Columnar Transition Research Articles

Equiaxed to Columnar Transition of Ti46Al8Nb0.5B Intermetallics during Directional Solidification

Equiaxed to Columnar Transition of Ti46Al8Nb0.5B Intermetallics during Directional Solidification

The Microstructure characteristic and Its influence on the stray grains of Nickel-based single crystal superalloys prepared by Laser directed energy deposition

The microstructure adjustment and the stray grains inhibition are important for the preparation of Nickel-based single crystal (SX) superalloys by laser directed energy deposition (L-DED). In this work, the single channel monolayer and single channel five-layers DD6 SX superalloys have been prepared by L-DED. The macroscopic simulations in the deposition processing and multiscale characterizations of deposition structure were used to analyze the formation and evolution mechanisms of unique precipitations and stray grains. Results shows that the larger laser power and higher powder feed rate have a positive impact on the epitaxial growth of columnar dendrite and the elimination of stray grains. The residual stress caused by the complex thermal cycles is not the direct reason that induce the formation of SGs, but the rapid movement of dislocations result in the formation of sub-grain boundary. According to the columnar transition to equiaxed theory (CET), temperature field simulation and analysis of element segregation behaviors, the element segregation results in the unique microstructure, include petal-shaped γ/γ′ eutectic and demarcation band with a thickness of a single γ/γ′ eutectic. Besides, the geometric size of γ/γ′ eutectic decrease with respect to the deposition height. With the change of height at deposition region, dendritic orientation deviations affect the uniformity of element segregation, resulting in variations in γ′ phase and γ/γ′ eutectic dimensions between dendritic core and the inter-dendrite zone at the microscopic level and manifesting as grain boundaries at the mesoscale. At the same time, the formation mechanisms of carbide and adjacent γ/γ′ eutectic leads to the misorientations between the dendrite core and the inter-dendrite zone, which also appears as stray grains at the mesoscopic scale.

Soft nanostructures out of star-shaped triazines with flexible amide spacers: liquid crystals with a cubic to columnar transition with memory effect, gels and supramolecular chirality

This work reports the synthesis and characterization of a new family of star-shaped tricarboxamides with C3-symmetry that have flexible amide spacers linking a tris(triazolyl)triazine core with three trialkoxyphenyl groups. The presence of amide groups allows the formation of intermolecular hydrogen bonds that reinforce π-stacking and van der Waals interactions, promoting liquid crystalline behavior, and self-assembly in solvents leading to organogels. As determined by polarized optical microscopy, differential scanning calorimetry and X-ray diffraction on powder samples, all the three reported molecules present a hexagonal columnar (Colh) phase stable at room temperature. Interestingly, they show a transition to a cubic micellar mesophase (BCC) at high temperatures. A mechanism for this transition, which is consistent with the fragmentation of columns to form supramolecular spheres, was elucidated from X-ray studies on aligned samples. Moreover, on cooling from the BCC to the columnar phase a preferential orientation of columns occurs, according to which each cubic domain gives rise to four hexagonal domains. The ability of the synthesized structures to aggregate in solvent media was studied in a variety of organic solvents, and all of them were able to gel 1-octanol at low concentrations. X-ray studies of gels and xerogels were carried out and showed a molecular organization consistent with Colh order. FTIR studies were carried out to analyze the formation of hydrogen bonds and the influence of the length of the flexible spacer in the liquid crystalline state and in the organogels. Furthermore, the presence of a stereogenic center in the flexible amide spacer leads to macroscopic chirality in the liquid crystal state and the organogels in 1-octanol as demonstrated by circular dichroism spectroscopy.

Influence of growth velocity on fragmentation during directional solidification of Al – 14 wt.% Sn alloy studied by in-situ synchrotron X-radiography

Directional solidification of Al – 14 wt.% Sn alloy was investigated by means of synchrotron X-radiography at the European Synchrotron Radiation Facility (ESRF). The dynamics of the solidification microstructure formation was studied in-situ and in real-time. In the present manuscript, a special attention is given to the influence of increasing growth velocity on the fragmentation phenomenon of dendrite arms, which has an important role and impact on the final grain structure. Fragmentations of both secondary dendrite arms and more strikingly part of primary trunks have been observed. The experiments show that the number of fragments increases concomitantly with the solidification growth rate until it reaches a limit value after which fewer or no more fragments are observed. The possible origins of the fragmentation are discussed in detail and the likelihood of each of these potential mechanisms is estimated to understand the reason of both primary and secondary arm detachment. It is shown that the fragmentation in a transient regime is correlated to the variations of the solute accumulation, latent heat rejection and the curvature of the attachment neck during solidification. Moreover, during these experiments, Columnar-to-Equiaxed Transition (CET) induced by the fragmentation phenomenon is observed. The velocity of the dendrite fragments due to the buoyancy force is a major parameter to control the CET and our results show that a reverse Equiaxed-to- Columnar Transition can even be observed. Once the growth velocity is of the same order or higher than the fragment flotation velocity, the fragments are trapped within the columnar dendritic microstructure, and CET cannot take place or the equiaxed regime cannot be sustained.

Microstructure evolution of laser cladding Inconel 718 assisted hybrid ultrasonic-electromagnetic field

In the present article, the hybrid ultrasonic-electromagnetic (UEM) physics field is introduced into the laser additive manufacturing Inconel 718. The effect of the hybrid UEM field on microstructure evolution is investigated. Obtained results show that when the hybrid UEM is applied, porosity reduces significantly, the columnar dendrites epitaxial growth behavior along the deposition direction depresses and the columnar transition to equiaxed grains improves. Moreover, the average microhardness of the coating increases from 244 HV0.2 to 271 HV0.2.

Columnar-equiaxed transition in continuous casting based on a micro-macro solidification model with long-range solutal mixing

A novel model is proposed to describe the columnar to equiaxed transition (CET) in continuous casting. The model bases on Rappaz and Thevoz’s solute diffusion model for equiaxed dendritic growth, combined with a 1-Dimensional solidification model normal to the slab surface. The unique feature of the proposed model is the combination with a mixing term between interdendritic and extradendritic melt, representing long-range solutal mixing by convection. The model can also be applied to predict equiaxed to columnar transition (ECT), i.e. the chill zone thickness. The model consists of modules such as nucleation, growth kinetics, solute and heat balance, and a solute mixing module. Nucleation is considered with a fixed nucleation undercooling. The growth kinetics of the dendrites are treated according to the LGK model. A finite difference scheme is employed for solving 1-Dimensional heat transfer equations and finally, volume averaged solute balance equations are solved in a staggered scheme. Mixing of inter- and extradendritic liquid is, as a first step, treated ideally fast. When applied to Fe-C binary system with the thermo-physical properties obtained from literature and CALPHAD simulations, the model successfully predicts recalescence, phase fraction evolution, and concentration profiles in different phases. Realistic boundary conditions of the continuous casting process are obtained from macroscopic FEM simulation.

Grafting density-induced smectic A to hexagonal columnar transition in mesogen-free isotactic liquid crystalline polyethers with n-dodecylsulfonyl side groups

Highly dipolar mesogen-free liquid crystalline polymers are excellent candidates for achieving large spontaneous polarization under relatively low electric fields for advanced electrical and optical applications. In this report, the effect of grafting density of n-dodecylsulfonyl side chains on the mesogen-free liquid crystalline self-assembly of comb-like isotactic poly(oxypropylene) was studied. When the grafting density was 80–100%, a stable 21 helical chain conformation was induced by the strong dipole-dipole interactions among the sulfonyl groups in the side chains. Consequently, a smectic A self-assembly was induced. When the grafting density decreased to 60–80%, a poor and possibly imbalanced 21 helical chain conformation was formed due to the frequent absence of the n-dodecylsulfonyl side chains along the main chain. As a result, a hexagonal columnar self-assembly was realized. This study demonstrated that a delicate interplay between the molecular defects and dipole-dipole interactions can lead to different self-assembly structures.

Microstructure evolution of Ti44Al alloy during directional induction heat treatment and its effect on mechanical properties

For realizing equiaxed to columnar transition, Ti44Al alloy was processed by directional induction heat treatment (DHT) and mechanical properties were tested. The results show that columnar grains can be obtained and elongation is improved at room temperature. During this heat treatment, Ti44Al alloy was heated in single β phase domain. The microstructure is consisted of fully lamellar colonies after DHT. The average lamellar spacing is decreased, and the orientation of lamellar colonies with angle of less than 60° is more than 76%. Moreover, grain boundary presents zigzag. Tensile testing shows that strength and elongation of heat-treated alloy are 217 MPa and 1.78% respectively, they are 253 MPa and 1.63% for as-cast Ti44Al alloy. The elongation is increased about 9.2% although tensile strength is decreased. Cleavage fracture is main way in as-cast Ti44Al alloy but a mixed fracture consisting of some cleavage planes and many tearing edges is in Ti44Al alloys after DHT. The fracture morphology shows that deterioration of strength is mainly resulted from the micropores that reserved from as-cast alloy, it means DHT could not eliminate the micropores. The improvement of ductility is caused by decreasing of lamellar spacing and changing orientation lamellar colonies.

Effects of Sr, Ce and P on the microstructure and mechanical properties of rapidly solidified Al[sbnd]7Si alloys

A series of Al7Si based alloys with trace addition of Sr, Ce or P were prepared using melt-spinning at two different high cooling rates, and the microstructure as well as the hardness of the ribbons was comprehensively studied. The results show that Sr or Ce induces the formation of columnar transition zone and modifies the morphology of eutectic Si phase from dispersed particle chains to fine fibers only at extremely high cooling rate. However, P has little effect on the microstructure of either α-Al or eutectic Si. Compared with conventional casting, the Al7Si ribbons exhibit extremely high hardness due to the fine dispersion of eutectic Si, the grain refinement and the supersaturation of Si. The hardness is further enhanced by Sr or Ce addition at extremely high cooling rate, especially for columnar and equiaxed zones, attributing to the modification effect on Si phase.

Sequential phase transformation of propeller-like C3-symmetric liquid crystals from a helical to ordered to disordered hexagonal columnar structure.

In this paper, we report thermally induced intercolumnar phase transitions of C3-symmetric liquid crystals (LCs) bearing a triazole-based propeller-like aromatic mesogen. Since the constituting aromatic rings are conjugated through rotatable single bonds, the mesogenic shape is tuneable depending on the degree of conformational motion. Molecule 1 with ninefold octyl peripheries shows a hexagonal columnar liquid crystalline phase transition from ordered mesogenic stacking to disordered mesogenic stacking upon heating. On the other hand, molecule 2 with sixfold octyl peripheries displays a helical hexagonal columnar phase with the P6/mmm space group at ambient temperature as well as the ordered and disordered hexagonal columnar phases at higher temperatures. The intracolumnar helical order can be understood by an interdigitated stacking of the propeller-like mesogens along the columnar axis and the optimized space-filling. Notably, all the intercolumnar phase transformations in this study are revealed as second-order transitions. The thermodynamic nature agrees well with the fact that the conformational motions of the C3-symmetric aromatic mesogen change abruptly with each columnar transition.

Shear-thinning and isotropic–lamellar–columnar transition in a model for living polymers

A model consisting of short polymer segments, capable of self assembling to form a branched network through the interaction of their ends, is considered. We investigate the mechanistic origins of a shear induced isotropic–lamellar–columnar transition, reported earlier in this model for living polymers. We find that these structures arise only in the presence of a potential barrier in the end point interaction between polymer segments. To elucidate the role played by long range solvent mediated forces, we have carried out simulations with and without the hydrodynamic interactions between the polymers and have seen that these structures are present in both these cases. We also show that the shear induced layering in these systems is a bulk phenomena and boundary interactions have little effect on the transition. The monomer volume fraction and viscosity of the background solvent are shown to have significant effects on determining the morphology of the flow-induced structures. We demonstrate that the structures obtained are indeed non-equilibrium states induced by the shear.

Study of Mechanical Properties of AZ91 Magnesium Alloy Welded by Laser Process Taking into Account the Anisotropy Microhardness and Residual Stresses by X-Ray Diffraction

The objective of this investigation was to study the mechanical properties of a magnesium alloy welded by a CO2 laser. Residual stresses were measured by X-ray diffraction. They were calculated by the classic sin2 ψ method in the isotropic zones by using the orientation distribution function (ODF) in the textured zones. The results demonstrated that laser welding results in the formation of several different zones with different microstructural and mechanical properties. Welding principally leads to a reduction in grain size and a new distribution of phases. The most remarkable observation was that of a superficial layer on the surface of the welded zone. This layer has a marked crystallographic texture, a reduction in the level of aluminum, and an elevated microhardness. These characteristics disappear at a depth of 200 μm under the welded zone. These modifications can be explained by the nature of the solidification, which occurs under nonequilibrium conditions resulting in an equiaxial columnar transition. This transition is evident also within the profile of residual tensile stresses, which are at their maximum at the interface between the superficial layer and the rest of the welded zone. These results are explained by the anisotropic properties of the textured layer in relation to the plasticity.

Field-induced columnar transition of biocompatible magnetic colloids: An aging study by magnetotransmissivity

The field dependence of the optical transmission of tartrate-coated and polyaspartate-coated magnetite-based aqueous colloids was studied. The colloidal stock samples were diluted to prepare a series of samples containing different particle volume fractions ranging from 0.17% up to 1.52% and measured at distinct times after preparation (1, 30, 120, 240, and 1460 days). We show that the magneto-transmissivity behavior is mainly described by the rotation of linear chains, at the low-field range, whereas the analysis of the data provided the measurement of the average chain length. Results also reveal that the optical transmissivity has a minimum at a particular critical field, whose origin is related to the onset of columns of chains built from isolated particle chains, i.e., due to a columnar phase transition. We found the critical field reducing as the particle volume fraction increases and as the sample's aging time increases. To investigate the origin of this phenomenon we used phase condensation models and Mie's theory applied to a chain of spheres and to an infinite cylinder. Possible implications for magnetophotonic colloidal-based devices and biomedical applications were discussed.

Phase diagram of hard colloidal platelets: a theoretical account

We construct the complete liquid crystal phase diagram of hard plate-like cylinders for variable aspect ratio using Onsager's second virial theory and employing the Parsons–Lee decoupling approximation to account for higher-body interactions in the isotropic and nematic fluid phases. The stability of the solid (columnar) state at high packing fraction is included by invoking a simple equation of state based on a Lennard–Jones–Devonshire cell model which has proven to be quantitatively reliable over a large range of packing fractions. By employing an asymptotic analysis based on the Gaussian approximation we are able to show that the nematic–columnar transition is universal and independent of particle shape. The predicted phase diagram is in qualitative agreement with simulation results.

Columnar phase structures of an organic-inorganic hybrid functionalized with eight calamitic mesogens.

A liquid-crystalline octapode, formed by laterally connecting calamitic mesogens to an inorganic silsesquioxane cube through flexible siloxane spacers, is studied using polarized light microscopy, differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The studies are extended to mixtures of the octapode with the respective monomer mesogens. The monomer and the octapode show a nematic phase. At lower temperatures, the octapode exhibits additionally a columnar hexagonal phase (6 lattice), which, on further cooling, undergoes a transition to a columnar rectangular phase (2 lattice). A similar phase-transition sequence is observed for mixtures of the octapode with moderate concentrations of the monomer. The columnar-columnar transition is discussed combining XRD and DSC results, and a possible model of the molecular self-organization is presented.

In SituImaging of Field-Induced Hexagonal Columns in Magnetite Ferrofluids

Field-induced structures in a ferrofluid with well-defined magnetite nanoparticles with a permanent magnetic dipole moment are analyzed on a single-particle level by in situ cryogenic transmission electron microscopy (2D). The field-induced columnar phase locally exhibits hexagonal symmetry and confirms the structures observed in simulations for ferromagnetic dipolar fluids in 2D. The columns are distorted by lens-shaped voids, due to the weak interchain attraction relative to field-directed dipole-dipole attraction. Both dipolar coupling and the dipole concentration determine the dimensions and the spatial arrangement of the columns. Their regular spacing manifests long-range end-pole repulsions that eventually dominate the fluctuation-induced attractions between dipole chains that initiate the columnar transition.

Liquid–crystal phase transitions in suspensions of plate–like particles

We study the liquid–crystalline phase behaviour of suspensions of hard colloidal platelets, in particular, the relationship between their size polydispersity and the stability of a nematic, columnar and smectic phase. A first–order nematic–to–columnar transition is observed for suspensions up to 25% polydispersity in platelet diameter (σD). However, at densities slightly above nematic–columnar coexistence, the columnar order appears to vanish in the case of σD = 25%, while columnar stability persists for σD = 17%. The observed tolerance of polydispersity in the columnar phase, being a two–dimensional crystal, seems remarkable in light of current predictions for the terminal polydispersity for hard–sphere crystallization and hard–rod smectic ordering.

Rod-like Polymers in Nematic Solvents: Mechanically and Electrically Driven Frederiks-like Transitions

A layer of short, grafted, main chain liquid crystalline polymers in a nematic solvent is confined to a slit. The polymers are modeled as bendable rods. This system can undergo a number of distortional transitions induced either by applied pressure or by an electric field. As well as Frederiks-like transitions the system can also exhibit columnar transitions in which the distorted domains are separated by undistorted regions. The coupling between the rods and nematic solvent implies that the system has an effective modulus due to both contributions. A combination of pressure and electric fields allows for the lowering of the Frederiks transition voltage to arbitrarily low values.

Transfer‐matrix theory of the crystal–columnar transition in polymeric liquid crystals

AbstractThe statistical–mechanical problem of the transition between crystalline and columnar phases in a main‐chain liquid–crystalline polymer is treated in a simple model in which only longitudinal motions of the polymer chains are permitted. A mean‐field approximation for the interchain potential is used to obtain a self‐consistent equation for the crystalcolumnar transition temperature. When applied to typical homopolymers this theory correctly predicts transition temperatures above the degradation temperature; when applied to a crude model of a random copolymer a temperature in the observed range is predicted.

Charge transfer induced nematic columnar phase in low molecular weight disc-like systems [1

Abstract Two large pentakis(phenylethynyl)phenyl alkyl ethers, without side chains as is usual for discotic systems, were synthesized in palladium catalysed one pot coupling reactions. These low molecular weight and non-liquid-crystalline electron donor compounds were doped with 2,4,7-trinitrofluorenone under the formation of charge transfer complexes. The induction of two types of enantiotropic mesophases (i.e. nematic and hexagonal columnar phases (Nc and Dho)) was observed. These charge transfer-induced mesophases were characterized by polarization microscopy, differential scanning calorimetry, and X-ray analysis; phase diagrams of the binary systems were also recorded as well as the hexagonal columnar to nematic columnar transition as a function of the length of the disturbing alkoxy group. It is considered possible now to study low molecular mass Nc phases by tailor-made two component mixtures on the basis of our easily accessible disc-shaped multiynes.