Lamellar Growth Research Articles

Formation mechanism of periodic layered structure between Fe–Cr–Si–B cast steel with high B content and molten Al

The periodic layered structures (PLSs) have been confirmed to be formed as the Fe–Cr–B cast steels containing about 3.0 wt% B were dipped into Al melt. However, there was no PLSs formed, in terms of a novel Fe–Cr–Si–B cast steel with B content much higher. The interfacial reaction between borides contained in this novel Fe–Cr–Si–B cast steel with 8.5 wt% B content and Al at 750 °C was studied using hot dip aluminizing, thermal diffusion treatment, and subsequent SEM, TEM, and 3DAP of the obtained specimens. The PLSs were formed in situ after a long period of incubation because of the high chemical stability induced by the overstoichiometric (Cr, Fe)2B. The formation of PLSs proceeded through an intermediate state which involved transformation from disordered particles to ordered Cr–Al–B intermetallic compound. Owing to the formation of the localized melt of FeAl3–Al eutectics, the understoichiometric Cr–Al–B intermetallic compound gradually drifted to agglomerate and grew under the effects of the driving force of the lamellar growth of MAB phase and the thermal flux, along with the drag of the overstoichiometric Cr–Al–B intermetallic compound due to the interlaced structure.

Superior photocatalytic performance of a novel flower-like Bi24Si2O40-Bi2O2SiO3 composite via organic-guided crystal growth and bandgap regulation

Aurivillius oxide layered materials are formed by bismuth oxide and pseu-do-perovskite layers, which usually presents a mica-like lamellar structure, that is easy to stack with each other, affecting the exposure of active sites, thereby reducing its photocatalytic performance. Herein, the organic modifier is used to adjust the morphology to obtain Bi24Si2O40-Bi2O2SiO3 (BSO) with three-dimensional flower-like structure. Furthermore, the optimization of morphology promotes the effective formation of heterostructures, organic-mediated morphology growth and the regulation of heterojunction effectively improves the activity of superoxide radicals (•O2−) and holes (h+). Therefore, the organic-modified MBSO-2 shows excellent photocatalytic performance, and its degradation rates of rhodamine B (RhB), ciprofloxacin (CIP) and norfloxacin (NOR) can reach 98.7%, 87.4% and 89.0%, respectively. Moreover, the great improvement of the degradation of RhB and NOR by MBSO-2 can be attributed to the important role played by its active species (h+ and •O2−), which shows that it is more conducive to improving the degradation of pollutants with h+ and •O2− as the degradation path. The modification strategy of organic-mediated lamellar growth provides a new idea for the morphology control of layered metal oxide photocatalysts and broadens its application.

Multiple kinetic processes of lamellar growth in crystallization of stretched lightly crosslinked polyethylene

The growth of polymer crystals is accompanied by the change of interior conditions as time evolves. Therefore, its underlying mechanism should be time-dependent. Here, we investigate the process of lamellar growth by measuring stretched lightly crosslinked high-density polyethylene samples with in situ small-angle X-ray scattering and wide-angle X-ray diffraction. Two analysis methods are used to track the lamellar radius. It is found that the lamellar growth sequentially undergoes three stages, i.e., the interface-controlled stage, the diffusion-controlled stage, and the crystal perfection stage through Ostwald ripening mechanism. The transition between the first and the second stages is related to the competition between the diffusion of chains towards the growth surface and the consumption of chains at the surface in the ideal interfacial kinetics. This transition is significantly affected by the yielding of the sample. The second transition corresponds to the depletion of crystallizable chains, and is directly reflected by the decrease of the polydispersity of lamella radius. These results highlight the importance of the chain consumption in determining the mechanism of lamellar growth.

Regulation of Human Platelet Adhesion with Ascorbic Acid.

Morphofunctional characteristics of human platelets in the presence of 0.1-5 mM ascorbic acid were studied. The platelet ability to form lamellae and the preservation of granules in platelets in suspension and during adhesion were evaluated. Ascorbic acid in concentrations of 0.1-1 mM induced no visible changes in platelet structure and did not affect their adhesion activity, but suppressed lamella growth and degranulation in adherent platelets in a dose-dependent manner. The maximum preservation of granules was revealed in the presence of 0.5 mM ascorbic acid (55% in 1 h from the moment of adhesion). In the presence of 2-5 mM ascorbic acid, spontaneous activation and degranulation of platelets was observed. Thus, ascorbic acid is capable of both suppressing and stimulating platelet activity. In concentrations of 0.5-1 mM ascorbic acid can be used to stabilize granules in adherent platelets.

Microstructure modification of twin-wire directed energy deposition-arc fabricated γ-TiAl alloy via deposition current control

Titanium aluminide has continuously attracted the attention of aerospace industry due to its low density and good performance at high temperatures. To overcome the room temperature brittleness induced manufacturing difficulty, additive manufacturing (AM) technology of titanium aluminide has been developing rapidly since last decade. Compared to powder-based AM methods such as electron beam melting (EBM), the twin-wire directed energy deposition-arc (TW-DED-arc) method for titanium aluminide fabrication possesses outstanding material deposition efficiency and much lower cost. However, because of the excessive arc deposition energy, the layer-band like microstructure of TW-DED-arc fabricated titanium aluminide is a great concern affecting the mechanical properties. In the present research, microstructure of as-fabricated TiAl alloy is modified by adjusting the deposition current of TW-DED-arc. Experimental results show that along with the increase of deposition current, the layer-band like microstructure consisting of dendritic grain region and fully lamellar colony region can be mostly homogenized. Although microhardness is decreased because of the inevitable lamella growth, tensile strength of TiAl deposit is on the other hand increased by eliminating the interdendritic γ-phase. In general, the present work has proved that deposition current adjustment is an effective method for homogenizing the microstructure of additively manufactured titanium aluminide.

Eutectic growth kinetics and microscopic mechanical properties of rapidly solidified CoCrFeNiMo0.8 high entropy alloy

Liquid CoCrFeNiMo0.8 high entropy alloy (HEA) was undercooled by up to 205 K and thus rapidly solidified with electromagnetic levitation technique. A kinetics transition from slow lamellar eutectic growth to rapid anomalous eutectic growth was observed, which showed a significant effect on crystal orientation and mechanical properties. At small undercoolings below 89 K only (σ-CrFeMo + FCC) lamellar eutectic growth proceeded. Once the undercooling exceeded this threshold, anomalous eutectic would grow directly and lead to an extra recalescence peak. The maximum growth velocity of lamellar eutectic was measured as 0.012 m s−1, whereas that of anomalous eutectic attained 2.547 m s−1 at the largest undercooling of 205 K. It was revealed that the eutectic growth mechanism transition resulted from the independent nucleation and cooperative branched growth of the two eutectic phases. An orientation relationship (OR) of {110}σ // {111}FCC and <111>σ // <110>FCC was determined between σ-CrFeMo and FCC phases by electron backscatter diffraction (EBSD) orientation analysis. Furthermore, mechanical measurements indicated that the hardness, compression strength and ductility of this HEA showed an increasing trend with the rise of liquid undercooling, which may be attributed to structural refinement and the volume fraction increase of anomalous eutectic.

3D phase-field simulations of lamellar and fibrous growth during discontinuous precipitation

3D phase-field simulations of lamellar and fibrous growth during discontinuous precipitation

In Situ Atomic Force Microscopy Tracking of Nanoparticle Migration in Semicrystalline Polymers.

We present in situ tracking of silica nanoparticle (NP) migration from a poly(ethylene oxide) (PEO) melt into interlamellar region using in situ atomic force microscopy (AFM). Our results confirm the previous hypothesis that NPs migrate into the interlamellar regions at crystallization growth rates smaller than a critical value under isothermal conditions. Under these slow crystallization conditions, bare silica NPs are rejected as defects by the growing crystal of PEO, and the in situ imaging on the large (50 nm) NPs helps track the migration into the amorphous zones. We extend this AFM technique to estimate lamellar growth rates that correlate with spherulite growth rates determined by polarized light optical microscopy (PLOM) but at smaller undercoolings than are typical for PLOM.

Polymer Crystallization with Configurable Birefringence in Double Emulsion Droplets

Biodegradable semicrystalline polymers are broadly used in microfluidic techniques to create microdroplets for various applications. However, it remains a fundamental challenge to modulate polymer crystallization and optical properties in microdroplets which have complicated spatial geometries. Here we investigate the crystallization of double emulsion droplets formed by poly(1,4-butylene adipate) (PBA), polycaprolactone (PCL), and their blends. Crystallization of these polymers drives the spherical liquid droplets to anisotropic solid microcapsules. Crystal growth shows a linear relationship with time, irrespective of flat-on, edge-on, or continuous twisting lamellar arrangement in the shell. We further reveal that the anisotropic shape and lamellar arrangement impart the crystallized microcapsule with orientation-sensitive birefringence. A representative feature is that all anisotropic microcapsules show two types of Maltese cross, depending on the microcapsule orientation. The PBA shell with flat-on lamellar arrangement always shows positive birefringence, while the PCL shell with edge-on lamellar growth may present negative or positive birefringence at specific orientation angles. Microcapsules of PCL/PBA blends are regulated by the composition of two polymers (fPCL), realizing the transition from PBA-dominated shell to PCL-dominated shell across fPCL = 0.5. We emphasize the peculiarities of crystallization behavior in double emulsion droplets by comparing with crystallization on plane substrate. This work may serve as a fundamental reference for understanding polymer crystallization and further expanding potential applications of semicrystalline polymers for optical microdevices.

Local Characterization of the Diffusion Process during Discontinuous Precipitation: A Review

Abstract The paper summarizes the recent studies on the diffusive mass transport accompanying the discontinuous precipitation (DP). The studies were performed by applying a local characterization of the kinetics of the DP reaction. In order to accomplish this, a technique of the analytical electron microscopy was used to measure the solute concentration profiles left behind the moving reaction front of single cells. Such an approach, combined with the information on the local values of the lamella thickness and growth rate of the cell, enabled for the first time the determination of local values of the grain boundary diffusivity in the systems Al–Zn, Ni–Sn, Cu–In, Ni–In and Co–Al. The obtained values were comparable to those resulting from the use of the global treatment of the diffusion problem for the DP reaction and the data on the radiotracer diffusion along stationary grain boundaries. This clearly indicates that there is no difference in the rate of diffusion along stationary and moving grain boundaries.

Simulation on in-situ crystal growth of lead-free solder Sn-57Bi alloy

Lead-free solder Sn-57Bi alloy is a good candidate material for electronic packaging. However, it is difficult to observe in-situ crystal growth process of Sn-57Bi alloy at high temperature and pressure environment. In this paper, based on the Monte Carlo method with the effect of volumetric and interfacial free energy taken into consideration quantitatively, a probability model for eutectic growth is established to simulate the in-situ crystal growth of Sn-57Bi alloy. The in-situ crystal growth of Sn-57Bi alloy at different undercoolings is studied. The simulation result shows the coupled growth of two phases in eutectic alloys, along with that lamellar spacing decreases and growth velocity increases with the increase of undercooling, agreeing with the reported experimental works on the eutectic growth. This work presents a good method to study the in-situ crystal growth process of alloys.

On the growth of graphite lamellae in a high Si GG20 cast iron

On the growth of graphite lamellae in a high Si GG20 cast iron

Effect of V on the microstructure and brittle-to-ductile transition of directionally solidified high-Nb TiAl alloy

The influence of V addition on the microstructure and high-temperature tensile performance of Ti46Al7Nb0.4W0.6Cr alloy, fabricated by using the cold crucible directional solidification method, is comprehensively investigated. The results show that V addition can refine the columnar grain width and promote the brittle-to-ductile transition (BDT) of the TiAl alloy. After the addition of 2 at.% V, the ultimate tensile strength and elongation increased at 973–1173 K. The fracture mode change from interlamellar to translamellar; this is attributed to the decrease in the angle between the lamellar alignment and growth direction. The BDT behaviour of V-containing TiAl alloy at 1073 K results from the transition from dislocation-dominated to twin-dominated deformation as indicated by transmission electron microscopic investigations.

Joule Heating and mechanical properties of epoxy/graphene based aerogel composite

While polymer composites are anticipated as a promising alternative for Joule heating materials, constructing a continuous conductive network of fillers remains an open challenge to obtain high-performance Joule heating properties with the high steady-state surface temperature at low power input, rapid heating/cooling rate, and excellent cycle stability. Herein, we designed a three-dimensional graphene-based aerogel, infiltrated epoxy into it, and demonstrated a key role of 2D materials-based three-dimensional filler on the anisotropic Joule heating performances of the final aerogel composite. The aerogel composite with the highest graphene loading of 4.7 wt% shows outstanding Joule heating performance with a steady-state temperature of 213 °C at a relatively low applied voltage of 5 V. Benefiting from the 3D graphene structure, the epoxy/graphene-based aerogel composites show an excellent electrical conductivity of 48.7 S/m and thermal conductivity of 1.1 W/mK along the parallel direction to the lamellar structure growth. The Joule heating performance of epoxy/graphene-based aerogel composite is correlated with the morphology, electrical conductivities, and thermal conductivities. Importantly, the induced steady-state temperature follows a linear relationship with both the electrical and thermal conductivities of materials. Mechanical properties of aerogel composite is also studied. The obtained results indicate that the epoxy/graphene-based aerogel composite can be a promising material for thermal management applications.

Microstructure and Formation Mechanism of β Eutectoid Decomposition in a γ‐TiAl Alloy with Ru Addition

A eutectoid decomposition of β→α + τ1 occurs during water quenching (WQ) and the corresponding formation mechanism in a γ‐TiAl alloy with Ru addition is studied. A novel morphology consisting of outer lamellar and inner net‐like eutectoid microstructure is observed. The α and τ1 products are determined to nucleate and grow with the same crystalline orientation in a eutectoid aggregation, respectively. No specific crystalline orientation is confirmed between α and τ1. The eutectoid reaction is found to be controlled by diffusion. Moreover, α and τ1 grow in a synchronous manner. The outer lamellar region absorbs Ti, but excludes both Ru and Al atoms to the center of the β matrix during the eutectoid reaction. Correspondingly, the high Ru and Al contents promote the sidewise growth of τ1 lamellae and result in the distinctions in microstructure with the outer lamellae.

Rod eutectic growth in bulk undercooled melts

This article proposes an analytical model to understand the rod growth of eutectic in the bulk undercooled melt. Based on the previous derivations of the lamellar eutectic growth models, relaxing the assumptions of small Péclet numbers, the model is derived by considering melt kinetic and thermal undercoolings. The intent of this model is to predict the transitions in eutectic pattern for conditions of the low and high growth velocities. In addition to investigation of the transition between lamellar and rod eutectic patterns, mathematical simplifications of solving Bessel function are presented as well, which is the most important priority to model calculation.

The smooth and notched three-point bending fatigue behavior of directionally solidified high-Nb TiAl alloy

To understand the fatigue behavior of directionally solidified (DS) TiAl alloys, the Ti46Al7Nb alloy fabricated by cold crucible DS method was investigated by microstructure characterization and three-point bending fatigue tests. Results show that the microstructure of the alloy is composed of α 2 /γ lamellae with β segregation, and the angle between lamellar arrangement and DS growth direction is 45°. Smooth and notched specimens were prepared to evaluate the fatigue performance of the TiAl alloy under pre-damage conditions. The finite element results indicate that the notched have more obvious stress concentration. The three-point bending fatigue limits of the smooth and notched specimens are 468 MPa and 213 MPa, respectively. The crack propagation path indicates that there are cleavage microcracks with different shapes around the main crack in the notched specimen, while the main crack in the smooth one is surrounded by many corrugated cracks perpendicular to the lamellar direction. The whole process of fatigue crack initiation and propagation under three-point loading is established. Fracture morphology shows that notched specimens are mostly brittle cleavage fracture along the lamellar facets, while the smooth are trans-lamellar fracture with plastic characteristics. The degree of fatigue deformation of the smooth specimen is greater than that of the notched one, which mainly reflects on high density dislocation and dislocation intersection, as well as more sub-grain boundaries and twins. • The high-Nb TiAl alloy with well-aligned lamellae was fabricated by CCDS. • The three-point bending fatigue limits of the smooth and notched specimens are 468 MPa and 213 MPa, respectively. • The corrugated secondary cracks formed around the main crack are conducive to the release of stress concentration. • The smooth specimen exhibits high-density dislocation intersection, as well as more sub-grain boundaries and twins.

Crystal orientation relationships and pattern evolution in directionally solidified Al-Cu-Mg ternary eutectic

Understanding the mechanisms of microstructure evolution during alloy solidification is crucial in designing multi-phase, multi-component systems. In the current work, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) techniques are deployed to carry out a detailed experimental investigation to better understand the correlation between crystallographic orientation relationships (ORs) and the microstructural pattern evolution of directionally solidified Al-Al2Cu-Al2CuMg ternary invariant eutectic. The behavior of the patterns in response to imposed low (6 K/mm) and high (25 K/mm) thermal gradients is also presented. Close examination of the microstructure revealed an unexpected modification of lamellar growth, with lamellar sheets being limited in length to approximately four times the width of one sheet, and the Al2Cu rod phase aligning along these pre-existing boundaries which correspond to low index crystallographic planes. Alignment of the pattern was found to increase with increasing thermal gradient. The results shed more light on somewhat contradictory literature data and provide an increased understanding of pattern evolution in multi-phase eutectic systems.

Gemological Characteristic Difference between Colorless CVD Synthetic Diamonds and Natural Diamonds.

CVD synthetic diamond plays an important role in the jewelry market due to its excellent performance and low cost. In this paper, colorless CVD synthetic diamonds produced by a Chinese company were investigated in detail with their gemological, spectroscopic, and luminescent properties compared with natural colorless diamonds. Compared with natural diamonds, CVD synthetic diamonds have high-order interference color and more apparent abnormal birefringence. The results of infrared spectra indicate that all the CVD samples are classified as type IIa, while the natural samples belong to type Ia. The CVD samples show lamellar growth and mottled luminescence pattern and have blue, orange red, purple red, and blue fluorescence, respectively, while most of the natural samples show blue fluorescence. CVD diamonds show lamellar growth structure, and natural diamonds show irregular ring-like growth structure. Thus, multiple methods combined with analysis are required to distinguish synthetic diamonds from natural diamonds. This work provides an experimental basis for the identification of CVD synthetic diamonds.

Effect of laser shock on lamellar eutectic growth: A phase-field study

Eutectic growth trajectory is a temperature-dependent important behavior that concerns interface stability and subsequent solidification microstructure. In this work, a multiphase-field lattice-Boltzmann method is employed to investigate the effect of thermal shock on eutectic evolution by imposing a laser spot inside the melt. Two morphology feature parameters, average growth velocity and average characteristic path deviation angle of the marginal lamellae, are introduced to quantify the out-of-equilibrium eutectic growth. A multilinear correlation between the feature parameters and the technical factors including laser power, spot radius, cooling rate, and thermal diffusivity capacity is summarized by multiple linear regression analysis. The presence of laser spot bends the solidification front by changing local temperature distribution. The local high-temperature region near the laser spot corresponds to a lower driving force and thus the depression of the solid-liquid interface. Extension to a more general model which can handle large undercooling and phase decomposition is also discussed. The laser shock, static or moving, can be regarded as an efficient tool to obtain the engineered patterns such as curved solidification front or tilt domain.