Showing papers on "Equiaxed crystals published in 2000"

Evolution of near-equiaxed microstructure in the semisolid state

Abstract: The microstructure of alloys with a near-equiaxed microstructure, produced by spray casting, magnetohydrodynamic (MHD) casting and the stress induced, melt activated (SIMA) process, as it evolves within short times in the semisolid state, is examined by rapid quenching and isothermal soaking experiments. Quenching experiments reveal the morphology and distribution of solid phase at high and medium volume fractions of liquid. At medium liquid content, the microstructure of spray-cast and SIMA alloys consists of discrete equiaxed grains uniformly dispersed in the liquid phase, while the corresponding microstructure of MHD-cast alloys exhibits extensive agglomerates consisting of incompletely spheroidized grains. The connectivity of solid phase and the formation of a solid skeleton in the semisolid state are discussed in terms of grain misorientation. Isothermal soaking experiments investigated grain growth and degree of spheroidization as a function of soaking time and liquid content in the semisolid state. Results demonstrated that MHD-cast microstructures are less equiaxed compared with SIMA and spray-cast alloys even after 5 min of soaking in the semisolid state. It is also shown that the grain growth rate is smaller in spray-cast alloys than in SIMA alloys. The role of coalescence and the effects of alloying elements are also discussed.

Dynamic recovery: sufficient mechanism in the hot deformation of Al (<99.99)

Abstract: A recent OIM study of the substructure in hot compressed Al has observed an increase in the fraction of boundaries both of 15–20° and above 20° as strain rises from 0.9 to 1.5. This was interpreted as evidence of continuous dynamic recrystallization being the mechanism for the steady state deformation. However, when the original grain boundaries and transition boundaries between deformation bands are discounted, the fraction of 15–20° boundaries is reduced to less than 20% and would be much lower if subboundaries less than 0.5° visible in TEM were taken into account. The present authors argue that dynamic recovery maintains the subgrains of constant size, low misorientation and equiaxed to produce a steady state and can permit a limited number of discrete segments with higher misorientation notably as temperature falls. Moreover, continuous dynamic recrystallization is not appropriate terminology because it is far from reaching the completion observed in other instances of continuous recrystallization.

Microstructure of ZrO2 Thermal Barrier Coatings Applied by EB-PVD

Abstract: The electron beam physical vapor deposition process (EB-PVD) provides distinctive coatings of a unique columnar microstructure for gas turbine components. The main advantage of this microstructure is its superior tolerance against stresses, erosion and thermoshock, thus giving it a major edge in lifetime. This paper describes recent investigations of microstructure and texture of yttria-partially stabilized zirconia (Y-PSZ) thermal barrier coatings. Close to the substrate a thin zone of varying thickness consisting of equiaxed ZrO2 grains is observed which can be explained by a geometrical model. A fourfold texture is formed on rotated substrates via evolutionary selection of the fastest growth directions. Rotation causes a layered structure of bent columns. Texture and morphology of the coatings are drastically changed under stationary deposition conditions.

A comparative characterization of near-equiaxed microstructures as produced by spray casting, magnetohydrodynamic casting and the stress induced, melt activated process

Abstract: The near-equiaxed microstructure of wrought and cast aluminum alloys as produced by the most common methods used to provide material for subsequent semisolid processing, is examined. More specifically, the grain size and degree of spheroidization of alloys produced by spray casting, magnetohydrodynamic (MHD) casting and the stress induced, melt activated (SIMA) process are characterized and compared. It is shown that the microstructure of alloys with the same composition differs significantly when produced by the three methods, showing the influence of the production method. Spray casting and the SIMA process result in a microstructure with perfectly equiaxed grains that is inherently suitable for semisolid processing, while MHD-cast alloys exhibit a non-uniform initial microstructure, with dendritic features dominant at the perimeter of the casting. Finally, the mechanisms responsible for the formation of near-equiaxed microstructure for each method are investigated.

XRD microstructural analysis of mullites obtained from kaolinite–alumina mixtures

Abstract: A microstructural study of mullite obtained by the reaction sintering of kaolinite–α–alumina mixtures in the range 1150–1700°C has been performed by using X-ray line profile analyses together with scanning and transmission electron microscopy equipped with microanalysis by energy dispersion (SEM-EDS, TEM-AEM). Two kinds of morphology corresponding to primary (elongated grains) and secondary (equiaxed grains) mullite have been observed. A bimodal crystallite size distribution has been detected through XRD microstructural analysis from 1300°C. The results obtained by this method are compared with SEM/TEM data.

Particle-stimulated nucleation of recrystallization for grain-size control and superplasticity in an Al-Mg-Si-Cu alloy

Abstract: Grain refinement of 6xxx aluminum alloys for superplasticity through particle-stimulated nucleation of recrystallization (PSN) has been a difficult task in the past due to the inhomogeneous nature of the precipitate distributions produced by traditional overaging heat treatment methods. Stretching prior to aging does not alleviate the problem. A new approach has been developed, wherein the interfaces of deformation bands caused by severe rolling were exploited as heterogeneous nucleation sites for precipitates in an Al–Mg–Si–Cu alloy. (US and International patents are pending.) This approach, combined with a two-step low–high heat treatment resulted in a homogeneous distribution of globular precipitates near 1 μm in diameter. In contrast to the globular shape, the plate-shape morphology was observed in the absence of pre-age deformation. Subsequent rolling and recrystallization resulted in a fine, uniform, equiaxed grain structure with an average grain diameter of 10 μm. The grain structure was weakly textured, statically stable, and superplastic above 500°C. A maximum strain rate sensitivity of 0.5 was achieved, with a corresponding maximum elongation of 375%.

Effects of the intensity and frequency of electromagnetic vibrations on the microstructural refinement of hypoeutectic Al-Si alloys

Abstract: An experimental apparatus that uses a superconducting magnet and enables the simultaneous application of an alternating electric field with a frequency of up to 50 kHz and a magnetic field of up to 10 T was designed and assembled. Electromagnetic vibrations were induced in Al-7 wt pct Si alloy during solidification by simultaneous application of the two fields. The thorough investigation, which was carried out over wide ranges of intensity (an electromagnetic pressure range of 0 to 2.25×105 Pa) and frequency (0 to 50 kHz), clarified the effects of the two main parameters on the microstructural refinement brought about by electromagnetic vibrations. Low-intensity vibrations changed the highly columnar dendritic structure into one composed of large, equiaxed dendrites. As the intensity, and consequently, the magnetic pressure were increased, at about 0.93×105 Pa, fine isolated grains started to appear and dominated the structure as the pressure was increased further. At low frequencies, the structure was one with large, equiaxed dendrites, which disintegrated to form a fine and homogeneous structure as the frequency was increased. At about 1.5 kHz, the trend reversed and the structure gradually became a completely columnar dendritic one at frequencies higher than 10 kHz. Metallographic observations showed that the cavitation phenomenon has been a main factor behind the observed microstructural refinement. The effects of mechanical vibrations of the experimental apparatus were also investigated and found to have no contribution to the observed effects.

Texture development of blanket electroplated copper films

Abstract: The transition from sputtered Al to electroplated Cu interconnects for future microelectronic devices has led to an interest in understanding the relationships between the microstructure and texture of Cu that might impact electrical performance, similar to what has been done for Al. Electroplated Cu undergoes a recrystallization at room temperature that is related to the presence of organic and inorganic additives in the plating bath. As plated, the Cu grains are small (approx. 0.1 μm) and equiaxed, but over a period of hours to days, recrystallization results in grains several microns in size. We observe a significant weakening of the strong as-plated (111) texture by x-ray diffraction pole figure measurements and an increase in the level of randomness. We propose that multiple twinning is the leading mechanism for this phenomenon.

Improved microstructure and properties of 6061 aluminum alloy weldments using a double-sided arc welding process

Abstract: Due to its popularity and high crack sensitivity, 6061 aluminum alloy was selected as a test material for the newly developed double-sided arc welding (DSAW) process. The microstructure, crack sensitivity, and porosity of DSAW weldments, were studied systematically. The percentage of fine equiaxed grains in the fully penetrated welds is greatly increased. Residual stresses are reduced. Porosity in the welds is reduced and individual pores are smaller. It was also found that the shape and size of porosity is related to solidification substructure. In particular, a weld metal zone with equiaxed grains tends to form small and dispersed porosity, whereas elongated porosity tends to occur in columnar grains.

Experimental Study of the Transition from Constrained to Unconstrained Growth during Directional Solidification

Abstract: Temperature measurements are carried out in 99.99 wt% aluminium and aluminium-silicon alloys. The experimental apparatus was initially built for the study of microporosity formation in aluminium alloys. 1) The construction is designed to obtain upward directional solidification by limiting lateral heat flow during cooling and suppressing fluid flow induced by the pouring sequence. Cooling occurs from the top part of the ingot, leading to the formation of a surface dendrite layer. In the 99.99 wt% aluminium, very few equiaxed grains sink down from the surface dendrite layer into the liquid. The density of the equiaxed dendritic grains is too low to block the columnar cellular front. Cooling curves show that, once superheat disappears (i.e., when no substantial thermal gradient remains in the liquid), the liquid is kept at an almost constant temperature during the growth of the columnar front and a small negative thermal gradient forms in the liquid ahead of the growing columnar front. It is concluded that the liquid is reheated by the growing columnar front. In the case of the aluminium-silicon alloys, a columnar-to-equiaxed transition (CET) is observed at almost two-thirds of the ingot length. The columnar length is found to increase slightly with decreasing the solute content. Recalescence is measured in a fully equiaxed region, while cooling rate, recorded by the thermocouple located just above the CET, remains negative.

Solidification parameters during the columnar-to-equiaxed transition in lead-tin alloys

Abstract: The columnar-to-equiaxed transition (CET) was studied in lead-tin alloys, which were solidified directionally from a chill face. The main parameters analyzed include the temperature gradients, solidification velocities of the liquidus and solidus fronts, and grain size. The transition was observed to occur when the temperature gradient in the melt decreased to values between −0.8 °C/cm and 1 °C/cm. In addition, there is an increase in the velocity of the liquidus front faster than the solidus front, which increases the size of the mushy zone. The size of the equiaxed grains increases with distance from the transition, an observation that was independent of alloy composition. The comparison with available analytical models and the observations indicate that the transition is the result of a competition between coarse columnar dendrites and finer equiaxed dendrites.

High Resolution EBSD Analysis of the Grain Structure in an AA2024 Friction Stir Weld

Abstract: The grain structures formed during friction stir welding of a typical aluminium aerospace alloy have been studied by high resolution EBSD analysis. The grain structures reflect the local deformation conditions, and due to the high temperatures and strains, show characteristics typical of different stages of dynamic recrystallisation. The large strains and high density of second phase particles in the nugget zone result in a very fine 2-3 μm equiaxed grain size. There is an abrupt transition in grain structure between the nugget and the TMAZ at the side of the weld. A very fine grain structure was also observed near to the top surface of the weld that was in contact with the tool shoulder. The nugget zone, onion ring structure was attributed to bands of different densities of second phase particles, rather than any significant difference in the local grain structure or texture.

Fabrication and bonding of copper sputter targets

Abstract: The method is used to fabricate pure copper sputter targets It includes first heating a copper billet to a temperature of at least 500° C The copper billet has a purity of at least 9999 percent Then warm working the copper billet applies at least 40 percent strain Cold rolling the warm worked copper billet then applies at least 40 percent strain and forms a copper plate Finally, annealing the copper plate at a temperature above about 250° C forms a target blank The target blank has equiaxed grains having an average grain size of less than 40 μm The grains of the target blank have (111), (200), (220) and (311) orientations with the amount of the grains having each of the orientations being less than 50 percent

Creep expansion of porous Ti-6Al-4V sandwich structures

Abstract: Low-density titanium alloy sandwich structures consisting of a porous core and fully dense face sheets can be produced by consolidating argon gas charged powder compacts followed by not rolling and annealing to expand the gas-filled pores. Little is known about the rate of pore expansion, its dependence upon temperature, and the morphological evolution of the pore shape during expansion. In situ eddy current and laser ultrasonic sensors have been combined with metallographic and texture measurements to measure the relative density, the elastic moduli, and the microstructural evolution of Ti-6Al-4V sandwich panels during the annealing stage of low-density core (LDC) processing. The eddy current data indicated that expansion began during, the heating phase, reached a maximum expansion rate (Δ) of 2 × 10−5 s−1 at approximately 685 °C, and had almost ceased (Δ < 1 × 10−6 s−1) after annealing for 4 hours at 920 °C. The elastic moduli were found to decrease with increasing temperature and volume fraction of porosity. The initial (as-rolled) microstructure consisted of a lamellar α + β microstructure with an α-phase lath thickness of 2.0 µm and contained a distribution of oblate-shaped pores with aspect ratios of up to 10. During the expansion process, it recrystallized into an equiaxed α + β structure with an α-phase grain diameter of 7.5 µm with spheroidal pores with aspect ratios of up to 3. The combination of the two sensors was found to enable the in situ determination of both the porous cores relative density and its elastic properties. These are the two material indices that govern the elastic response of a sandwich structure.

Substructural changes during hot deformation of an Fe-26Cr ferritic stainless steel

Abstract: Dynamic softening and substructural changes during hot deformation of a ferritic Fe-26Cr stainless steel were studied. The flow stress increased to reach a steady state in all the cases and the steady-state stress decreased with decreasing Z, the Zener-Hollomon parameter. A constant subgrain size was observed to correspond to the steady-state flow and the steady-state subgrain size increased with decreasing Z. Substructure examinations revealed that elongated, pancake-shaped subgrains formed in the early stage of deformation. Straight sub-boundaries and equiaxed subgrains developed progressively with strain, leading eventually to a stable substructure at strains greater than 0.7. During deformation at 1100 °C, dynamic recrystallization occurred by the migration and coalescence of subboundaries. Dynamic recovery dominated during deformation at 900 °C, resulting in the formation of fine equiaxed subgrains. Based on microstructural observations, the process of substructural changes during hot deformation was described by a schematic diagram.

Effect of preform microstructure on the hot working mechanisms in ELI grade Ti–6Al–4V: transformed β v. equiaxed (α + β)

Abstract: Processing of Ti–6Al–4V includes hot working above and below the β transus and the various stages of manufacture involve preforms with either transformed β (Widmanstatten colony type) or equiaxed (α + β ) microstructure. For achieving defect free products during manufacture, it is important to understand the response of these two preform microstructures to the imposed hot working conditions. In this paper, the influence of the preform microstructure on the hot working mechanisms of extra low interstitial (ELI) grade Ti–6Al–4V has been studied with the help of hot compression experiments conducted in the temperature range 750–1100°C and strain rate range 0·001–100 s-1. The data have been analysed using the standard kinetic approach as well as the more recent approach of processing maps. In the α–β range, the stress–strain behaviour of transformed β preform is marked by a higher flow stress and a continuous flow softening while the equiaxed (α + β ) preform exhibits steady state flow at lower strain...

Structural and dimensional control in micromachined integrated solid state gas sensors

Abstract: The application of electron microscopy for quality control in the fabrication of solid state integrated tin oxide gas sensors is described. Among the parameters that influence the performance and mechanical stability of these devices are the thickness, grain size, porosity and point defect (oxygen vacancy) distribution of the active SnO2 layer and the intrinsic stresses and interfacial adhesion in the support layers. The sputtered SnO2 films were highly crystalline with a columnar growth structure and equiaxed grains of mean diameter ∼10 nm. HREM studies revealed evidence of CSP defects, which act as traps for free carriers and can therefore be expected to influence the conductivity of the oxide layer. Increasing the thickness of the film from 300 to 600 nm lead to an increase in the sensitivity to low concentrations of NO2 (∼1 ppm) by up to a factor of two. Sensitivity is also affected by surface roughness and film porosity, which increase the effective area on which gas molecules can be adsorbed.

Observation and modeling of dynamic recrystallization in high-strain, high-strain rate deformation of metals

Abstract: The microstructural evolution inside shear bands was investigated experimentally and analytically. A fine recrystallized structure (grains with 0.05-0.3 mu m) is observed in Ti, Cu, 304 stainless steel, Al-Li, and Ta, and it is becoming clear that a recrystallization mechanism is operating. The fast deformation and short cooling times inhibit grain-boundary migration; it is shown that the time is not sufficient for migrational recrystallization. A rotational mechanism is proposed and presented in terms of dislocation energetics. This mechanism necessitates the stages of high dislocation generation and their organization into elongated cells. Upon continued deformation. the cells become sub-grains with significant misorientations. These elongated sub-grains break up into equiaxed grains with size of approximately 0.05-0.3 mu m. It is shown that grain-boundary reorientation can operate within the time of the deformation process.

Corrosion behavior of Ti-6Al-4V alloy welded by scanning electron beam

Abstract: The corrosion behavior of Ti-6Al-4V alloy welded by SEE (scanning electron beam) is described in this paper. Solidification macrostructure of weld was transformed from 650-mu m columnar crystals to 100-mu m equiaxed crystals by SEE. Anodic polarization behavior of the weld was different following the change in solidification macrostructure. Anodic polarization curve of 150-mu m equiaxed-grain weld coincided with that of base metal, which indicated a best state of equal corrosion tendency between the base and the weld metal. Alloy element homogeneity in the equiaxed solidification structures made the weld more corrosion resistant than the columnar solidification structures. Fine microstructure and a small quantity of dislocations have a good effect on uniform corrosion resistance of the weld, but coarse microstructure, a spinodal decomposition of the metastable beta-phase and a large quantity of dislocations were related to the inferior resistance either to uniform corrosion or to local corrosion. (C) 2000 Elsevier Science S.A. All rights reserved.

Measurement of grain size and size distribution in engineering materials

Abstract: Methods to characterise grain size distribution in engineering materials are evaluated, and compared with measurements on two representative materials, an equiaxed aluminium alloy and a heterogeneo...

Effect of β-Si3N4 starting powder size on elongated grain growth in β-Si3N4 ceramics

Abstract: The microstructural evolution of pressureless sintered silicon nitride ceramics prepared from different particle sizes of β-Si 3 N 4 as starting powders, has been investigated. When the specimen prepared from as-received β-powder of 0.66 μm in average size, was sintered at 1850°C, equiaxed β-Si 3 N 4 grains were observed. As the size of the initial β-powder went down to 0.26 μm, however, the growth of elongated grains was enhanced, which resulted in a whisker-like microstructure similar to that made from α-starting powder. When the sintering temperature was increased to 2000°C, the elongated grains were also developed even in the specimen made from 0.66 μm β-powder. The observed results were discussed with relation to the two dimensional nucleation and growth theory for faceted crystals. In addition, fracture toughness of the specimen consisting of elongated grains, which was prepared from finer powders, increased.

The effect of microstructural banding on failure initiation of HY-100 steel

Abstract: Microstructural banding of a hot-rolled HY-100 steel plate was accentuated by cooling slowly from the austenite region, which resulted in alternating layers of soft, equiaxed ferrite, and hard “granular ferrite.” The segregation of substitutional alloying elements such as Ni and Cr was identified as the main cause for the microstructural banding. Such banding induces anisotropic flow behavior at large strains, with deformation constrained by “pancake-shaped” bands of the hard granular ferrite. Tensile tests of circumferentially notched HY-100 specimens were performed in order to explore the stress dependence of failure in the slow-cooled as well as the quenched and tempered conditions. The failure behavior of the slow-cooled, microstructurally banded material exhibited a pronounced susceptibility to a void-sheet mode of failure. However, the absence of carbides within the equiaxed ferrite delays void coalescence and material failure to higher strains than in a quenched and tempered microstructure, despite the increased susceptibility to shear localization.

Microstructure and Hall-Petch behavior of Fe-Co-based Hiperco© alloys

Abstract: The microstructure and hardness of Fe–Co-based Hiperco© alloys were investigated. Scanning electron microscopy revealed elongated grains in the as-received (cast and cold-rolled) alloys, and samples containing Nb had second phase precipitates. Annealing of alloys for 1 to 3 h at temperatures in the range 700 to 800 °C resulted in grains becoming equiaxed. In the Nb-containing alloys, the original precipitates dissolved and new precipitates appeared. The rate of grain growth decreased with increasing Nb content, suggesting that Nb or Nb-containing precipitates were responsible for a reduction in the grain boundary mobility. The hardness as a function of grain size in the annealed samples of all of the alloys could be plotted on the same Hall–Petch curve, indicating that the yield strength of these alloys is governed by the grain size, independent of the alloy composition and volume fraction of precipitates.

An investigation of the effect of fatigue deformation on the residual mechanical properties of Ti-6Al-4V ELI

Abstract: Tensile properties, hardness, and Charpy impact toughness of Ti-6Al-4V extralow interstitial (ELI) with equiaxed α and Widmanstatten α structures at various stages of fatigue were investigated. Fatigue crack initiation characteristics of the same alloy were also investigated in this study. In the equiaxed α structure, fatigue cracks initiated mainly at the interface between primary-α grains, while in the Widmanstatten α structure, they initiated across α plates at an angle of around 45 deg to the stress axis. Specimens with the Widmanstatten α structure fractured before adequate fatigue hardening was achieved because a multitude of microcracks readily formed. Specimens with the equiaxed α structure fractured after adequate fatigue hardening developed. Tensile strength, 0.2 pct proof stress, and hardness increased clearly with increasing stress cycles and fatigue steps, particulary in the low-cycle fatigue (LCF) region, while impact toughness and elongation showed a reverse trend. It is suggested, therefore, that the dislocation density multiplies more rapidly near the specimen surface during the early stages of fatigue, while during the later stages of fatigue, dislocation density increases near the center of the specimen. Also, the dislocation multiplication will continue until saturation of the entire specimen has occurred.

A method for studying weld fusion boundary microstructure evolution in aluminum alloys

Abstract: Aluminum alloys may exhibit a variety of microstructures within the fusion zone adjacent to the fusion boundary. Under conventional weld solidification conditions, epitaxial nucleation occurs off grains in the heat-affected zone (HAZ) and solidification proceeds along preferred growth directions. In some aluminum alloys, such as those containing Li and Zr, a nondendritic equiaxed grain zone (EQZ) has been observed along the fusion boundary that does not nucleate epitaxially from the HAZ substrate. The EQZ has been the subject of considerable study because of its susceptibility to cracking during initial fabrication and repair. The motivation of this investigation was to develop a technique that would allow the nature and evolution of the fusion boundary to be studied under controlled thermal conditions. A melting technique was developed to simulate the fusion boundary of aluminum alloys using the Gleeble{reg{underscore}sign} thermal simulator. Using a steel sleeve to contain the aluminum, samples wee heated to incremental temperatures above the solidus temperature of a number of alloys. In Alloy 2195, a 4Cu-1Li alloy, an EQZ could be formed by heating in the temperature range approximately from 630--640 C. At temperatures above 640 C, solidification occurred by the normal epitaxial nucleation and growth mechanism. Fusion boundary behaviormore » was also studied in Alloys 5454-H34, 6061-T6 and 2219-T8. Nucleation in these alloys was observed to be epitaxial. Details of the technique and its effectiveness for performing controlled melting experiments at incremental temperatures above the solidus are described.« less