, Fe3O4, which was alloyed with Ni if nickel-dip pre-treatment was adopted and was compatible with α-Fe, crystallized in the ground coat near the glass-metal interface. Independent of nickel-dip pre-treatment the spinel crystals were extensively developed at the edges of the specimen slab, but were fewer and smaller in the centre of the flat faces. The quantity and size of crystals increased with the firing time. Within the titania-opacified cover coat (fired subsequently at 800°C for 3 min) equiaxed TiO2 crystals (anatase with a minor amount of rutile) were distributed uniformly. The TiO2 crystals grew inwards to some extent into the ground coat and became needle-like with their long axes aligned diagonally to the interface. The devitrification of enamel was affected significantly by diffusion across the interfaces which resulted in a saturation with oxide components.

Orientation distributions and melting behaviour of extended and folded-chain crystals in gel-drawn, ultra-high-molecular-weight polyethylene

Abstract: X-ray diffraction (XRD) studies on the orientation distributions of the 110 and 100 planes in gel-drawn samples of ultra-high-molecular-weight polyethylene (UHMWPE reveal the complex character of the crystal-orientation distribution in the samples. The orientation distribution is composed of two contributions differing in width. The dependence of the contents of both components on the draw ratio, as well as their behaviour during melting of the sample, are studied. The character of the dependencies observed indicate that each component of the orientation distribution can be associated with a separate assembly of crystals occurring in the sample, both having the same orthorhombic crystal structure. The broad component corresponds to only slightly oriented folded-chain crystals, while the narrow component corresponds to almost perfectly oriented extended-chain crystals being composed of stretched macromolecules. The content of the folded-chain crystals decreases with an increase in the draw ratio, whereas the content of the extended-chain crystals increases. During melting, folded-chain crystals melt directly to an amorphous phase, while the extended-chain crystals undergo crystal-crystal transition forming a hexagonal phase.

Mechanisms of Uniform Corrosion of Zirconium Alloys in Water and Steam

Abstract: Zirconium alloys are used in key components in nuclear reactors, e.g. fuel cladding tubes in pressurised water reactors. Current trends towards extended burnup of the nuclear fuel in these reactors have accentuated the demand for Zr alloys with higher uniform corrosion resistance under irradiation and lower hydrogen absorption. The first part of this work was performed to investigate the matrix composition, that was proposed to influence the uniform corrosion resistance of Zircaloy, the traditional fuel cladding material. The first direct measurements were made of the concentrations of the alloying elements Fe, Cr and Ni in the matrix in five Zircaloy materials, using atom probe microanalysis. In all five materials the matrix was highly depleted with respect to alloying elements. Only 100-200 wt.ppm of Fe, Cr and Ni remained in the matrix, which is approximately 10% of the bulk composition. The measured concentrations correspond well to recent diffusion and solubility data for Zircaloys. In the second part of this study the oxide microstructure was related to the corrosion resistance and hydrogen uptake of zirconium alloys with different composition and heat treatments. The initial oxidation in air or water of Zr-4 needles was studied with atom probe analysis and transmission electron microscopy (TEM). The grain size of the thin oxide layers was approximately 5 nm, and the oxide composition was ZrO. Thin foil cross-sections were prepared of the metal-oxide interface and of the oxide layers grown in static autoclave at 400 °C steam on Zircaloy-4 and Zr-0.5Sn- 0.53Nb. Oxides in the thickness range 1-42 µm were examined with TEM. For comparison, one low-annealed Zr-4 material with high uniform corrosion rate was also investigated. In the metal-oxide interface of all materials, small columnar ZrO 2 grains (A? 15 x 100 nm) were observed growing in direct contact with the metal. No amorphous layer was found in the oxide-metal interface or between columnar grains. Thin oxide layers (pre-transition, A? 1 µm) were dense, while thick (post transition) oxide layers contained some cracks close to equiaxed grains. On the low-annealed Zr-4, thick oxide layers contained a high number of equiaxed grains, and intergranular pores and cracks were frequent even close to the interface. The metal-oxide interface had a less ordered structure, with shorter columnar or equiaxed oxide grains. In post-transition oxide layers of the Zr-Sn-Nb alloy (which had a lower relative hydrogen uptake) a crystalline 20-100 nm thick intermediate layer in the metal-oxide interface was found.