Showing papers on "Equiaxed crystals published in 2004"

Development of dynamic recrystallization theory

Abstract: Although hot working had been defined as deformation above the recrystallization temperature (determined after cold working), it was only about 1965 that dynamic recrystallization (DRX) was confirmed to be occurring during the deformation; two decades were required to clarify the similarities to, and the differences from, static recrystallization. In classical discontinuous DRX in Cu, Ni, and γ-Fe, successive necklaces of new grains cause work softening; however in steady-state, the nuclei are uniformly distributed as reestablished dislocation structure limits growth. In high recovery metals at high strains, the grain boundary (GB) serrations meet across the elongated thinned grains thus pinching them off into almost equiaxed grains containing a substructure, thus geometric DRX.

Microstructural evolution of a Ti–6Al–4V alloy during β-phase processing: experimental and simulative investigations

Abstract: The microstructural evolution of a Ti–6Al–4V alloy during thermomechanical processing in the β-phase field was investigated using both experimental and modelling methods. The experimental results show that dynamic and/or metadynamic recrystallization occurred when the alloy was processed in the β-phase field. A model embedding fundamental metallurgical principles of dynamic recrystallization (DRX) within the cellular automaton (CA) method was able to simulate quantitatively and topographically the microstructural evolution and the flow stress–strain relationship during the thermomechanical processing. In the simulation, the dislocation density variation and the grain growth kinetics of each dynamically recrystallizing grain (R-grain) was calculated on the physical model of DRX, and the plastic flow curve was calculated directly from the dislocation density variation of the matrix grains and the R-grains. The equiaxed growth of R-grains was simulated using the CA method. The influence of strain rate and temperature on the microstructural evolution and the flow stress during dynamic recrystallization was studied, and the results compared with experiments.

Grain Refinement at the Nanoscale Via Mechanical Twinning and Dislocation Interaction in a Nickel-Based Alloy

Abstract: A nanostructured surface layer was formed on an Inconel 600 plate by subjecting it to surface mechanical attrition treatment at room temperature. Transmission electron microscopy and high-resolution transmission electron microscopy of the treated surface layer were carried out to reveal the underlying grain refinement mechanism. Experimental observations showed that the strain-induced nanocrystallization in the current sample occurred via formation of mechanical microtwins and subsequent interaction of the microtwins with dislocations in the surface layer. The development of high-density dislocation arrays inside the twin-matrix lamellae provides precursors for grain boundaries that subdivide the nanometer-thick lamellae into equiaxed, nanometer-sized grains with random orientations.

Mechanical Properties Related to Microstructural Variation of 6061 Al Alloy Joints by Friction Stir Welding

Abstract: The microstructural change related with the mechanical properties of a friction stir welded 6061 Al alloy has been investigated under various welding conditions. Frictional heat and plastic flow during friction stir welding produced fine and equiaxed grains in the stir zone, macroscopically upset and elongated grains in the thermo-mechanically affected zone caused by dynamic recovery and recrystallization. The heat-affected zone, characterized by coarse precipitates, was formed beside the weld zone. Hardness distribution near the weld zone was strongly related to the behavior of precipitates and dislocation density. Especially, hardness of the SZ at a higher tool rotation speed was higher than that of a lower tool rotation speed due to higher density of spherical shaped re-precipitates. The joint strength was approximately 200 MPa which was lower than that of the base metal, 270 MPa, because softening region was formed around the weld zone.

A fixed grid front-tracking model of the growth of a columnar front and an equiaxed grain during solidification of an alloy

Abstract: In a new model of alloy solidification in a square mold, the interface being followed by a front-tracking technique is representative of a curve joining the tips of growing solid dendrites. The coupled heat equation is solved via an Eulerian control-volume formulation. In the absence of convection, the nucleation and nonequilibrium growth of both a front of columnar grains and a single equiaxed grain have been modeled and animated. This is a major step toward the computationally efficient complete direct numerical simulation of the developing grain structure in a casting process.

Microstructural Development of Calcium alpha‐SiAlON Ceramics with Elongated Grains

Abstract: Silicon nitride (Si3N4) and SiAlONs can be self-toughened through the growth of elongated β-Si3N4/β-SiAlON grains in sintering α-SiAlONs usually retain an equiaxed grain morphology and have a higher hardness but lower toughness than β-SiAlONs The present work has demonstrated that elongated alpha-SiAlON grains can also be developed through pressureless sintering alpha-SiAlONs with high-aspect-ratio grains in the calcium SiAlON system have exhibited significant grain debonding and pull-out effects during fracture, which offers promise for in-situ-toughened α-SiAlON ceramics

Formation of nanostructure in Al produced by a low-energy ball milling at cryogenic temperature

Abstract: Nanocrystalline (nc) Al powders produced by ball milling at a low-energy level and cryogenic temperature (i.e., cryomilling) were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average grain size ( d ) in the 8 h milled Al was 26 nm. The development of the nanostructure appears to be dominated by the total level of the microstrain generated by the deformation process. Two types of distinctive nanostructures were identified: primarily a random dispersion of equiaxed grains with typical diameters of 10–30 nm, and a less frequently observed lamellar structure with a length range of 100–300 nm and an average width close to d . The morphology of the mixed nanostructures in the cryomilled sample shows a large number of high angle grain boundaries (HAGBs) formed by a grain subdivision mechanism. Microstructural evidence for the formation of nanostructure by recrystallization mechanism is also discussed.

Geometric and Crystallographic Characterization of WC Surfaces and Grain Boundaries in WC-Co Composites

Abstract: Electron backscattered diffraction has been used to determine the orientation of WC crystals in a WC-Co composite and atomic force microscopy has been used to measure the shapes of planar sections of the same crystals. A stereological analysis has been used to determine that {101¯0} prism facets and the {0001} basal planes are the WC surfaces that are most frequently in contact with Co. Further, the WC habit is an approximately equiaxed trigonal prism bound by three prism facets and two basal facets. An analysis of ∼15,600 grain boundaries shows that certain interfaces occur with a frequency that is much higher than would be expected in a random distribution and that the grain boundary habit planes also have {101¯0} and {0001} orientations. Eleven percent of all the observed WC-WC interfaces are 90° twist boundaries about [101¯0]. Two types of boundaries with a 30° rotation about [0001], a twist and an asymmetric tilt, comprise 3% of the population.

Codeposition and Microstructure of Nickel—SiC Composite Coating Electrodeposited from Sulphamate Bath

Abstract: Ni–SiC composite coatings were electroplated from a nickel sulphamate solution with a SiC suspension, with and without the addition of monovalent thallium ions. Without thallium ions, the incorporated SiC particles did not modify the columnar grain structure of the nickel matrix. Conversely, the nickel matrix exhibited an equiaxed grain structure; meanwhile, the amount of codeposited SiC was markedly increased by adding thallium ions to the solution. Additionally, the density of lattice defects associated with nickel grains substantially decreased when the columnar grains were replaced by equiaxed grains. These distinct grain structures associated with the various coatings were discussed in terms of the inhibition of nickel electrocrystallization by thallium ions as well as the absorption and reduction of the various ionic species absorbed on the surface of the SiC particles.

Dynamic deformation behavior and ballistic impact properties of Ti-6Al-4V alloy having equiaxed and bimodal microstructures

Abstract: Effects of microstructural morphology on dynamic deformation behavior and ballistic impact properties of Ti-6Al-4V alloy plates were investigated in this study. Dynamic torsional and ballistic impact tests were conducted on equiaxed and bimodal microstructures, which were processed by different heat treatments, and then the test data were analyzed in relation to microstructures and tensile properties. According to the dynamic torsional test data, maximum shear stress and fracture shear strain of the bimodal microstructure were higher than those of the equiaxed microstructure, and the possibility of the adiabatic shear band formation was more likely in the equiaxed microstructure than in the bimodal microstructure. In the ballistically impacted region of the equiaxed microstructure, a number of adiabatic shear bands and cracks were observed to be formed along plastic flow lines, and delamination occurred because of cracking along the flow lines or shear bands. In the case of the bimodal microstructure, shear bands were found in limited areas near the penetrated surface without occurring delamination, and their number was smaller than that of the equiaxed microstructure. Thus, ballistic performance of the bimodal microstructure was better than that of the equiaxed microstructure, which was consistent with the dynamic torsional test results.

Coalescence of equiaxed grains during solidification

Abstract: The tendency of low-concentration alloys to develop hot tears during solidification is strongly related to the ability of the primary solid phase to form a coherent network, which can withstand significant tensile or shear stresses. Coalescence, i.e. the transformation of two impinging solidification fronts into a solid bridge (or grain boundary), has been simulated for a low-concentration, grain-refined aluminium alloy. Based on a front-tracking microsegregation model and a sharp solid–liquid interface hypothesis, the globulitic grain structure of the alloy has been approximated in two dimensions by a Voronoitessellation with a limited number of straight segments representing the grain boundaries. Near the end of solidification, the liquid film between two neighbouring grains is undercooled by an amount proportional to the grain boundary energy, which is itself a function of the relative misorientation of the grains. As main results, the model can predict, during cooling, the thickness and the spatial repartition of the intergranular liquid films, the fraction of bridged grain boundaries and the gradual transformation of a continuous interdendritic liquid film into a fully coherent solid.

Modeling of macrosegregation and solidification grain structures with a coupled cellular automaton - Finite element model

Abstract: Extension of a coupled Cellular Automaton (CA) - Finite Element (FE) model is presented for the prediction of solidification grain structures and macrosegregation. Following the validation procedure proposed by Ahmad et al. [Metallurgical and Materials Transactions 29A (1998) 617-630], applications of the model to the experiments conducted by Hebditch and Hunt are proposed [Metallurgical Transactions 5 (1974) 1557-1564]. The algorithm and the numerical implementation of the coupling between the CA and FE methods are first validated by considering a single grain that develops with almost no undercooling. Such a calculation is shown to retrieve the solution of a purely FE method calculation in which the grain structure is not accounted for. Several applications of the model are then presented in order to quantify the effects of the grain structure on the final macrosegregation map. In particular, the eqffect of the undercooling of the columnar front, the presence of equiaxed grains nucleated in the undercooled liquid, as well as the transport and sedimentation of equiaxed grains are investigated. The differences between the segregation maps predicted with the CAFE model and the purely FE model are presented. Although good validation is reached with the experimental segregation profiles, it is concluded that refined experimental data are required to further validate the predictions.

Influence of temperature and hydrostatic pressure during equal-channel angular pressing on the microstructure of commercial-purity Ti

Abstract: The paper presents the results of investigations aimed at the processing ultrafine-grained (UFG) structures in commercial-purity Ti by equal-channel angular pressing (ECAP) at low temperatures (200, 300 °C). The most important processing factors influencing the formation of equiaxed UFG structures are studied. It is shown that the decrease of ECAP temperature and the increase of hydrostatic pressure in deformation zone contribute to the formation of finer grains.

Anisotropic Grain Growth in α‐Al2O3 with SiO2 and TiO2 Additions

Abstract: Microstructural changes caused by doping α-Al2O3 with small amounts of SiO2 and TiO2 added singly or together were investigated. When they were sintered at 1450°C for 120 min, singly doped samples developed equiaxed microstructures, but codoped material developed an anisotropic microstructure that contained platelike grains with an average aspect ratio of 3.4. The development of anisotropy thus resulted from a cooperative effect of silicon and titanium. Amorphous material was present at most grain boundaries in the silicon-doped sample. In the codoped sample, only boundaries that exhibited a basal facet were penetrated by amorphous material. Energy dispersive X-ray spectroscopy analysis showed strong titanium enrichment at the edges of platelets. Additional experiments demonstrated that the volume fraction of highly anisotropic platelike grains interspersed with equiaxed grains could be adjusted by using varying amounts of titanium with a constant amount of silicon content. The fracture toughnesses of such materials increased as the structure became more anisotropic.

Characterization of Atomic Layer Deposited WN x C y Thin Film as a Diffusion Barrier for Copper Metallization

Abstract: The film properties of WNxCy films deposited by atomic layer deposition (ALD) using WF6 ,N H 3, and triethylboron source gases were characterized as diffusion barrier for Cu metallization. It is noted that the as-deposited film shows an extremely low resistivity of about 350 µΩ-cm with a film density of 15.37 g/cm 3 . The film composition measured from Rutherford backscattering spectrometry shows W, C, and N of approximately 48, 32, and 20 at.%, respectively. Transmission electron microscopy analyses show that the as-deposited film is composed of face-centered-cubic phase with a lattice parameter similar to both β-WC1-x and βW2N with an equiaxed microstructure. The barrier property of this ALD-WNxCy film at a nominal thickness of 12 nm deposited between Cu and Si fails only after annealing at 700 o Cf or 30 minutes while the sputter-deposited Ta (12 nm) and ALD-TiN (20 nm) fail at 650 and 600 o C, respectively. It is thought that the superior diffusion barrier performance of ALD-WNxCyfilm is the consequence of both nanocrystalline equiaxed grain structure and the formation of high density film.

Processing, Microstructure, and Strength of Alumina–YAG Eutectic Polycrystals

Abstract: Dense polycrystalline eutectics of alumina and yttrium aluminum garnet (YAG) were fabricated by hot-pressing powders of pulverized arc-melted buttons at homologous temperatures of 0.9Teu–0.93Teu (where Teu is the eutectic temperature). The eutectic microstructure of the arc-melted buttons was retained after densification, although the grain boundaries were decorated with equiaxed grains of alumina and YAG ∼1–5 μm in size; possible causes for their formation have been discussed. A comparison of the measured strength of the polycrystalline eutectics (274 ± 61 MPa) with grain size and fracture toughness suggests that the strength-limiting flaws are significantly smaller than the mean grain size and larger than the mean eutectic spacing.

Quasi-static and dynamic deformation behavior of Ti–6Al–4V alloy containing fine α2-Ti3Al precipitates

Abstract: Widmanstatten and equiaxed microstructures containing very fine 2 particles were obtained by aging a Ti–6Al–4V alloy, and their quasi-static and dynamic deformation behavior was investigated in comparison to that of unaged microstructures Quasi-static and dynamic torsional tests were conducted on them using a torsional Kolsky bar, and torsionally deformed areas beneath fracture surfaces were observed to investigate various microstructural factors determining the deformation behavior and effects of 2 precipitation The dynamic torsional test results indicated that maximum shear stress and fracture shear strain of the aged Widmanstatten and equiaxed microstructures were higher than those of the unaged microstructures The number of voids initiated in the aged Widmanstatten and equiaxed microstructures was five times greater than those in unaged microstructures because of the 2 precipitation This indicated that the aging treatment had a homogenizing effect, ie, less likelihood of developing a region of concentrated strain that preceded the adiabatic shear band formation, thereby reducing the possibility of the adiabatic shear band formation Fine 2 precipitation by aging was effective in the improvement of quasi-static and dynamic torsional properties and in the reduction of the adiabatic shear banding, which provided a new idea to improve ballistic performance of Ti alloy armor plates © 2003 Elsevier BV All rights reserved

An analysis of the effect of grain refinement on the hot tearing of aluminium alloys

Abstract: Using the CAST hot tearing rig, grain refinement was found to delay the onset of strength development and load transfer in the mushy zone and reduce the severity of hot tearing. A modified RDG model for dendritic equiaxed grains predicted that grain refinement reduces the hot tearing susceptibility by delaying strength development and interdendritic feeding. However, a modified RDG model for cellular equiaxed grains suggested that reducing the grain size should also decrease the permeability of the mush consequently increasing the hot tearing susceptibility. Over grain refinement leading to hot tearing has been observed elsewhere. It was concluded that a fine equiaxed, dendritic grain morphology was optimum to reduce hot tearing.

Effect of the Electromagnetic Vibration Intensity on Microstructural Refinement of Al-7%Si Alloy

Abstract: The static magnetic field and the alternating electric field were simultaneously imposed on the melt of Al-7%Si hypo-eutectic alloy, and primary α-dendrite particles were refined by electromagnetic vibrations imposed during solidification. The refinement mechanism of primary α-dendrite particles was quantitatively investigated in terms of vibration intensity. When the vibration intensity was increased, coarse equiaxed dendrite particles decreased, and primary α-dendrites approached a globular shape that size was about 25 μm in diameter. By contrast, dendrite arm spacing (DAS) remained intrinsically unchanged even if vibration intensity was changed with electric current intensity. Refinement of primary α-dendrite particles was likely to be caused by collapse of dendrite arms due to the micro-explosion and stirring of the melt.

Microstructural development of HP9/4/30 steel during partial remelting

Abstract: Semi-solid processing, also known as thixoforming, is a forming process that shapes metal components in their semi-solid state. Prior to forming, the microstructure of the alloy consists preferably of solid metal spheroids in a liquid matrix. This paper describes the microstructural development within the semi-solid zone of a typically banded high performance HP9/4/30 steel through a direct partial remelting process from as-received and as-deformed conditions. Partial remelting was carried out at temperatures between 1430 and 1470°C. Liquation occurred initially at the grain boundaries, then also along the segregation bands. With increasing time and hold temperature, these "columns" broke down into shorter, more equiaxed segments, offering a better chance of being thixoformed. The microstructures revealed distinct polygonal cells at 1430°C that changed to more rounded solid grains with diminishing sharp edges at 1450°C, followed by smaller truncated cell structures due to the liquation of the bands at 1460°C and 1470°C. The partial remelting procedures carried out in this study are from material that is in a recrystallised state. Thixoforming from this recrystallised state is shown to be successful. This indicates a widening of the range of potential routes to thixoformable microstructures.

Superplasticity at Room Temperature in Zn-22Al Alloy Processed by Equal-Channel-Angular Extrusion

Abstract: Equal-Channel-Angular extrusion (ECAE) has been conducted at room temperature in order to develop an ultrafine-grained structure in a Zn-22 mass% Al eutectoid alloy. The microstructures had an equiaxed and homogeneously distributed Al-rich and Zn-rich duplex-phase and its average grain size was about 0.35 mm. This alloy exhibited a large elongation of 240% albeit at room temperature and a high strain rate of 10 � 2 s � 1 . A grain-size-dependent phenomenon was also observed at lower strain rates. The activation energy was close to that for grain boundary diffusion. It was apparent that the deformation behavior at room temperature is consistent with that observed in conventional superplasticity. From the results, it was concluded that the dominant deformation mechanism was grain boundary sliding accommodated by dislocation movement controlled by grain boundary diffusion.

Mechanical properties and microstructures of 1100 aluminum subjected to dry machining

Abstract: Commercial purity 1100 aluminum samples with an initial grain size of 4.6 μm were subjected to dry orthogonal cutting, i.e., without using cutting fluid. The strain distributions in the deformed material ahead of the cutting tool tip were estimated using a metallographic method based on the observation of shear angles. Local flow stress values were estimated using microhardness measurements. Deformation structures developed in the material ahead of the tool tip were investigated at four different locations using transmission electron microscopy (TEM). The microstructure in the primary deformation zone (PDZ) was characterized by elongated subgrains, extending in the direction of the strain gradient. The average thickness of the subgrains was 380 nm and their width was 730 nm. The hardness of the PDZ was 80 kg/mm 2 , which represented an increase of 80% from the initial hardness. The strains in the machined chips (MC) reached a high value of 2.3. The corresponding microstructure consisted of a mixture of elongated subgrains with dimensions similar to those observed in the PDZ, and recrystallized equiaxed grains with diameters ranging between 25 and 75 nm. This was accompanied with an 8% decrease in the hardness of the MC compared to that of the PDZ. Grain growth occurred in the secondary deformation zone (SDZ) resulting in large equiaxed grains of 1.2 μm in diameter. The use of cooling fluid during machining restricted grain growth in the SDZ by limiting the grain size to 360 nm.