Evolution Of Cellular Structure Research Articles

The Role of Dislocation Type in the Thermal Stability of Cellular Structures in Additively Manufactured Austenitic Stainless Steel.

The ultrafine cellular structure promotes the extraordinary mechanical performance of metals manufactured by laser powder-bed-fusion (L-PBF). An in-depth understanding of the mechanisms governing the thermal stability of such structures is crucial for designing reliable L-PBF components for high-temperature applications. Here, characterizations and 3D discrete dislocation dynamics simulations are performed to comprehensively understand the evolution of cellular structures in 316L stainless steel during annealing. The dominance of screw-type dislocation dipoles in the dislocation cells is reported. However, the majority of dislocations in sub-grain boundaries (SGBs) are geometrically necessary dislocations (GNDs) with varying types. The disparity in dislocation types can be attributed to the variation in local stacking fault energy (SFE) arising from chemical heterogeneity. The presence of screw-type dislocations facilitates the unpinning of dislocations from dislocation cells/SGBs, resulting in a high dislocation mobility. In contrast, the migration of SGBs with dominating edge-type GNDs requires collaborative motion of dislocations, leading to a sluggish migration rate and an enhanced thermal stability. This work emphasizes the significant role of dislocation type in the thermal stability of cellular structures. Furthermore, it sheds light on how to locally tune dislocation structures with desired dislocation types by adjusting local chemistry-dependent SFE and heat treatment.

Growth of defects around grain boundary and evolution of cellular structure during isothermal aging process in 2:17-type SmCo permanent magnets

The cellular structure consisted of cell boundary phase (1:5H phase) and cell phase (2:17R phase) is crucial for 2:17-type SmCo magnets to obtain high performance. In this study, the growth of defects around grain boundary and the evolution of cellular structure during isothermal aging process are systematically investigated. Further analysis determines the effects of these microstructural changes on magnetic properties. With the isothermal aging time increasing from 0.5 to 12 h, the width of 1:5H phase-free zones (PFZs) around the grain boundary expands from approximately 100 to 200 nm. Simultaneously, the cellular structure inside the grain becomes more complete, and the Cu content in the 1:5H phase is increased. However, as the isothermal aging time further extends to 36 h, accompanied by the formation of substantial amounts of Cu-rich phases along the grain boundaries, the width of PFZs around the grain boundary increases to approximately 300 nm. Moreover, the width of the 1:5H phase widens, and it even disappears, leading to incomplete cellular structure and a low Cu content in the 1:5H phase. The broad PFZs and incomplete cellular structure negatively impacts the coercivity and squareness of the magnet. Finally, an optimal performance of Br = 10.8 kG, Hcj = 29.7 kOe, (BH)max = 26.7 MGOe, and SF = 49.9 % is obtained for the magnet aged 12 h. These findings indicate that the defects around the grain boundary can be effectively controlled and form complete cellular structure by adjusting the isothermal aging time, which may provide an important clue for developing high-performance and low-cost 2:17-type SmCo magnets.

Pressure fluctuation and cellularization characteristics of 2-ethylfuran spherical expanding flame

The pressure fluctuation and cellularization characteristics of 2-ethylfuran (2-EF) liquid biofuel were investigated. The study was based on experiments in a constant volume combustion chamber (CVCC) under different initial temperatures (373, 403, and 433 K), initial pressures (1, 2, and 4 bar), and equivalence ratios (0.7–1.4). The spectrum analysis of the explosion pressure showed that the amplitude of pressure fluctuations was more intense with an increase in the equivalence ratio and initial pressure. Moreover, a three-dimensional reconstruction algorithm was constructed to analyze the evolution of cellular structures. It was found that both crack expansion and cell formation caused the irregular evolution of the flame front, which was more obvious at high equivalence ratios and initial pressures. Finally, the flame’s pressure fluctuation before reaching the quartz window was analyzed and combined with flame cellularization, which showed that the early pressure curve and cellularization parameters (cell number and crack length) had similar fluctuations, and the amplitude of the pressure fluctuation was influenced mainly by the initial pressure and the extent of flame cellularization.

Formation and Evolution of Cellular Structures in Wedge-Induced Oblique Detonations

Numerical simulation of an oblique detonation induced by a wedge is performed to investigate the formation and evolution of the oblique cellular detonation structure and the quantitative comparison of the cellular structure of a normal and an oblique detonation. The compressible reactive Euler equations are solved using the seventh-order WENO scheme on an adaptive mesh based on the open source program Adaptive Mesh Refinement in Object-oriented C++ (AMROC). The numerical results show that there are two sets of transverse waves, the left-running transverse waves (LRTW) and the right-running transverse waves (RRTW), which form the oblique cellular detonation structure. Although both sets of transverse waves are convected downstream, they propagate at almost the same relative velocity in the opposite direction. The LRTWs start in the transition zone because of the detonation instability. However, the RRTWs form due to the interaction between the deformed detonation front and the reflected shock wave from the wedge. For the same degree of overdrive, numerical simulations reveal that the characteristic cell size of an oblique detonation is almost the same as that of a normal detonation.

On the origin and evolution of cellular structures in CoCrFeMnNi high entropy alloy fabricated by laser powder bed fusion

On the origin and evolution of cellular structures in CoCrFeMnNi high entropy alloy fabricated by laser powder bed fusion

Bimodal Cellular Structure Evolution in PBAT Foams Incorporated by Carbon Nanotubes and Graphene Nanosheets

Bimodal Cellular Structure Evolution in PBAT Foams Incorporated by Carbon Nanotubes and Graphene Nanosheets

A general mechanism of grain growth ─Ⅰ. Theory

The behaviors of grain growth dominate the formation of the microstructure inside polycrystalline materials and thus strongly influence their practical performances. However, grain growth behaviors still remain ambiguous and thus lack a mathematical formula to describe the general evolution despite decades of efforts. Here, we propose a new migration model of grain boundary (GB) and further derive a mathematical expression to depict the general evolution of grain growth in the cellular structures. The expression incorporates the variables influencing growth rate (e.g. GB features, grain size and local grain size distribution) and thus reveals how the normal, abnormal and stagnant behaviors of grain growth occur in polycrystalline systems. In addition, our model correlates quantitatively GB roughening transition with grain growth behavior. The general growth theory may provide new insights into the GB thermodynamics and kinetics during the cellular structure evolution.

Enhanced thermal stability of the cellular structure through nano-scale oxide precipitation in 3D printed 316L stainless steel

• The thermal stability of the cellular structure in the 316L stainless steel could be improve by the nano-scale oxide precipitation. • Oxides at different heat treatment temperature were identified. • The relationship between the evolution of cellular structure and the oxide precipitates was investigated. The cellular structure is one of the reasons for the improvement of 316 L performance which fabricated by additive manufacturing technology. However, the cellular structure tends to disappear at high temperatures. In this study, oxide nanoparticles were introduced into the 316 L matrix to improve the thermal stability of the cellular structure, this material was successfully fabricated by mechanical alloying followed with laser-powder-bed-fusion (LPBF). The thermal stability of the cellular structure in 316 L was tested with isothermal heat treatment at 1073 K, 1173 K and 1273 K, respectively. The microstructural stability of the cellular was characterized with scanning electron microscopy, transmission electron microscopy (TEM), electron backscatter diffraction, X-ray diffraction measurements and Vickers microhardness testing. The cellular structure was stable at 1073 K due to the dispersed precipitates pining the dislocations at its boundary. The chemical composition of the oxide precipitates with sizes in the range of 10–200 nm was examined by TEM combined with an energy dispersive spectrometry. This work provided a method to enhance the thermal stability of the microstructure of the additively manufactured stainless steel 316 L.

Experimental investigation on the effect of equivalence ratio on the development of cellular structure of E30-air mixture

Ethanol is a renewable, clean fuel and is usually blended with gasoline to be used as an alternative fuel for internal combustion engines. In the present study, the instability development and cellular structure evolution of a spherically expanding flame of E30 (gasoline blended with ethanol of 30% liquid volume) are investigated. By processing the flame images, the equivalent diameters of cells on the flame surface are calculated to describe and explain the development of the cellular structure. The results show that with the propagation of flame, the size of the cellular structure generally increases first and then decreases. Eventually, the mean equivalent diameter of cells tends to a constant value, and these small cells are uniformly distributed over the entire flame surface. The effect of equivalence ratio on the flame instability is studied as well. As the equivalence ratio increases, the cellular structure development is advanced and the cell size is smaller, which indicates that the flame instability is enhanced. The critical wave number is almost the same, while the critical Peclet number approximately followed a linear decrease. The experimental measurement of the sensitivity of Pecr to equivalence ratio was verified by the theoretical results and the linear equation is proposed.

Velocity fluctuation and cellular structure of near-limit detonations in rough tubes

Detonation limits are characterized by a decrease in the propagation velocity, cellular structures to lower unstable modes and an increase in the velocity fluctuation of the detonation. The increase in the average velocity deficit as the limits are approached is not a sensitive change since the failure of the detonation can occur at a relatively small velocity deficit of the order of 20%. A more sensitive indication of the onset of detonation limits is the lowering of the unstable mode (i.e., towards single-headed spin) and the large longitudinal fluctuation of the detonation velocity. In this paper, recent results are reported for the aforementioned near-limit detonation characteristics for a number of detonable mixtures and tube diameters for both smooth and rough tubes. Mixtures include H2, C2H2, C3H8, CH4 fuels with both O2 or N2O as oxidizers. Tube diameters were 25.4 mm, 38.1 mm, 50.8 mm and 76.2 mm. To investigate the effect of wall roughness on the limits phenomena in tubes, wire spirals with different diameters were inserted into the different diameter test tubes. Regularly spaced photodiodes (IF-950C) along the tube were used for velocity measurements and smoked mylar foils were inserted into the tube for the measurement of the cellular structure. Results confirm that the cellular structure evolution towards the lower unstable modes follows well the observed increase in velocity fluctuation; the subsequent detonation failure defined by the absence of cells occurs also at high-velocity fluctuation and an abrupt increase in the average velocity deficit.

Numerical Studies on Propagation Mechanisms of Gaseous Detonations in the Inhomogeneous Medium

Numerical simulation of propagation mechanisms of gaseous detonations in the inhomogeneous medium is studied by using the reactive Euler equations coupled with a two-step chemical reaction model. The inhomogeneity is generated by placing artificial temperature perturbations with different wavelengths and amplitudes. The motivation is to investigate the effect of artificial perturbations on the evolution or amplification of cellular instability. The results show that, without artificial perturbations, a planar ZND detonation can evolve into a fully-developed cellular detonation after a distance because of the amplification of the cellular instability. With the artificial perturbations evolved in, at the early stage, the artificial perturbations control the transverse wave spacing by suppressing the amplification of the cellular instability. However, after a steady-state, the cellular instability starts to amplify itself again and eventually transits to a fully-developed cellular detonation. It is demonstrated that the presence of the artificial perturbations delays the formation of the cellular detonation, and the increase of instability factor can slow down this delay. It is also found that, if the wavelength of the artificial perturbations is close to the transverse wave spacing of the cellular detonation in the homogeneous medium, synchronization of these two factors occurs, and hence a cellular detonation with extremely regular cell pattern is immediately formed. The temperature discontinuity causes the front to be more turbulent with the presence of weak triple-wave structure locally besides the natural transverse waves. The artificial perturbations can increase the intrinsic instability, and hence changes the propagation mechanism of the detonation front. In contrast, large artificial perturbations could prohibit the propagation but reduce cellular instability. It is concluded that the competition of artificial perturbations with intrinsic detonation instability dominates the evolution of cellular structures of the detonation front.

Detonation propagation over a wedge with two channels in a duct

An experimental study was performed in a rectangular duct to investigate the detonation diffraction process in stoichiometric methane-oxygen mixtures. A wedge was placed in the duct to form two channels between the wedge and the channel walls, i.e., an upper channel and a divergence channel. The upper channel heights were 27mm, 47mm and 67mm, and the deviation angles (α) were 60 ° and 30 °. On the bottom wall and side wall, soot foils were placed to record the cellular structure evolution. For the larger deviation angle (α =60 °), the re-initiation mechanisms can be varied, which are affected by the upper channel height. Two overdriven detonation regions are generated behind the wedge on the bottom wall as well as the side wall for more sensitive mixtures (characterized by smaller detonation cell sizes). However, for insensitive mixtures, the re-initiation process seems to be more complicated. More regions containing finer cells can be observed on the foils. In case of α =30 °, the detonation re-initiates at the divergence wall and propagates toward the side wall. Universally, the re-initiation is facilitated as the upper channel height increases, and equivalently, the re-initiation distance is reduced.

Effects of bluff bodies on the propagation behaviors of gaseous detonation

In this study, we report an experimental investigation of detonation propagation in a tube filled with bluff bodies. Experiments were carried out in a 6 m long rectangular cross-section (112 mm × 107 mm) tube. Firstly, the effect of an array of cube bodies (60 mm × 60 mm × 60 mm) on the detonation propagation characteristics were studied. Hydrogen, ethylene and acetylene mixed with air and hydrogen-oxygen diluted with argon were used as the test mixtures. Evenly spaced photodiodes were mounted on the top wall to recorded the optical signals, from which the detonation velocity could be determined. Soot foils were adopted in hydrogen-oxygen-argon mixture to record the evolution of the cellular structure. The results show that the flame accelerates rapidly in the obstructed tube. The critical conditions for deflagration to detonation transition (DDT) are found to be consistent with L/λ > 7, where L is the modified characteristic geometrical size for the tube with repeated cubes and λ is the detonation cell size. The long soot foils indicate that the detonation modes are influenced significantly by the bluff bodies. Near the limits, only some traces of transverse waves and shock waves can be observed, making the distinction between choked flame and quasi-detonation blurred. Secondly, the effect of a single bluff body on the detonation diffraction was investigated. Stoichiometric hydrogen-oxygen and those diluted with argon at sub-atmospheric pressures were used as the test mixtures. In the vicinity of the bluff body, a soot foil was used to record the detonation cellular structure evolution, from which the effect of the geometry of the bluff body on the diffraction and re-initiation processes was studied. Three transition regimes are observed: (1) a detonation propagates continuously over the bluff body; (2) the cellular structure initially fails and then recover due to the two symmetrical diffraction waves collision or interaction between shock waves and the bottom wall; (3) a detonation quenches and then re-initiate by flame acceleration. The re-initiation distance was found to be dependent on mixture sensitivity (chemical length scale) and the geometry of the bluff bodies (physical length scale).

Complex cellular structure evolution of polystyrene/poly (ethylene terephthalate glycol-modified) foam using a two-step depressurization batch foaming process

In order to decrease the cell size and maintain very high volume expansion ratio simultaneously, a methodology for the preparation of complex cellular structure (CCS) in polystyrene/poly(ethylene terephthalate glycol-modified) (PS/PETG) blend using two-step depressurization pressure batch foaming process was proposed. First, the optimum foaming temperature for PS and PS/PETG blend, respectively, were confirmed by one-step depressurization foaming process. Then, the CCS in PS and PS/PETG blending foam were fabricated by two-step depressurization foaming process at the optimum foaming temperature. The rheological properties of PS and PS/PETG blend were tested by dynamic rotational rheometer. The dispersion morphologies and foam morphologies were observed by scanning electron microscope. The lowest densities of PS and PS/PETG blending foams were obtained at the temperature of 136℃. The interfaces between PS and PETG could act as nucleation sites for the cell nucleation, which were helpful to fabricate the CCS. The CCS could be controlled by tuning the degree of the first-step depressurization and the holding time. The results showed that the large cells could be beneficial to decrease the foam density and the presence of small cells was beneficial to increase the cell number.

Growth and setting of gas bubbles in a viscoelastic matrix imaged by X-ray microtomography: the evolution of cellular structures in fermenting wheat flour dough.

X-ray tomography is a relevant technique for the dynamic follow-up of gas bubbles in an opaque viscoelastic matrix, especially using image analysis. It has been applied here to pieces of fermenting wheat flour dough of various compositions, at two different voxel sizes (15 and 5 μm). The resulting evolution of the main cellular features shows that the creation of cellular structures follows two regimes that are defined by a characteristic time of connectivity, tc [30 and 80 min]: first (t ≤ tc), bubbles grow freely and then (t ≥ tc) they become connected since the percolation of the gas phase is limited by liquid films. During the first regime, bubbles can be tracked and the local strain rate can be measured. Its values (10(-4)-5 × 10(-4) s(-1)) are in agreement with those computed from dough viscosity and internal gas pressure, both of which depend on the composition. For higher porosity, P = 0.64 in our case, and thus occurring in the second regime, different cellular structures are obtained and XRT images show deformed gas cells that display complex shapes. The comparison of these images with confocal laser scanning microscopy images suggests the presence of liquid films that separate these cells. The dough can therefore be seen as a three-phase medium: viscoelastic matrix/gas cell/liquid phase. The contributions of the different levels of matter organization can be integrated by defining a capillary number (C = 0.1-1) that makes it possible to predict the macroscopic dough behavior.

Evolution of cellular structures on oblique detonation surfaces

In this study, numerical simulations using the inviscid Euler equations with one-step Arrhenius chemistry model are carried out to investigate the effects of activation energy and wedge angle on the stability of oblique detonation surfaces. Two kinds of cellular structure are studied, one is featured by a single group of transverse waves traveling upstream, referred to as LRTW (left-running transverse waves), and the other is featured by additional RRTW (right-running transverse waves). The present computational simulation reveals the formation of un-reacted gas pockets behind the cellular oblique detonation. Numerical smoked foil records are produced to show the emergence of the two types of transverse waves and the evolution of the unstable cellular structure of the oblique detonation. The transverse wave dynamics, including the colliding, emerging and splitting types, are found to be similar to the normal detonation propagation, demonstrating the instability mechanism is originated from the inherent instability of cellular detonations. Statistical analysis on the cellular structure is carried out to observe quantitatively the influences of activation energy and wedge angle. Results from the parametric study show that high activation energy and low wedge angle are favorable to the LRTW formation. However, the condition for the RRTW formation is more complex. In the case of low activation energy, small wedge angle is beneficial to the RRTW formation, as to the LRTW formation. In contrary, for high activation energy, there appears one moderate wedge angle favoring the RRTW formation and giving the shortest length between the onset of both LR and RR transverse waves. For quantitative comparison, we analyze the variation of two distances with the wedge angle, one is between the detonation initiation and LRTW formation points, and the other between LRTW and RRTW formation points. Results show the latter is relatively less pronounced than the former, indicating the RRTW formation depends mainly on the activation energy and the generation of LRTW.

Evolution of cellular structures duringGe1−xSixsingle-crystal growth by means of a modified phase-field method

We have studied the evolution of cellular structures in Ge 1-x Si x single-crystal growth as a function of process parameters. Because these structures are much larger than those occurring during the solidification of metals, we developed a modified phase-field method, which is able to handle these structure within reasonable computer times using the real material parameters. The model has been tested for computing equilibrium shapes of crystals, dendritic growth, and cellular growth of Ni x Cu 1-x. We also performed classical molecular dynamics calculations in order to compute the diffusion coefficients of Si and Ge in melts of various compositions.

Modeling Mesoscale Cellular Structures and Drizzle in Marine Stratocumulus. Part I: Impact of Drizzle on the Formation and Evolution of Open Cells

Abstract A new modeling framework is used to investigate aerosol–cloud–precipitation interactions and dynamical feedbacks at the mesoscale. The focus is on simulation of the formation and evolution of cellular structures that are commonly seen in satellite images of marine stratocumulus clouds. Simulations are performed at moderate resolution in a 60 × 60 km2 domain for 16 h to adequately represent the mesoscale organization associated with open cells and precipitation. Results support the emerging understanding that precipitation plays a critical role in the formation and evolution of open cells. Evaporation of raindrops generates a dynamic response that manifests itself in cellular organization of updrafts and downdrafts and promotes and sustains the formation of an open cellular structure in cloud fields. Vertical motion in open-cell centers with thin clouds is minimal. It is shown that a mean surface rain rate as low as 0.02 mm day−1 is, for the case considered, sufficient to promote the formation of open cells. The maximum dimension of individual open cells ranges between 5 and 30 km. Individual cells grow at a mean rate of between 5 and 10 km h−1. Irregularity in the shape of open cells is caused by formation of new precipitating regions at the cell walls and interference with neighboring cells, which erode, and eventually eliminate, the old cells. The typical lifetime of large individual open cells is about 2 h, close to that observed by radar, although a collection of open cells as a whole may last for tens of hours.

Growth kinetics by means of phase-field methods in applied crystal growth

We have studied the evolution of cellular structures in GeSi single crystal growth as a function of process parameters. Because these structures are much larger than those occurring during the solidification of metals, we developed a modified phase-field method, which is able to handle these structure within reasonable computer times using the real material parameters.

Protein Expression in Liposomes

Compartmentalization is one of the key steps in the evolution of cellular structures and, so far, only few attempts have been made to model this kind of “compartmentalized chemistry” using liposomes. The present work shows that even such complex reactions as the ribosomal synthesis of polypeptides can be carried out in liposomes. A method is described for incorporating into 1-palmitoyl-2-oleoyl-sn-3-phosphocholine (POPC) liposomes the ribosomal complex together with the other components necessary for protein expression. Synthesis of poly(Phe) in the liposomes is monitored by trichloroacetic acid of the 14C-labelled products. Control experiments carried out in the absence of one of the ribosomal subunits show by contrast no significant polypeptide expression. This methodology opens up the possibility of using liposomes as minimal cell bioreactors with growing degree of synthetic complexity, which may be relevant for the field of origin of life as well as for biotechnological applications.