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Showing papers on "Equiaxed crystals published in 1998"


Journal ArticleDOI
TL;DR: In this article, the influence of hot working variables and microstructure on failure via fracture or flow-localization controlled processes is summarized for both conventional and isothermal hot working processes.
Abstract: The hot workability of conventional titanium alloys and titanium aluminides is reviewed. For both alloy classes, the influence of hot working variables and microstructure on failure via fracture or flow-localization controlled processes is summarized. The occurrence of wedge cracking and cavitation during bulk forming of α / β alloys with Widmanstatten microstructures or γ titanium aluminides with lamellar or equiaxed structures, is examined. In particular, the effects of grain size, grain boundary second phases and process variables on failure are presented. Observations and models of flow localization and cavitation processes which lead to failure during low strain rate, superplastic, tensile-type deformation of titanium and titanium aluminide alloys with fine equiaxed structures, are also described. In the area of flow-localization-controlled failure during bulk forming, the occurrence of shear bands and other flow nonuniformities during both conventional and isothermal hot working processes is reviewed. The influence of material properties, such as flow softening rate and strain rate sensitivity and process variables, which lead to temperature and hence flow nonuniformities, is examined. The flow localization concepts are illustrated for several hot working processes.

222 citations


Journal ArticleDOI
TL;DR: In this article, a microdendritic (second-phase), equiaxed, cell structure was friction-stir welded with a compositionally modified 1100 aluminum alloy.

205 citations


Journal ArticleDOI
TL;DR: In this article, the tensile and creep properties of orthorhombic alloys in the Ti-Al-Nb system were reviewed with emphasis on tensile properties of ternary alloys.

138 citations


Journal ArticleDOI
TL;DR: In this paper, the crystallization of an industrial mold slag was quantified using a single hot thermocouple technique which, when combined with a video camera based observation system, allowed observation of the onset and growth of the crystals which were precipitated from the melt.
Abstract: The conditions under which crystallization develops in a mold slag must be understood in order to select or design a mold flux for use in the continuous casting of steels. In this paper, the crystallization of an industrial mold slag was quantified using a single hot thermocouple technique which, when combined with a video camera based observation system, allowed observation of the onset and growth of the crystals which were precipitated from the melt. The beginning of crystallization was determined by direct observation and the growth rate of crystals were measured by frame by frame image analysis of recordings of the progress of crystallization. Isothermal experiments were performed at different temperatures and a Time-Temperature-Transformation (TTT) diagram was determined for this industrial mold slag. X-ray diffraction of quenched samples was used to determine the type of crystalline phases that were precipitated. The TTT diagram was divided into two separate regions which corresponded to the precipitation of dicalcium silicate (Ca 2 SiO 4 ) at temperatures over 1 050 °C and of Cuspidine (Ca 4 Si 2 O 7 F 2 ) at temperatures below 1 050°C. The evolution crystal fraction was described by Avrami's equation. This work indicates that industrial mold slags are easily undercooled, that crystallization occurs throughout the melt, that crystals grow initially as equiaxed dendrites and that the onset of crystallization is a function of cooling rate and must be described by either TTT or CCT curves.

126 citations


Journal ArticleDOI
TL;DR: In this paper, a clear relationship between the microstructure and grain orientation has not been established in polycrystalline copper, and this has been the aim of the present study.

125 citations


Journal ArticleDOI
TL;DR: In this article, a wrought aluminium alloy was cast from its liquidus temperature to obtain a fine structure of equiaxed, non-endritic primary grains, which were characterized by image analysis which showed that the average grain size in two dimensions was ∼44 μm and the average circularity factor was ∼18.
Abstract: A wrought aluminium alloy 2618 was cast from its liquidus temperature to obtain a fine structure of equiaxed, nondendritic primary grains. In contrast, the structures obtained from casting the alloy from above its liquidus temperature displayed the usual dendritic grains. The primary grains were characterized by image analysis which showed that the average grain size in two dimensions was ∼44 μm and the average circularity factor was ∼18, indicating that the grains were both fine and globular. Some particles were also viewed in three dimensions and this showed mostly round and smooth surfaces although some protrusions could be seen to emerge on some particles. The misorientation angles between adjacent particles were measured by the electron back-scattered diffraction (EBSD) and most were >15°, indicating genuinely separate grains. The grains in the as-cast material remained fine for up to 30 min at 610°C during reheating, although particle coalescence led to eventual coarsening. The liquidus cast billets were thixoformed at 610°C and the microstructures after forming were examined. It is concluded that the liquidus casting is capable of producing thixotropic materials without resorting to stirring, grain refining or severe deformation and reheating.

100 citations


Journal ArticleDOI
TL;DR: In this article, the effects of current pulsation frequency on weld bead microstructure, hardness, and tensile properties in AA8090 type aluminium-lithium alloy sheets were studied.
Abstract: Effects of current pulsation frequency on weld bead microstructure, hardness, and tensile properties in AA8090 type aluminium-lithium alloy sheets were studied. It was observed that the structure in the as solidified weld was predominantly columnar in the case of the conventional (i.e. continuous current) gas tungsten arc welding process. The grain structure became finer and more equiaxed with the introduction of current pulsation. Moreover, there was an optimum frequency range over which the grain refinement was a maximum. The same optimum frequency range corresponded with maxima in hardness, ultimate tensile strength, and percentage elongation. Tensile strength increased, in general, after solution treatment and aging (STA). The best combination of tensile properties was achieved for welds deposited under a 6 Hz pulse frequency in the STA condition.

90 citations


Journal ArticleDOI
TL;DR: In this article, a growth model for nanocrystalline AlN is presented, in which the equiaxed AlN nanoparticles precipitate at or near the surface of coalesced liquid Al droplets, whereas the AlN nanowhis...
Abstract: Aluminum and N2 (1 atm) react at 900−1100 °C to give AlN. The nitridation reactions of four types of starting Al powdernanocrystalline Al of two different origins, and 2-μm and 20-μm Al powders from commercial sourcesare described. All four Al powders undergo nitridation to give nanocrystalline AlN; however, only the nanocrystalline Al powders undergo nitridation completely at the low temperatures employed. The nanocrystalline AlN is formed in two principal particle morphologies, equiaxed nanoparticles and nanowhiskers, in relative amounts that depend on reaction conditions and the starting Al powders. Added AlCl3 is an effective promoter of nanowhisker growth. Either AlN morphology may be produced with a high selectivity; samples containing predominately equiaxed nanoparticles or predominately nanowhiskers are readily obtained. The results are consistent with a growth model in which the equiaxed AlN nanoparticles precipitate at or near the surface of coalesced liquid Al droplets, whereas the AlN nanowhis...

83 citations


Journal ArticleDOI
TL;DR: In this paper, a thermomechanical process was developed to produce ultrafine (1 μm) equiaxed ferrite grains in hot-rolled steel strip, which is applicable to a wide range of steel chemistries, including low and high carbon and microalloyed steels.
Abstract: A new thermomechanical process has been developed to produce ultrafine (1 μm) equiaxed ferrite grains in hot rolled steel strip. This process is remarkably simple and is applicable to a wide range of steel chemistries, including low and high carbon and microalloyed steels. Strips are reheated to produce a coarse austenite grain size, then rolled in a single pass at or just above the austenite to ferrite transformation temperature. It is suggested that the observed refinement is due to strain induced transformation from austenite to ferrite. The requirements for this appear to be high strain induced by shear in the strip surface layers, and thermal gradients created by heavy quenching of the strip surface by the work rolls. The yield strength was markedly higher than conventionally processed strip, although there was little work hardening even though total elongation of over 20% was achieved.

66 citations


Journal Article
TL;DR: On heating, individual plate-like pseudohexagonal particles of kaolinite undergo dehydroxylation forming metakaolin at about 550°C. At higher temperatures (1200°C) cristobalite and elongated mullite crystals nucleate, and eventually at 1300°C the particles lose their face-on pseudo-hexagonal habit and ball up as mentioned in this paper.
Abstract: On heating, individual platelike pseudohexagonal particles of kaolinite undergo dehydroxylation forming metakaolin at about 550°C. Between 900 and 1000°C, nucleation of nanometre scale γ-Al 2 O 3 spinel crystals occurs while the original hexagonal particle morphology is retained. At higher temperatures (1200°C) cristobalite and elongated mullite crystals nucleate, and eventually at 1300°C the particles lose their face-on pseudohexagonal habit and ball up. Dense kaolinite, clay derived bodies show similar phase and morphology evolution : after 3 h at 900°C, elongated edge-on, and more equiaxed face-on, particles of metakaolin are observed. These closely packed, platelike, amorphous particles retain the individual kaolinite particle morphology. Light and dark contrast in dense bodies appears to arise from phase separation into alumina and silica rich regions. Between 900 and 1000°C nanometre crystals of γ-Al 2 O 3 spinel nucleate within these regions and, after 3 h at 1200°C, randomly orientated mullite and cristobalite crystals nucleate from the amorphous phase.

65 citations


Journal ArticleDOI
TL;DR: In a given casting, the mechanism of grain nucleation affects the subsequent development of the grain structure as discussed by the authors, and the mechanism for the formation of equiaxed grains in the absence of grain refining additions is unclear.
Abstract: In a given casting, the mechanism of grain nucleation affects the subsequent development of the grain structure. Despite being well documented in the literature, the mechanism for the formation of equiaxed grains in the absence of grain refining additions is unclear. Controversy still remains regarding the various theories of equiaxed grain formation, none of which have been thoroughly verified. In this paper, previous theoretical and experimental work is critically reviewed. It is concluded that all of the proposed mechanisms can operate depending on the alloy composition, casting conditions and the types of nucleating substrates present in the melt.

Journal ArticleDOI
TL;DR: In this article, the macroscopic and microscopic subdivisions are discussed in terms of the local deformation modes and slip amplitudes and their relation to the behavior of rolled A1 crystals.
Abstract: Pure aluminum crystals of cube orientation have been deformed in plane-strain compression to strains of unity using a channel die. The macrostructures and microstructures were characterized in three dimensions by a range of metallographic techniques including optical, scanning, and transmission electron microscopy. Particular attention was paid to quantifying global textures and local variations in crystal orientation by means of X-ray pole figures, automatic electron back-scattered diffraction (EBSD) and semiautomatic transmission electron microscope (TEM) Kikuchi line analysis. Cube crystals are observed to break up into macroscopic deformation bands aligned along the elongation direction and strongly disorientated by rotations mostly, but not uniquely, about the transverse direction. The bands develop deformation substructures of dislocation boundaries or, in certain cases, of intersecting dislocation boundaries which have characteristic microtexture signatures of alternating lattice rotations. The transition regions between the bands are composed of equiaxed dislocation cells which accommodate continuous orientation gradients over distances of about 20 µm. Compared to the behavior of rolled A1 crystals, the macroscopic bands are observed to lie in different planes, but the microscopic subdivisions and microtextures developed in the channel die and in rolling are very similar. The origins of the macroscopic and microscopic subdivisions are discussed in terms of the local deformation modes and slip amplitudes and their relation to the behavior of rolled crystals.

Journal ArticleDOI
TL;DR: In this article, the authors compared theoretical works concerning the effect of convection on the growth of isolated dendrites with experiments on NH 4 Cl settling equiaxed crystals and found that no major discrepancy has been found to prevent the application of the theories in question to moving equiaaxed crystals.

Journal ArticleDOI
TL;DR: In this article, the authors examined the creep strength of Nb3Al-base alloy with two types of different microstructures, equiaxed grains and directionally elongated grains.

Journal Article
TL;DR: In this article, a unified heterogeneous nucleation mechanism is proposed to describe equiaxed zone formation in Al-Cu-Li welds, which is shown to form preferentially at the fusion boundary of arc welds and may have important implications with respect to both weld cracking susceptibility and structural integrity.
Abstract: The effects of welding conditions, composition and solidification substrate have been systematically studied in an effort to determine the nature of nondendritic equiaxed grain formation in Al-Cu-Li welds. The equiaxed zone (EQZ) in these alloys forms preferentially at the fusion boundary of arc welds and may have important implications with respect to both weld cracking susceptibility and structural integrity. Over a wide range of weld heat input, achieved by varying weld travel speed and current during autogenous gas tungsten arc welding, it was not possible to eliminate this zone, although its width and the grain size varied considerably. The solidification substrate had a profound effect on EQZ formation. The EQZ did not form when solidification occurred from an as-cast weld metal substrate. Under other substrate conditions, including the as-cast/solution heat treated condition, an EQZ was always present. Both lithium and zirconium influence EQZ formation, with zirconium exhibiting a more dominant effect. At low levels of lithium and zirconium (0.5 Li, 0.03 Zr), an EQZ was not observed. A unified heterogeneous nucleation mechanism is proposed to describe EQZ formation in Al-Cu-Li welds.

Journal ArticleDOI
TL;DR: In this article, the microstructure, crystallographic texture and grain boundary distributions were studied in as received and cold-rolled samples of ultrafine-grained (UFG) copper produced by severe plastic deformation.
Abstract: Ultrafine-grained (UFG) copper produced by severe plastic deformation demonstrates an unusual combination of high strength and rather high ductility at room temperature. To obtain the UFG microstructure, the equal channel angular extrusion technique was used in the current work. The microstructure, crystallographic texture and grain boundary distributions were studied in as received and cold-rolled samples. The microstructures and textures generated by rolling of the UFG sample were compared with those formed in identically rolled conventional coarse-grained copper. Rolling to 83° resulted in drastic changes of all microstructural parameters owing to the transformation of the microstructure with initially equiaxed grains into a banded microstructure with mainly elongated and subdivided grains having orientations typical of a rolling texture. Additional rolling led to the strengthening of the main texture components. The emergence of cube oriented {001}(100) grains was distinguished in the rolled ...

Journal ArticleDOI
TL;DR: In this article, a method is described for calculating changes in the surface area and edge per unit volume as a function of common deformations encountered in the production of steels, where each grain in the undeformed material is represented by a tetrakaidecahedron, a shape which is a realistic representation of equiaxed grains.
Abstract: The deformation of a polycrystalline material leads to changes in the amount of grain surface and grain edge per unit volume. These parameters are of importance in kinetic theory since both surfaces and edges are heterogeneous nucleation sites. In the present study a method is described for calculating changes in the surface area and edge per unit volume as afunction of common deformations encountered in the production of steels. Unlike previous analyses, each grain in the undeformed material is represented by a tetrakaidecahedron, a shape which is a realistic representation of equiaxed grains. There are some interesting results which are compared with previous work.

Journal ArticleDOI
TL;DR: In this article, a process based on liquid phase sintering and subsequent annealing for grain growth is presented to obtain the in situ enhancement of toughness of SiC-30 wt, 50 wt%, and 70 wt% TiB2 composites.
Abstract: A process based on liquid phase sintering and subsequent annealing for grain growth is presented to obtain the in situ enhancement of toughness of SiC–30 wt%, 50 wt%, and 70 wt% TiB2 composites. Its microstructures consist of uniformly distributed elongated α-SiC grains, relatively equiaxed TiB2 grains, and yttrium aluminium garnet (YAG) as a grain boundary phase. The composites were fabricated from β-SiC and TiB2 powders with the liquid forming additives of Al2O3 and Y2O3 by hot-pressing at 1850°C and subsequent annealing at 1950°C. The annealing led to the in situ growth of elongated α-SiC grains, due to the β→α phase transformation of SiC, and the coarsening of TiB2 grains. The fracture toughness of the SiC–50 wt% TiB2 composites after 6 h annealing was 7.3 MPa m1/2, approximately 60% higher than that of as-hot-pressed composites (4.5 MPa m1/2). Bridging and crack deflection by the elongated α-SiC grains and coarse TiB2 grains appear to account for the increased toughness of the composites.

Journal ArticleDOI
TL;DR: In this paper, the effect of phase morphology on fatigue crack growth (FCG) resistance has been investigated in the case of an α-β titanium alloy, where the microstructures comprising metastable β matrix are observed to yield higher FCG resistance than those for transformed β matrix, irrespective of primary α phase morphology (equiaxed or elongated).
Abstract: Effect of phase morphology on fatigue crack growth (FCG) resistance has been investigated in the case of an α-β titanium alloy. Fatigue crack growth tests with on-line crack closure measurements are performed in the microstructures varying in primary α (elongated/equiaxed/Widmanstatten) and matrix β (transformed/metastable) phase morphologies. The microstructures comprising metastable β matrix are observed to yield higher FCG resistance than those for transformed β matrix, irrespective of primary α phase morphology (equiaxed or elongated). But, the effect of primary α phase morphology is dictated by the type of β phase (transformed or metastable) matrix. It is observed that in the microstructures with metastable β matrix, the equiaxed primary α as second phase possesses higher FCG resistance as compared to that of elongated α morphology. The trend is reversed if the metastable β matrix is replaced by transformed β phase. The fatigue crack path profiles are observed to be highly faceted. The detailed fractographic investigations revealed that tortuosity is introduced as a result of cleavage in α or β or in both the phases, depending upon the microstructure. The crack closure concept has been invoked to rationalize the phase morphology effects on fatigue crack growth behavior. The roughness-induced and plasticity-induced crack closure appear to be the main mechanisms governing crack growth behavior in α-β titanium alloy.

Journal ArticleDOI
TL;DR: In this article, a fully-lamellar γ-titanium aluminide alloy containing carbon and silicon additions has been found to have superior creep resistance and the aging characteristics of this alloy, which also contains Nb, Cr, and W, have been studied by means of transmission electron microscopy techniques and are correlated to the post-crept microstructures.

Journal ArticleDOI
TL;DR: In this paper, roughness-induced crack closure in the alloy Ti-2.5 wt% Cu at low Δ K (d a /d N −8 m cycle −1 ) was examined.
Abstract: Fatigue crack closure and fracture surfaces in the alloy Ti-2.5 wt% Cu at low Δ K (d a /d N −8 m cycle −1 ) were examined. For a solution-annealed equiaxed microstructure as well as for a fine and a coarse lamellar microstructure, roughness-induced crack closure occurs due to serrated crack path, which influences the crack growth rates significantly. The fracture surface roughness is found to be a measure of roughness-induced crack closure in lamellar microstructures where planar slip prevails. The height distribution and tilt angle distribution of the crack path facets are the decisive factors for roughness-induced crack closure. In the solution-annealed equiaxed microstructure, ductile fracture areas by the side of crystallographic facets can act as obstacles to crack tip sliding along planar slip planes, causing less mode II displacement and thus resulting in a much lower level of crack closure.

Patent
24 Jul 1998
TL;DR: In this paper, a brake rotor manufactured from (α+β) titanium alloy will fulfill the thermal stress requirements when an equiaxed grain structure is imposed on the alloy.
Abstract: Modern brake rotors require enhanced resistance to thermal stress in order to withstand vigorous operating conditions. A brake rotor manufactured from (α+β) titanium alloy will fulfill the thermal stress requirements when an equiaxed grain structure is imposed on the alloy. The equiaxed grains can preferably range from 300 μm to 3 mm in size. The equiaxed grain structure is attained by heat treating the brake rotor at the β phase transformation temperature, followed by quenching. When Ti-6Al-4V titanium alloy is used to form the brake rotor, β phase transformation temperature is 1000° C. the heat treatment temperature range is 986-1200° C. The preferable heat treatment for Ti-6Al-4V alloy is 1050° C. for 2 hours.

Journal ArticleDOI
TL;DR: The n-type 0.1 wt% SbI3-doped Bi2Te2.15 compounds were fabricated by hot extrusion in the temperature range 300-510°C under an extrusion ratio of 20:1.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the solidification behavior of Al-Cu-SiCp composites under multidirectional solidification conditions and found that an increase in particle volume fraction and/or increase in cooling rate reduced the extent of macrosegregation of reinforcements.
Abstract: The present investigation is aimed at understanding the solidification behaviour of Al-Cu-SiCp composites under multidirectional solidification conditions. Macro- and microstructures of composites solidified using small and large size moulds have been studied. It was observed that an increase in particle volume fraction and/or an increase in cooling rate reduced the extent of macrosegregation of reinforcements in the composites. In the case of composites solidified in large size permanent moulds, the differential cooling rate along the transverse direction led to the formation of a conical-shaped particle-depleted zone in the bottom part of the casting. Columnar grains were observed in the particle-depleted zone, while the particle-containing region exhibited equiaxed grain morphology. SiC particles did not facilitate heterogeneous nucleation of matrix phases. In the absence of convection, SiC particles promoted grain refinement in the matrix. However, when there was a significant amount of convection, the presence of SiC particles led to a coarser grain structure. Matrix dendrite arm spacing (DAS) was not significantly altered by the presence of SiC particles.

Patent
07 Oct 1998
TL;DR: In this paper, the authors proposed an Al alloy material for high speed motion parts, which is composed of an aluminum alloy having a composition consisting of 1.5-7.0% Cu, 0.01-2.0%, and the balance aluminum with inevitable impurities.
Abstract: PROBLEM TO BE SOLVED: To provide an Al alloy material for high speed motion parts, having high high-temperature characteristics and excellent in machinability. SOLUTION: The Al alloy forging material is composed of an aluminum alloy having a composition consisting of 1.5-7.0% Cu, 0.01-2.0% Mg, and the balance aluminum with inevitable impurities. Further, the microstructure after solution heat treatment has θ'-phases and/or Ω-phases and is composed of equiaxed recrystallized grains 1 of ≤500 μm grain size, and 1000 hr creep rupture strength and high temperature proof stress are regulated to ≥250 N/mm2 and ≥280 N/mm2, respectively. COPYRIGHT: (C)2000,JPO

Journal ArticleDOI
TL;DR: In this article, the introduction of larger seeds into fine α-SiC powders accelerated the grain growth of some α -SiC grains and resulted in self-reinforced microstructure consisting of large elongated grains and small equiaxed grains.

Journal ArticleDOI
TL;DR: In this paper, the role of creep deformation twinning in near-gamma duplex TiAl alloys is investigated in terms of their dependence on stress, temperature and strain.

Journal ArticleDOI
TL;DR: In this paper, the appearance of the fatigue fracture surface and crack growth curve have been examined for a Ti-2.5Cu alloy with different micro-structures (two equiaxed and two lamellar microstructures).
Abstract: The appearance of the fatigue fracture surface and crack growth curve have been examined for a Ti-2.5Cu alloy with different microstructures (two equiaxed and two lamellar microstructures), and for TIMETAL 1100 with a lamellar microstructure. With increasing AK, a slope change in the crack growth curve correlates with a transition in the fracture surface appearance (induced by a fracture mode transition); this being found in each microstructure. The microstructure size that controls the fatigue fracture is found to be the grain size for equiaxed microstructures and the lamella width for lamellar microstructures. The transitional behaviour can be interpreted in terms of a monotonic plastic zone size model in microstructures having a coarse microstructure size and in terms of a cyclic plastic zone size model for microstructures having a fine microstructure size.


Journal ArticleDOI
TL;DR: In this article, X-ray texture measurements and Orientation Imaging Microscopy (OIM) were used to study the growth of fine equiaxed and columnar grains.