Showing papers on "Equiaxed crystals published in 1992"

The coupled zone of rapidly solidified AlSi alloys in laser treatment

Abstract: Laser-melted tracks were produced on AlSi samples containing between 15.5 and 26 wt% Si with the resultant solidification rates being measured by taking a longitudinal section through the centre of the laser trace. The Al-rich boundary of the coupled zone, i.e.the growth rate-concentration limit at which the transition from fibrous AlSi eutectic to α-Al dendrites plus interdendritic eutectic takes place, has been experimentally determined for concentrations of Si varying from 15.5 to 20 wt%. Supposing that the growing structure, for a given growth rate, is the one having the higher growth temperature, good agreement is found with the more recent microstructural growth models when kinetic effects are taken into account. For concentrations of Si higher than 20 wt%, primary Si crystals imbedded in equiaxed eutectic grains are observed which replace columnar eutectic and dendritic growth.

Effect of microstructure on

Abstract: Good candidates to replace the conventional titanium alloys and nickel-based superalloys currently employed in gas turbine engines are the titanium aluminides. These ordered structures are attractive because of their good modulus retention at high temperatures, low densities, and improved creep resistance. The purpose of this research was to investigate how the micro- structure of the titanium aluminide, Ti-24Al-11Nb (at. pct), influences the creep rate. Study of the stress dependence of the steady-state creep rate has shown that microstructures with a coarse α2 lath size, in general, display improved creep resistance over fine-lathed structures. Specimens with an equiaxed α2-phase microstructure exhibit much greater creep rates at all stresses and temperatures, implying that dislocation slip length, proportional to α2 phase dimension, does not always control creep in these materials and that texture may adversely affect creep resis- tance. When microstructures and dislocation substructures were investigated by transmission electron microscopy (TEM), observations of crept specimens indicated that the majority of dis- locations have a-type Burgers vectors and are located in the α2 phase.

Mechanical Properties and Microstructure of Si3N4 Matrix Composite with Nano-Meter Scale SiC Particles

Abstract: Si3N4 matrix composites with dispersed SiC particles were prepared by hot-pressing compacts of powder mixtures of sub-micron size SiC and Si3N4 particles. SiC particles in nano-meter scale were dispersed uniformly inside Si3N4 grains. After adding SiC particles, the configuration of β-Si3N4 grains changed from equiaxed/platelet-like to rod-like structure. At 20vol% SiC, grains were fine and equiaxed. With increasing SiC content, the α- to β-phase transformation was suppressed, and relative density was decreased. On the other hand, the bending strength reached a maximum value at 10vol% SiC and fracture toughness was maximum at 5vol% SiC. These changes are related closely to the shape of Si3N4 grains and relative density. Young's modulus was minimum at 5vol% SiC. This seems to be caused by the disorder of the interface structure between SiC and Si3N4. Vickers hardness was constant up to 10vol% SiC and increased beyond it.

Superplastic behavior of two-phase titanium aluminides

Abstract: A two-phase Ti(57 at. pct)-Al(43 at. pct) alloy with an initial lamellar microstructure was thermomechanically processed to form an equiaxed fine-grained structure. The fine-grained (- L = 5 μm) material was superplastic in the temperature range 1000 °C to 1100 °C, exhibiting a stress exponent of about 2 with a tensile ductility of 275 pct. The rate-controlling deformation mechanism is proposed to be grain boundary sliding accommodated by slip controlled by lattice diffusion in TiAl. At room temperature, the lamellar and fine-grained materials exhibit the same compressive yield stress. The compressive strain to failure, however, for the fine-grained material was about 28 pct compared to 6 pct for the lamellar material.

Grain Defect Formation During Directional Solidification of Nickel Base Single Crystals

Abstract: Grain defect formation during solidification of nickel base single crystals has been studied in a series of alloys over a wide range of imposed thermal gradients and growth rates. Solute-driven convective instabilities, associated with high levels of Re and W and/or low levels of Ta, promote the formation of “freckle” defects as well as dendrite fragmentation and nucleation of misoriented grains. As the imposed positive thermal gradients at the solid-liquid interface decrease, grain nucleation events become more frequent until a general columnar to equiaxed transition is encountered. Transitions in solidification behavior and are shown to be consistent with changes in dendrite morphology and operative regimes of grain defect formation are characterized in terms of primary dendrite arm spacings. Suuerallovs 1992 Edited by S.D. Antolokch, R:W. Stusrud, R.A. MacKay, D.L. Anton, T. Khan, R.D. Kissinger, D.L. Klarstrom The Minerals, Metals & Materials Society, 1992

Densification of Reaction‐Synthesized Titanium Carbide by High‐Velocity Forging

Abstract: A novel process for the manufacture of dense titanium carbide is described. Titanium carbide is produced by the reaction synthesis method, while densification and near-net shaping is accomplished by a high-velocity forging step. Disks with 10-cm diameter were produced with densities over 96% of the theoretical density. The major problem encountered in this study has been thermal shock. Use of insulation and furnace cooling has decreased the severity of this problem. Optical and scanning electron microscopy observations of the resulting microstructure reveal equiaxed grains with an average size of 44 μm. Quasi-static and high-strain-rate compressive strength measurements yield values greater than 1.7 and 2.2 GPa, respectively. The morphologies of thermally induced (slow) and rapidly propagating cracks were characterized and the fracture modes were found to be intergranular and transgranular, respectively. The addition of Ni (5 and 25 wt%) yielded a ceramic–metal composite with a favorable microstructure.

Microstructural observations on the oxidation of γ′-Ni3Al at low oxygen partial pressure

Abstract: Microstructural development during the oxidation of (001)-oriented γ′-Ni3Al single crystals at 1223 K under an oxygen partial pressure of 4 × 10−19 atm has been studied by electron microscopy. After 1 min oxidation, TEM cross sections revealed a continuous 4 nm thick film of γ-Al2-O3 with equiaxed 20 nm protrusions into the metal. Pre-thinned foils oxidized for 6 min and studied in plan view in the TEM showed that the scale consisted of 20 nm γ-Al2O3 grains oriented to the metal such that (111) γ-Al2O3 (001) γ′-Ni3Al. Continued oxidation resulted in thickening of the γ-Al2O3 scale, no grain growth, and the development of a plane metal/γ-Al2O3 interface. Depletion of Al from the adjoining metal resulted in a well-defined disordered zone of NiAl solid solution between the Al2O3 scale and the γ′-Ni3Al. After 5 h oxidation large, randomly oriented α-Al2O3 grains nucleated at the metal/γ-Al2O3 interface, growing inward and transforming from the γ-Al2O3 outward. The α-Al2O3 contained intragranular and intergranular voids. The γ-Al2O3 exhibited a high density of planr defects and the interface between the γ-Al2O3 and α-Al2O3 contained many voids. Voids at the metal/oxide interface were never observed.

Oxidation resistant superalloy castings

Abstract: The oxidation resistance of a superalloy casting such as an equiaxed, directionally solidified, or single crystal casting, is improved by melting, pouring, or casting the alloy so as to react with a magnesium or calcium-bearing ceramic material. Magnesium or calcium is introduced into the alloy through a controlled reaction between the alloy and the magnesium or calcium-bearing ceramic material.

Microstructure and Wear Characteristics of an Al–4Cu–20Pb Alloy Produced by Spray Deposition

Abstract: Al-4Cu-20Pb alloy was spray deposited at a gas pressure of 14 MPa and melt Bow rate of 16 kg min -1 The microstructure of the preform exhibited equiaxed morphology with grain size varying from 20-25 μm Uniform distribution of submicron size lead particles were observed in the Al-matrix Wear testing, employing a pin-on-disc type set-up, revealed considerably low wear rate of the alloy compared to that of similar alloys reported in the literature

Recovery and recrystallization of cold-rolled polysynthetically twinned (PST) crystals of TiAl

Abstract: The recovery and recrystallization behavior of cold-rolled polysynthetically twinned (PST) crystals of TiAl with a nearly stoichiometric composition was investigated as a function of reduction in thickness and subsequent annealing time at 900 and 1000°C. The change in microhardness was found to occur in two stages regardless of the amount of reduction; the first stage associated with the decrease in dislocation density and the second stage related with the annealing-out of deformation twins introduced in TiAl lamallae during rolling. However, the recrystallization mode was found to vary depending on the amount of reduction in thickness. In specimens rolled to less than 20% reduction in thickness, the full recovery in hardness occurs preserving the lamellar structure. In contrast, in specimens rolled to more than 40%, a structure composed of equiaxed grains of TiAl is obtained after recrystallization. A mechanism is proposed to interpret the unusual recrystallization behavior of PST crystals of TiAl cold-rolled to less than 20%.

Finite-element thermal modeling of casting microstructures and defects

Abstract: Finite-element thermal modeling offers the capability of simulating casting solidification conditions. By combining thermal modeling and inspection, a defect map can be established to enable prediction of casting microstructures and defects. Gamma-prime size, dendrite-arm spacing, equiaxed grain size, shrink, hot tearing, misrun, and other features can all be predicted.

Microstructure and crack-shape effects on the growth behavior of small fatigue cracks in Ti24Al11Nb

Abstract: The effects of four different microstructures in the titanium aluminide alloy Ti24Al11Nb (at.%) on the fatigue crack growth behavior of small surface cracks and large cracks have been investigated. The four microstructures were a Widmanstatten basketweave, a Widmanstatten aligned colony, an equiaxed primary α2 in a Widmanstatten matrix and a completely equiaxed α2 structure. Small cracks were found to develop arbitrary shapes owing to the effects of microstructure. The crack shapes (aspect ratios) were measured using a laser interferometric and photomicroscopic system, and these measurements allowed accurate calculation of crack growth rates at the surface position as well as at the depth position for the surface cracks. After accounting for the continuous variation in crack shape in crack growth rate calculations, the trends in small-crack growth rates agreed reasonably well with the corresponding large-crack growth rates. While the crack growth rates at depth positions for small cracks correlated well with large-crack data in the basketweave microstructure, crack growth rates at surface positions correlated well with the corresponding large-crack data in the other microstructures. The microstructural factors that may be responsible for this behavior are discussed.

Sources of acoustic emission during fatigue of Ti-6al-4V : effect of microstructure

Abstract: The fundamentals of acoustic emission (AE) analysis of fatigue cracking were applied to Ti-6AI-4V. The effect of microstructure on the characteristics of the AE events generated and the failure mechanisms which produced AE in Ti-6AI-4V were established. Lamellar microstructures generated one to two orders of magnitude more emission than equiaxed microstructures. The combination of larger grain size, more continuous 0t/L3 interfaces, more tortuous crack-front geometry, cleavage and intergranular fracture in lamellar microstructures accounts for the greater amount of emission. For lamellar microstructures, most AE events were generated in the upper 20% of the stress range, whereas in equiaxed microstructures, most events were generated at lower stresses. Most AE events were generated during crack opening and also at low stresses. AE events having high level intensities were also generated at stresses other than the peak stress. This is because in titanium alloys, which have both high strength and toughness, AE events are generated from both plastic zone extension and crack extension.

Novel equiaxed magnetite media with high coercivity and thermal stability

Abstract: Equiaxed magnetite media with coercivity of up to 1560 Oe, saturation and residual magnetizations of up to 83 and 50 emu/g, respectively, and squareness ratio of up to 0.61 have been developed by the modification of Mn and Co, and by proper annealings. They were prepared by coprecipitation of Fe, Mn, and Co ions from an aqueous solution, by suitable alkali such as triethylamine, to adjust the pH, followed by calcination, reduction, oxidation, step‐annealings, and final reduction. Oxidation leads to a defect spinel ferrite structure, whose coercivity can be greatly enhanced by a step‐annealing treatment, and high coercivity can be retained after the final reduction to magnetite, whose coercivity can not be raised at all by the step‐annealing. These media have a temperature coefficient of coercivity as low as 0.20–0.33%/°C, hence are potential candidates for high density recording.

Method for preparing silicon nitride based ceramics

Abstract: Si 3 N 4 based ceramics are prepared by molding, calcining and sintering a powder mixture of columnar-shaped Si 3 N 4 particles having a particular size, a minor quantity of sintering aids and equiaxed Si 3 N 4 particles. The ceramics have a structure containing a specific quantity of coarse columnar grains grown from the coarse columnar-shaped particles as seed crystals, and superior in wear resistance, resistance to chipping and thermal resistance, and useful as wear resistant components or cutting tools. The ceramics can be produced without recourse to hot press sintering by normal sintering or sintering under a gas pressure, or further by applying HIP.

Modelling of dendritic solidification using finite element method

Abstract: Computer based numerical modelling of solidification is being increasingly used in an effort to develop and improve casting processes, a long term goal of this work being the prediction of microstructural features such as grain shape, size, and dendrite arm spacings throughout a casting. In a numerical heat flow model this can be achieved only through the inclusion of the kinetics of nucleation and dendrite growth. In the present paper strategies for including columnar and equiaxed solidification kinetics into a finite element model are reviewed. A detailed model for columnar solidification is then presented together with results obtained from calculations on an Al–5 wt-%Cu alloy and a multicomponent nickel based superalloy. It is shown that the inclusion of a dendrite tip undercooling is important, particularly in systems having a low Stefan number. Furthermore, the thermal histories in the superalloy can only be accurately calculated if an experimentally determined solid fraction versus temperat...

Effect of Composition and Stirring on the Grain Structure of Aluminium Alloys solidified in Ingots

Abstract: Solidification experiments have been performed on a series of industrial aluminium alloys in a rectangular vertical ingot mould, in which an electromagnetic linear motor activated a recirculating flow. Results include macrographs, grain size measurements, and high resolution temperature records on 14 thermocouples. The refinement effect is clearly demonstrated. It is strong when stirring changes from zero to some moderate level, then its increment decreases asymptotically. In most of the temperature records, the recalescence behaviour due to the formation of equiaxed crystals can be followed in detail. Under good stirring conditions, equiaxed crystallisation starts simultaneously in all the volume elements and all of them follow the same thermal history, which explains the very good uniformity in grain structure. Non-uniform or coarse grain structures which are observed when stirring conditions are poor can be correlated to dissimilarities in the thermal history of the different volume elements. A...

The transition from columnar to equiaxed dendritic growth in proeutectic, low-volume fraction copper, Pb−Cu alloys

Abstract: Lead, 17.1, 11.2, and 5 volume fraction copper (14, 9, and 4 wt pct Cu) alloys have been directionally solidified at constant growth velocities ranging from 1 to 100 μm s−1. Serially increasing the growth velocity within this range results in a graded microstructural transition from fully columnar, albeit segregated, copper dendrites in a lead matrix to one consisting only of equiaxed grains. The imposed velocity necessary to effect fully equiaxed growth is found to drop rapidly as the volume fraction of copper is decreased. Factors which complicate the controlled, directional solidification of these alloys are discussed and the experimental results are interpreted in view of, and seen to be in qualitative agreement with, Hunt’s theory on the transition from columnar to equiaxed growth of dendrites.

Influence of Macro/Microstructure on the Toughness of 'All Weld' Multipass Submerged Arc Welded C-Mn Steel Deposits

Abstract: 'All weld' deposited rectangular blocks of C-Mn steel were prepared by the multipass submerged arc process, using different welding currents (500-750 A) and speeds (40-60 cm/min). A filler wire of 4 mm diameter and a basic agglomerated flux were employed. Varying the welding parameters influenced the macrostructure comprising coaxial dendrites and reheat refined regions whose amounts were determined by quantitative metallography. The influence of dendrite content, on the toughness as influenced by temperature and orientation of the specimens (LT, TS and SL) was assesed both in the as welded and in the heat treated (873 K) conditions.An increase in the area fraction of dendrites beyond about 37% adversely affected the toughness which was found to be lowest in the LT direction. A post weld heat treatment carried out at 873 K improved the overall toughness due to spheroidization of cementite, especially within the refined equiaxed regions, resulting from 'reheating' during multipass deposition.

Analytical microscopy of a γ-TiAl composite containing a carbide phase

Abstract: Results are presented from microstructural observations of a carbide-containing gamma-TiAl composite Ti-44.5Al-6.64C (at. pct), prepared as an ingot by nonconsumable arc melting and casting. Results of SEM and TEM observations and WDS and EDAX analyses showed that the microstructure of the Ti-44.5Al-6.64C composite consisted of a matrix of twinned gamma with alpha-2 in lamellar and equiaxed forms. The alloy had a uniform distribution of a ternary carbide phase which occurred in elongated and equiaxed morphologies. The crystal structure of the carbide phase and the relative concentrations of Al and Ti were consistent with the published hexagonal structure of the AlTi2C and AlTi2N phases; however, the ternary carbide did not have any crystallographic correspondence with the gamma matrix. 19 refs.

Silicon Nitride Ceramics—Alloy Design

Abstract: Properties of silicon nitride ceramics depend on the phases present and their microstructure. In order to obtain ceramics with optimum properties, knowledge of phase relationships in silicon nitride-metal oxides systems and the mechanism and kinetics of microstructure development are necessary. There are four major stable silicon nitride phases in the Si3N4-metal oxides systems: β-Si3N4, α’-SiAlON, AlN-polytypoids and silicon oxynitride. The morphologies of these phases are different. The β-Si3N4 grains are elongated hexagonal rods; the α’-SiAlON grains are equiaxed; the AlN-polytypoids are platelets; and the silicon oxynitride are equiaxed. Ceramics with combinations of these phases would have different microstructures and properties. The system Si3N4-SiO2-AlN-Al2O3-YN-Y2O3 will be used to demonstrate the concept of the alloy design. Kinetics of microstructural development will also be discussed.

Planar to equiaxed transition in the presence of an external wetting surface

Abstract: First time experimental evidence is presented for the steady-state growth of equiaxed grains during directional solidification in the presence of a positive temperature gradient. The evidence is shown for transparent grains in the succinonitrile-acetone system, which are induced to grow on wetting surfaces (restraints). The conditions for equiaxed to dendrite transitions on the restraint are recorded, and a simple model is developed to explain this effect.

Surface hardening of Ti-6Al-4V by electrolytic hydrogenation

Abstract: Disclosed is a process of surface hardening of Ti-6A1-4V alloy that can be performed by electrolytic charging in an acid solution, subsequent solution treatment, followed by dehydrogenation to obtain an equiaxed alpha grain in transformed beta matrix. Surface hardnesses of the processed specimens are better than that of the mill-annealed specimen. The depth of hardened layer depends on the charging time.

The Indentation Fracture Resistance of Self–Reinforced Mullites

Abstract: A materials science goal for structural ceramics researches is to develop materials with ever-increasing fracture toughnesses. Mullite(3Al2O3·2SiO2) is a promising structural ceramic that develops a duplex microstructure of large acicular, or needle-like crystals within an equiaxed finer grain size matrix during grain growth. By selecting the appropriate thermal treatments, one can develop this duplex microstructure consisting of quite large “fiber-like”(acicular or needle-like) crystals in an equiaxed fine grain size matrix. The sizes of the acicular grains and their volume fraction maybe controlled by the annealing process that promotes grain growth. This microstructure is a special kind of self-reinforced fiber composite in which the material is reinforced by its own fibrous crystals. Thus it is possible to prepare a monolithic, yet fiber reinforced mullite structure having different mechanical properties.

Si3N4-TiN Porous Ceramics (Part 1)

Abstract: Porous ceramics were produced by pressureless sintering green compacts composed of Si3N4 and TiN powders without additives in nitrogen gas atmosphere at 1700°C. The microstructure and electrical-properties of the Si3N4-TiN porous ceramics in a humid atmosphere were investigated. The following results were obtained: (1) The pore size distribution showed a sharp peak at 2000A. (2) Rod-like shaped crystals of Si3N4 and equiaxed crystals of TiN distributed uniformly in the matrix. A large number of Si3N4 fibers of about 5μm were observed for 50wt% TiN. (3) The pore volume and specific surface area decreased proportionally with increasing amount of TiN. However, anomaly in the pore volume and specific surface area was observed at 50wt% TiN. (4) A sharp decrease in the impedance was observed in a dry atmosphere from 1×107 to 1×103Ω⋅cm for ceramics containing 40-60wt% TiN. (5) The impedance dependence of humidity was observed for ceramics in impedance higher than 105Ω⋅cm (less than 40wt% TiN content). In a high humid atmosphere, the impedance decreased with increasing specific surface area.

Scale development on impure nickel at high temperatures

Abstract: The growth of scales on impure Ni at 1373 and 1473 K was examined by microscopy and by oxygen-tracer-imaging techniques. The outward transport of cations was the basic rate-controlling step, and on the simple initial columnar scales such cation movement was the only kinetic process. However, oxygen movement was essential for the continued growth of more complex scales. Three types of sub columnar oxide were observed: (1) relatively coarse equiaxed crystals, (2) fume-like crystals, and (3) oxide that filled the pores between the crystals of the first two types. The oxygen for the first two types was provided by dissociation of the base of the columnar layer and also from voids that developed in the columnar layer. Oxygen for the third type came directly from the atmosphere following scale cracking. Such cracks healed after forming. The origin of the major features of scales was accounted for.