Showing papers on "Equiaxed crystals published in 2006"

Columnar to equiaxed grain transition in as solidified alloys

Abstract: The generally reported observations pertinent to any proposed interpretation of the columnar to equiaxed transition (CET) in as solidified alloys are initially considered. The review then proceeds to consider the proposed mechanisms of equiaxed grain formation, the influence of alloy and processing conditions on the CET, criteria for the termination of columnar growth and, finally, deterministic/stochastic models for predicting the CET. In conclusion, the present level of understanding of the CET and current modelling capabilities are summarised and assessed.

Crystal fragmentation and columnar-to-equiaxed transitions in Al-Cu studied by synchrotron X-ray video microscopy

Abstract: Recent improvements in detectors combined with the eminent brightness and collimation offered with modern synchrotron sources open the way forin situ X-radiographic investigations of solidification fundamentals and phenomena in real alloys at resolutions approaching regular video microscopy. Here, the authors present observations on dendrite fragmentation from columnar fronts in Al-Cu and subsequent transport phenomena. From directional solidification experiments it has been found that the tendency for crystal fragments to detach by remelting of branch roots in the mush dendrite network strongly depends upon the relative buoyant and settling motions of crystal fragments and mush liquid, respectively. At the copper concentrations studied (20 to 30 wt pct), primary aluminum dendrites are lighter than the melt and solidification experiments parallel and anti-parallel to gravity show significant differences in detachment tendency. The experimental results compare well with three different models proposed for fragmentation at different mush locations; however, the results also demonstrate that these models differ substantially both with respect to detachment frequency and the ability for detached fragments to cause eventual columnar to equiaxed transitions. Under particular conditions it has been found that crystal fragmentation could lead to an alternating mesoscale segregation.

A three-phase model for mixed columnar-equiaxed solidification

Abstract: A three-phase model for mixed columnar-equiaxed solidification is presented in this article. The three phases are the parent melt as the primary phase, as well as the solidifying columnar dendrites and globular equiaxed grains as two different secondary phases. With an Eulerian approach, the three phases are considered as spatially coupled and interpenetrating continua. The conservation equations of mass, momentum, species, and enthalpy are solved for all three phases. An additional conservation equation for the number density of the equiaxed grains is defined and solved. Nucleation of the equiaxed grains, diffusion-controlled growth of both columnar and equiaxed phases, interphase exchanges, and interactions such as mass transfer during solidification, drag force, solute partitioning at the liquid/solid interface, and release of latent heat are taken into account. Binary steel ingots (Fe-0.34 wt pct C) with two-dimensional (2-D) axis symmetrical and three-dimensional (3-D) geometries as a benchmark were simulated. It is demonstrated that the model can be used to simulate the mixed columnar-equiaxed solidification, including melt convection and grain sedimentation, macrosegregation, columnar-to-equiaxed-transition (CET), and macrostructure distribution. The model was evaluated by comparing it to classical analytical models based on limited one-dimensional (1-D) cases. Satisfactory results were obtained. It is also shown that in order to apply this model for industrial castings, further improvements are still necessary concerning some details.

Microstructural effects of AZ31 magnesium alloy on its tensile deformation and failure behaviors

Abstract: An Mg AZ31 alloy with modified microstructures was investigated to determine microstructural effects on room temperature mechanical properties acquired from low strain rate (∼10 −3 s −1 ) tensile testing to failure. Three distinct microstructures were generated via heat treatment, viz.: (1) single phase, fine equiaxed grains; (2) single phase, coarse grains with twins; (3) fine, equiaxed grains decorated with Mg 17 (Al,Zn) 12 grain-boundary precipitates. Each microstructure was separately characterized with optical microscopy, X-ray crystallography, fractography, and hardness measurement prior to tensile testing. Tensile coupons fabricated from each microstructure were then elongated to failure in a miniature stage, and true stress–true strain curves were computed with the digital image correlation (DIC) technique. Initial yield point, ultimate tensile strength and maximum elongation were also computed and examined within the context of key features of each microstructure to infer the mechanism of plastic deformation in tension and whether or not the potential for improved room temperature formability exists.

The microstructure, tensile properties, and creep behavior of Mg-Zn alloys containing 0-4.4 wt.% Zn

Abstract: This paper describes the microstructure, tensile, and tensile–creep behavior of a series of Mg–Zn alloys ranging from 0 to 4.4 wt.% Zn. The microstructures consisted of equiaxed hexagonal-close-packed grains with fine precipitates preferentially located at grain boundaries. Some of the microstructures contained fine laths within the equiaxed grains. The finest grain sizes were observed for a Zn composition of 4 wt.%. Tensile experiments were performed at room temperature and 150 °C while creep experiments were conducted at 150 °C for applied stresses between 30 and 50 MPa. The greatest tensile and creep resistance was exhibited by Mg–4.1Zn which contained 0.2 wt.% Y. The measured creep exponent for the Mg–4.1Zn alloy was 4.2, suggesting dislocation climb as the dominant creep mechanism. Overall, Zn proved to be a potent grain refiner and strengthener for Mg where 4 wt.% appeared to be the optimal Zn content for tensile and creep strengthening over the range of alloying additions examined.

Development of adiabatic shear bands in annealed 316L stainless steel: Part II. TEM studies of the evolution of microstructure during deformation localization

Abstract: The evolution of adiabatic shear localization in an annealed AISI 316L stainless steel has been investigated and was reported in Part I of this paper (Met. Trans. A, 2006, Vol. 37A, pp. 2435–446). In the present research (Part II), a comprehensive transmission electron microscopy (TEM) examination was conducted on the microstructural evolution of shear localization in this material at different loading stages. The TEM results indicate that elongated subgrain laths and an avalanche of dislocation cells are the major characteristics in an initiated band. Development of the substructures within shear bands is controlled by dynamic recovery and continuous dynamic recrystallization. The core of shear bands was found to consist of fine equiaxed subgrains. Well-developed shear bands are filled with a mixture of equiaxed, rectangular, and elongated subgrains. The equiaxed subgrains, with a typical size less than 100 nm, are postulated to result from either the breakdown and splitting of subgrain laths or the reconstruction of subcells.

Effect of Carbon Addition on Microstructure and Mechanical Properties of a Wrought Co-Cr-Mo Implant Alloy

Abstract: The wrought Co–Cr–Mo alloys with C contents of 0.02, 0.09 and 0.18% (mass%) were fabricated by hot-forging process to study the influence of carbon contents on the microstructures and mechanical properties. The microstructures of Co–29Cr–6Mo–0.02C and Co–29Cr– 6Mo–0.09C consist of equiaxed uniform grains which contain stacking faults, twins and " martensite bands. No carbide found at inter- and intragranular region. Co–29Cr–6Mo–0.18C consists of irregular grain sizes and carbide found at inter- and intra-granular region. The carbide in Co– 29Cr–6Mo–0.18C was identified as M23C6 type carbide from the XRD pattern analysis. It is found that the amount of stacking fault and " martensite are strongly dependent upon the C content. The density of stacking faults and " martensites observed in Co–29Cr–6Mo–0.09C

Numerical simulation of microstructure evolution of Ti-6Al-4V alloy in vertical centrifugal casting

Abstract: A multiscale model is developed for simulating the microstructure evolution during solidification processes of Ti-6Al-4V alloy in vertical centrifugal casting, which combines the 3D finite difference method (FDM) at the macroscale with a 2D cellular automaton (CA) model at the microscale. The macro model is used to simulate the fluid flow, mass, heat and species transfer throughout the casting under centrifugal conditions. The micro model is used to predict the nucleation and growth of microstructures for the vertical central plane. A semi-coupled scheme between the 3D macroscopic and 2D microscopic calculations is adopted. Based on the CA cell, a modified method for simulating the shrinkage cavity is given. With the proposed model, numerical simulations are performed to investigate the influences of mould rotation speed, superheat, and mould material on microstructure formation. Calculated results reveal that the equiaxed zone is found to expand with increasing the mould rotation speed, as well as decreasing melt superheat and heat diffusivity of mould. The role of rotation speed is much greater than that of superheat and mould material in centrifugal casting. The underlying mechanisms responsible for those physical phenomena are discussed.

Refinement of Solidification Microstructure and Austenite Grain by Fine Inclusion Particles

Abstract: The effect of deoxidation products of Ce2O3, ZrO2 and MgO particles on solidification microstructure has been studied in Fe–10mass%Ni, Fe–0.20mass%C–0.02mass%P and Fe–0.50mass%C–1mass%Mn alloys. The degree of the equiaxed crystallization is explained by the lattice misfit parameter between γ (or δ)-Fe and oxide. The single-phase solidification microstructure of Fe–10mass%Ni and Fe–0.50mass%C–1mass%Mn alloys is well related to austenite grain boundaries under the inhibition of grain growth by pinning. The correspondence between solidification structure and initial austenite grain has been studied in two-phases solidification of Fe–0.15 (or 0.30)mass%C–1mass%Mn–1mass%Ni alloy. The γ-grain size decreases with decreasing the lattice misfit parameter between γ-Fe and oxide and increases with decreasing the Zener pinning force. The number of γ-grains to that of primary δ-grains per unit area in a cross section increases with decreasing the aforementioned lattice misfit parameter, indicating that more than one nucleation event per δ-grain occurs at δ-ferrite grain boundary during δ to γ transformation.

Microstructural characterization of Al-7075-T651 chips and work pieces produced by high-speed machining

Abstract: High-speed orthogonal machining produced a series of Al-7075-T651 chips and work pieces for microstructural charcterization. The orthogonal machining conditions included two rake angles, three cuttings speeds, and a constant feed rate. The chips and work pieces were characterized using optical metallography, transmission electron microscopy (TEM), and microhardness measurements. TEM observations indicated growth of the η′ precipitates and showed recrystallized equiaxed grains within the shear bands of the chips. Microhardness profiles of the work pieces showed a decrease in hardness at the work surface. The thickness of this deformation layer increased with cutting speed. Microstructural characterizations were correlated with calculated temperature–time profiles to investigate possible mechanisms for the observed microstructure changes.

The Effect of Grain Refinement and Cooling Rate on the Hot Tearing of Wrought Aluminium Alloys

Abstract: Using modifications to the Rappaz-Drezet-Gremaud hot tearing model, and using empirical equations developed for grain size and dendrite arm spacing (DAS) on the addition of grain refiner for a range of cooling rates, the effect of grain refinement and cooling rate on hot tearing susceptibility has been analysed. It was found that grain refinement decreased the grain size and made the grain morphology more globular. Therefore refining the grain size of an equiaxed dendritic grain decreased the hot tearing susceptibility. However, when the alloy was grain refined such that globular grain morphologies where obtained, further grain refinement increased the hot tearing susceptibility. Increasing the cooling decreased the grain size and made the grain morphology more dendritic and therefore increased the likelihood of hot tearing. The effect was particularly strong for equiaxed dendritic grain morphologies; hence grain refinement is increasingly important at high cooling rates to obtain more globular grain morphologies to reduce the hot tearing susceptibility.

Modeling the relationship between microstructural features and the strength of WC–Co composites

Abstract: A two-dimensional finite element method (FEM) was used to predict the stress–strain distributions and the fracture strengths of WC–Co composites with carbide grain sizes from 1.4 to 5.3 μm and carbide volume fractions from 0.7 to 0.9. Stress–strain distributions were calculated in plane sections of microstructures mapped by orientation imaging microscopy. An effective fracture energy was set so that the measured strength of each material was reproduced by the simulation. This model was then used to simulate the properties of hypothetical microstructures to investigate the influence of independent variations in microstructural characteristics on strength. The results indicate that composites with minimum contiguity, containing highly angular, and equiaxed carbide grains with a narrow size distribution should have the maximum strength. Of these parameters, contiguity is the most influential.

Microstructural development of adiabatic shear bands in ultra-fine-grained low-carbon steels fabricated by equal channel angular pressing

Abstract: Microstructural development of adiabatic shear bands formed in ultra-fine-grained low-carbon steels fabricated by equal channel angular pressing (ECAP) was investigated in this study. Dynamic torsional tests were conducted on four steel specimens, two of which were annealed after ECAP, using a torsional Kolsky bar. The ECAP’ed specimen consisted of fine equiaxed grains of 0.2 μm in size, which were slightly coarsened and had an equiaxed shape after annealing. Some adiabatic shear bands were observed at the gage center of the dynamically deformed torsional specimen, and their width was narrower in the ECAP’ed specimen than in the 1-h annealed specimen. Detailed transmission electron microscopic analysis on adiabatic shear bands indicated that very fine equiaxed grains of 0.05–0.2 μm in size were developed within the adiabatic shear band, and that cell structures were formed in the shear band flank by partitioning elongated ferrites. These phenomena were explained by dynamic recovery and recrystallization due to the highly localized plastic deformation and temperature rise occurring in the shear band. The temperature rise in the shear band formation process was estimated to be above 540 °C by observing spheroidized cementites inside pearlite grains.

Modeling of Macrosegregation and Solidification Grain Structures with a Coupled Cellular Automaton—Finite Element Model

Abstract: A coupled Cellular Automaton (CA)-Finite Element (FE) model is presented for the prediction of solidification grain structures coupled with the calculation of solid and liquid flow induced macrosegregation. The model is applied to simulate the solidification of a Pb-48wt%Sn alloy in a rectangular cavity cooled down from only one of its vertical boundaries. The algorithm and the numerical implementation of the coupling between the CA and FE methods are first validated by considering a single grain developing with no undercooling. Such a CAFE simulation is shown to retrieve the solution of a purely FE method simulation for which the grain structure is not accounted for. Several applications of the model are then presented to quantify the effects of the grain structure on the final macrosegregation map. In particular, the effect 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. Although good validation is reached when comparing computed and measured segregation profiles available in the literature for the chosen configuration, it is concluded that refined experimental data are required to further validate the predictions of a coupled CAFE model.

Grain structure evolution in a 6061 aluminum alloy during hot torsion

Abstract: The grain structure development of 6061 aluminum alloys deformed at elevated temperatures in torsion is presented here and related to industrial processing scenarios. Samples of a 6061 aluminum alloy representing two variations in chemistry have been studied at different values of effective strain, ɛeff., strain rate, e ˙ , and temperature, T. Rapid quenching techniques were implemented to ensure preservation of the deformed microstructure after deformation ceased. It was determined that at effective torsion strains greater than 2.5, a fine, equiaxed grain structure develops and may be related to a type of dynamic recrystallization (DRX). The final grain size increases with increasing deformation temperature.

Interfacial microstructure and strength of the dissimilar joint Ti3Al/TC4 welded by the electron beam process

Abstract: Dissimilar weld joints in Ti3Al/TC4 were welded using the electron beam (EB) process. The microstructure evolution characterizations of the joints was investigated by means of OM, SEM, XRD, TEM and the tensile strengths of the joints were tested. The microstructure of the weld metal of every joint was identical. The structures were characterized by martensite, appearing coarse equiaxed grains. There existed grain coarsening and martensitic transformation in Ti3Al/HAZ and TC4/HAZ. With the increase of heat input, the grain size was significantly raised, yet the composition of the weld metal was independent of heat input. The highest tensile strength of the joints can reach 831 MPa, equaled almost to 92% of that of Ti3Al-based alloy.

Origin of Texture Development in Barium Bismuth Titanate Prepared by the Templated Grain Growth Method

Abstract: Microstructure development was examined for BaBi4Ti4O15 prepared by the templated grain growth method, and the origin of texture development was discussed. The microstructure development in a compact composed of a platelike template and equiaxed matrix grains was characterized as follows: (1) the template grains thickened at an early stage; (2) the matrix grains changed their shape from equiaxed to platelike, and simultaneously, the plate faces aligned parallel to those of template grains; and (3) a group of large grains with mutually parallel alignment was formed by prolonged heating at high temperature. Texture developed during these microstructural changes, and process (2) made the greatest contribution toward texture development.