Anisotropy Of Interfacial Energy Research Articles

Orientation dependent spheroidization response and macro-zone formation during sub β-transus processing of Ti-6Al-4V alloy

In the present study, sub β-transus static annealing was carried out on a (α+β)-warm-rolled (700 °C, 90% thickness reduction) Ti-6Al-4V alloy at 900 °C for various durations (15, 30, 45, 60, 75 and 90 min). The key observation was that the spheroidization response of the constituent α-colonies from the starting warm-rolled material differs significantly during subsequent (α+β)-annealing treatment. While some of the α-colonies underwent an early morphological conversion from lamellar to equiaxed, some others stayed stable for prolonged durations. This unique phenomenon has been examined, for the first time, from an orientation perspective by coupling slip activation, boundary formation, and interfacial energy anisotropy of individual α-colonies with their response to static spheroidization. Orientation of the α-colonies in reference to the loading directions dictate the nature of slip activation (single versus multiple slip; basal or prism <a> slip plus pyramidal 〈c+a〉 slip) during prior (α+β)-rolling. During subsequent static (α+β)-annealing, this factor translates into the relative ease of boundary splitting and thermal grooving for them. Formation of longitudinal boundaries, then set the order of spheroidization for diffusion based coarsening processes during long term annealing. Anisotropy in interfacial energy, owing to the loss of coherency during prior deformation (warm-rolling), creates further orientation dependency in the spheroidization sequence. An immediate consequence of this orientation dependent spheroidization response is realized in the formation of the macro-zones for two-phase Titanium alloys after secondary thermomechanical processing. Some remedial processing strategies are highlighted therein.

A review: applications of the phase field method in predicting microstructure and property evolution of irradiated nuclear materials

Complex microstructure changes occur in nuclear fuel and structural materials due to the extreme environments of intense irradiation and high temperature. This paper evaluates the role of the phase field method in predicting the microstructure evolution of irradiated nuclear materials and the impact on their mechanical, thermal, and magnetic properties. The paper starts with an overview of the important physical mechanisms of defect evolution and the significant gaps in simulating microstructure evolution in irradiated nuclear materials. Then, the phase field method is introduced as a powerful and predictive tool and its applications to microstructure and property evolution in irradiated nuclear materials are reviewed. The review shows that (1) Phase field models can correctly describe important phenomena such as spatial-dependent generation, migration, and recombination of defects, radiation-induced dissolution, the Soret effect, strong interfacial energy anisotropy, and elastic interaction; (2) The phase field method can qualitatively and quantitatively simulate two-dimensional and three-dimensional microstructure evolution, including radiation-induced segregation, second phase nucleation, void migration, void and gas bubble superlattice formation, interstitial loop evolution, hydrate formation, and grain growth, and (3) The Phase field method correctly predicts the relationships between microstructures and properties. The final section is dedicated to a discussion of the strengths and limitations of the phase field method, as applied to irradiation effects in nuclear materials.

Degradation of perpendicular magnetic anisotropy in CoFeB film after H2 plasma irradiation

The degradation of the perpendicular magnetic anisotropy (PMA) of a Ta cap/MgO/CoFeB stack film after H2 plasma irradiation was investigated to clarify the magnetic damage induced during the hydrogen-containing plasma etching (e.g., CO/NH3 or CH3OH plasma etching). When the CoFeB layer thickness was 1.2 nm, the out-of-plane coercivity decreased from 1.0 to 0.15 mT after H2 plasma irradiation, and the perpendicular anisotropy field also decreased, from 220 to 90 mT. The dependence of the reduction in the PMA on the CoFeB thickness revealed that the interfacial anisotropy energy was reduced from 0.98 to 0.79 mJ/cm2 after H2 plasma irradiation. These results clearly indicate that hydrogen-containing plasma etching has the potential to degrade the PMA during MgO/CoFeB film patterning.

Perpendicular magnetic anisotropy of CoFeB\Ta bilayers on ALD HfO2

Perpendicular magnetic anisotropy (PMA) is an essential condition for CoFe thin films used in magnetic random access memories. Until recently, interfacial PMA was mainly known to occur in materials stacks with MgO\CoFe(B) interfaces or using an adjacent crystalline heavy metal film. Here, PMA is reported in a CoFeB\Ta bilayer deposited on amorphous high-κ dielectric (relative permittivity κ=20) HfO2, grown by atomic layer deposition (ALD). PMA with interfacial anisotropy energy Ki up to 0.49 mJ/m2 appears after annealing the stacks between 200°C and 350°C, as shown with vibrating sample magnetometry. Transmission electron microscopy shows that the decrease of PMA starting from 350°C coincides with the onset of interdiffusion in the materials. High-κ dielectrics are potential enablers for giant voltage control of magnetic anisotropy (VCMA). The absence of VCMA in these experiments is ascribed to a 0.6 nm thick magnetic dead layer between HfO2 and CoFeB. The results show PMA can be easily obtained on ALD high-κ dielectrics.

Interfacial free energy and stiffness of aluminum during rapid solidification

Using molecular dynamics simulations and the capillary fluctuation method, we have calculated the anisotropic crystal-melt interfacial free energy and stiffness of aluminum in a rapid solidification system where a temperature gradient is applied to enforce thermal non-equilibrium. To calculate these material properties, the standard capillary fluctuation method typically used for systems in equilibrium has been modified to incorporate a second-order Taylor expansion of the interfacial free energy term. The result is a robust method for calculating interfacial energy, stiffness and anisotropy as a function of temperature gradient using the fluctuations in the defined interface height. This work includes the calculation of interface characteristics for temperature gradients ranging from 11 to 34 K/nm. The captured results are compared to a thermal equilibrium case using the same model and simulation technique with a zero gradient definition. We define the temperature gradient as the change in temperature over height perpendicular to the crystal-melt interface. The gradients are applied in MD simulations using defined thermostat regions on a stable solid-liquid interface initially in thermal equilibrium. The results of this work show that the interfacial stiffness and free energy for aluminum are dependent on the magnitude of the temperature gradient, however the anisotropic parameters remain independent of the non-equilibrium conditions applied in this analysis. The relationships of the interfacial free energy/stiffness are determined to be linearly related to the thermal gradient, and can be interpolated to find material characteristics at additional temperature gradients.

Seaweed eutectic-dendritic solidification pattern in a CoCrFeNiMnPd eutectic high-entropy alloy

A CoCrFeNiMnPd eutectic high-entropy alloy was designed and prepared. A unique eutectic-dendritic solidification pattern was observed in the as-cast alloy. Heretofore, this kind of pattern was rarely reported no matter in binary or multi-component alloys. Through a series of experimental characterization and theoretical analysis, it could be concluded that the sluggish diffusion effect of high-entropy alloys makes the interface deviate considerably from the local non-equilibrium condition and the alternate arrangement of CoCrFeNiPd-rich FCC and Tetragonal Mn7Pd9 lamellae results in weak interface energy anisotropy, thus forming the seaweed eutectic-dendrite. Application of the designing rules of high-entropy alloys to the current work indicates that mere consideration of the weighted average physical properties of constituent elements is not sufficient to the design of high-entropy alloys with a single solid-solution phase.

Eutectic colony formation in systems with interfacial energy anisotropy: A phase field study

Instability of a binary eutectic solidification front to morphological perturbations due to rejection of a ternary impurity leads to the formation of eutectic colonies. Whereas, the instability dynamics and the resultant microstructural features are reasonably well understood for isotropic systems, several experimental observations point to the existence of colonies in systems with anisotropic interfaces. In this study, we extend the understanding of eutectic colonies to anisotropic systems, where certain orientations of the solid-liquid or solid-solid interfaces are associated with a lower free energy than the others. Through phase field simulations in 2D and 3D, we have systematically probed the colony formation dynamics and the resulting microstructures, as functions of the pulling velocity and the relative orientation of the equilibrium interfaces with that of the imposed temperature gradient. We find that in 2D, stabler finger spacings are selected with an increase in the magnitude of anisotropy introduced, either in the solid-liquid or in the solid-solid interface. The fingers have a well-defined orientation for the case of anisotropy in the solid-liquid interface, with no fixed orientations for the lamellae constituting the colony. For the case where anisotropy exists in the solid-solid interface, the lamellae tend to orient themselves along the direction of the imposed temperature gradient, with tilted solid-liquid interfaces from the horizontal. The 3D simulations reveal existence of eutectic spirals which might become tilted under certain orientations of the equilibrium interfaces. Our simulations are able to explain several key features observed in our experimental studies of solidification in Ni-Al-Zr alloy.

Formation mechanism of PbTe dendritic nanostructures grown by electrodeposition

The formation mechanism of PbTe dendritic nanostructures grown at room temperature by electrodeposition in nitric acid electrolytes containing Pb and Te was investigated. Scanning electron microscopy and transmission electron microscopy analyses indicated that the PbTe dendritic nanostructures were composed of triangular-shaped units surrounded by {111} and {110} planes. Because of the interfacial energy anisotropy of the {111} and {110} planes and the difference in the current density gradient, the growth rate in the vertical direction of the (111) basal plane was slower than that in the direction of the tip of the triangular shape, leading to growth in the tip direction. In contrast to the general growth direction of fcc dendrites, namely <100>, the tip direction of the {111} basal plane for our samples was <112>, and the PbTe dendritic nanostructures grew in the tip direction. The angles formed by the main trunk and first branches were regular and approximately 60°, and those between the first and second branches were also approximately 60°. Finally, the nanostructures grew in single-crystalline dendritic form.

Solidification behavior of Co-Sn eutectic alloy with Nb addition

(Co76Sn24)100-xNbx (x = 0, 0.5, 0.8, 1.0) eutectic alloy melts were solidified at small undercooling for investigating the effect of Nb addition on microstructural development. With increasing Nb content, the solidification interface transits from eutectic seaweed (x = 0) to eutectic dendrite (x = 0.5) as a response to the change in the anisotropy of interfacial energy. Coupled eutectic growth can no longer be maintained within the main stems at x = 0.8 because the enrichment of Nb in the liquid ahead of the interface causes a significant difference in growth velocity between the α-Co and β-Co3Sn2 phases and, as such, Co3Sn2 doublons form. For x = 1.0, the difference in growth kinetics between the two eutectic phases is so large that divorced eutectic growth takes place on a large scale. Letting the undercooling prior to solidification increase, the eutectic interface morphology of (Co76Sn24)99.5Nb0.5 eutectic alloys returns from dendritic pattern back to factual seaweed pattern at more than 38 K undercooling and then transits to compact seaweed pattern at more than 181 K undercooling. The eutectic growth velocity slightly increases at low and intermediate undercooling but obviously decreases at large undercooling due with Nb addition.

Effect of Nb on the Growth Behavior of Co3Sn2 Phase in Undercooled Co-Sn Melts

The growth behavior of the primary β-Co3Sn2 phase in (Co67Sn33)100−x Nb x (x = 0, 0.5, 0.8, 1.0) hypereutectic alloys at different melt undercoolings was investigated systematically. The growth pattern of the β-Co3Sn2 phase at low undercooling changes with the Nb content from fractal seaweed (x = 0, 0.5) into dendrite (x = 0.8) and then returns to fractal seaweed (x = 1.0) as a response to the changes in interface energy anisotropy and interface kinetic anisotropy. As undercooling increases, the dendritic growth of the β-Co3Sn2 phase in (Co67Sn33)99.2Nb0.8 alloy gives way to fractal seaweed growth at an undercooling of 32 K (−241 °C). At larger undercooling, the fractal seaweed growth is further replaced by compact seaweed growth, which occurred in the other three alloys investigated. The growth velocity of the β-Co3Sn2 phase slightly increases at low and intermediate undercooling but clearly decreases at larger undercooling due to the Nb addition. The growth velocity sharply increases as the growth pattern of the Co3Sn2 phase transits from fractal seaweed into compact seaweed.

Phase Field Simulations of Autocatalytic Formation of Alpha Lamellar Colonies in Ti-6Al-4V

We present phase field simulations incorporating contributions due to chemical free energy, anisotropic interfacial energy, and elastic energy due to transformation strain, to demonstrate the nucleation and growth of multiple variants of alpha from undercooled beta in Ti-6Al-4V under isothermal conditions. A new composite nucleation seeding approach is used within the phase field simulations to demonstrate that the presence of a pre-existing strain field can cause the nucleation of specific crystallographic variants of alpha based on minimization of local elastic strain energy. Under conditions where specific combinations of elastic strains exist, for example in the vicinity of one or more pre-existing alpha variants, the nucleation of a new alpha variant is followed by the successive nucleation of the same variant in the form of a lamellar colony by an autocatalytic mechanism. At a given thermodynamic undercooling, the colony structure was favored at a nucleation rate that was low enough to allow sufficient growth of previously nucleated variants before another nucleus formed in their vicinity. Basket weave morphology was formed at higher nucleation rates where multiple nuclei variants grew almost simultaneously under evolving strain fields of several adjacent nuclei.

Observations of fractal patterns induced on surface of chalcogenide glass

Properties of chalcogenide glasses were studied. After heat treatment, fractal patterns were found on surfaces of Ge23Se67Sb10 and Ge30Se60Sb10 glasses. According to box counting method, the fractal dimensions are determined as 1.58–1.61, which are similar to that of the fractal crystal GeSe2. By analysis, the main factors for the formation of the fractal patterns are the glass structure and the weak interface energy anisotropy. Comparing the fractal pattern with compact seaweed pattern in undercooled Co-Sn melt, the growth mechanism of the fractal pattern in glass is similar to that in undercooled melt, i.e. the larger undercooling leads to the weaker of the interfacial energy anisotropy, which leads to the formation of fractal structure.

Smectic-like bâtonnets in nematic/twist-bend nematic biphasic samples

ABSTRACTWe have studied a mixture of the twist-bend nemogenic dimer CB7CB with rod-like nematic molecules, which exhibits nematic (N) and twist-bend nematic (NTB) phases and a very large biphasic coexistence range. At the N-NTB transition, we observe the nucleation of highly anisometric NTB droplets which are very similar to the classic smectic A (SmA) bâtonnets. These observations confirm the recently proposed close analogy between the NTB and SmA phases, on the basis of their identical macroscopic symmetry. As for their smectic analogues, the NTB bâtonnets are fluid in two dimensions; they easily merge when brought into contact and they are solid-like in that they did not flow along their optic axis. The observed fluidity and low viscosity show that the NTB phase is indeed a nematic phase, i.e. an anisotropic fluid, rather than a soft crystal or glassy state. Unlike their smectic analogues, the NTB bâtonnets have almost perfect symmetry of revolution and the axis of the NTB helix is uniformly aligned parallel to the long axis of the bâtonnet. The large aspect ratio of the bâtonnets, typically ≈ 10–30, indicates a very strong anisotropy of the N-NTB interfacial energy, W2/W0 ≈ 200–2000, and suggests that the anchoring energy differs from the usual Rapini–Papoular form.

Domain Architectures and Grain Boundaries in Chemical Vapor Deposited Highly Anisotropic ReS2 Monolayer Films.

Recent studies have shown that vapor phase synthesis of structurally isotropic two-dimensional (2D) MoS2 and WS2 produces well-defined domains with clean grain boundaries (GBs). This is anticipated to be vastly different for 2D anisotropic materials like ReS2 mainly due to large anisotropy in interfacial energy imposed by its distorted 1T crystal structure and formation of signature Re-chains along [010] b-axis direction. Here, we provide first insight on domain architecture on chemical vapor deposited (CVD) ReS2 domains using high-resolution scanning transmission electron microscopy, angle-resolved nano-Raman spectroscopy, reflectivity, and atomic force microscopy measurements. Results provide ways to achieve crystalline anisotropy in CVD ReS2, establish domain architecture of high symmetry ReS2 flakes, and determine Re-chain orientation within subdomains. Results also provide a first atomic resolution look at ReS2 GBs, and surprisingly we find that cluster and vacancy defects, formed by collusion of Re-chains at the GBs, dramatically impact the crystal structure by changing the Re-chain direction and rotating Re-chains 180° along their b-axis. Overall results not only shed first light on domain architecture and structure of anisotropic 2D systems but also allow one to attain much desired crystalline anisotropy in CVD grown ReS2 for the first time for tangible applications in photonics and optoelectronics where direction-dependent dichroic and linearly polarized material properties are required.

Anomalous α-Mg Dendrite Growth During Directional Solidification of a Mg-Zn Alloy

Abstract Dendritic morphology was investigated in a directionally solidified magnesium-zinc alloy using synchrotron X-ray tomography and electron backscattered diffraction. Unexpectedly, primary dendrites grew along $$ {\langle}21\overline{3}1 {\rangle}$$ ⟨ 21 3 ¯ 1 ⟩ , rather than the previously reported $${\langle} 11\overline{2} 0{\rangle} $$ ⟨ 11 2 ¯ 0 ⟩ and $${\langle} 22\overline{4}5 {\rangle}$$ ⟨ 22 4 ¯ 5 ⟩ directions. Further, seven asymmetric sets of side branches formed, instead of six-fold symmetric arms, evolving with three coexisting morphologies per trunk of: traditional, seaweed structure, and free growth. The anomalous growth is attributed to the imposed thermal gradient and zinc-induced interfacial energy anisotropy variations.

Evolution of the interfacial magnetic anisotropy in MgO/CoFeB/Ta/Ru based multilayers as a function of annealing temperature

We report the effect of annealing temperature on the dynamic and static magnetic properties of MgO/CoFeB/Ta/Ru multilayers. Angular resolved ferromagnetic resonance measurement results show that the as-deposited film exhibits in-plane magnetic anisotropy, whereas in the annealed films the magnetic easy-axis is almost along the direction perpendicular to the plane of the layers. The extracted interfacial anisotropy energy, Ki, is maximized at an annealing temperature 225∘C, in agreement with the vibrating sample magnetometry results. Although the magnetization is not fully out-of-plane, controlling the degree of the magnetization obliqueness may be advantageous for specific applications such as spin-transfer oscillators.

Pattern formation during interfacial reaction in-between liquid Sn and Cu substrates – A simulation study

Heteroepitaxial formation of η-Cu6Sn5 intermetallic compound (IMC) on unidirectional Cu has been found to exhibit remarkable microstructural features and provide opportunities for microstructure control of microelectronic interconnects, but the crystallographic origin and formation mechanism of such morphological patterns are still unclear. Here we investigate pattern formation during interfacial reactions in between liquid Sn and two types of unidirectional Cu substrates, (001) and (111), by using a combination of crystallographic analysis, theory of microelasticity and phase-field simulations. The simulation results show that each symmetry-related η-Cu6Sn5 variant grows along the direction of best atomic site matching with the substrates. No spatial alignment can be achieved when the growth takes place without heteroepitaxial constraints. In addition, anisotropy in interfacial energy between the liquid Sn and IMC is found to lead to faceted morphology. Further, the habit planes of η-Cu6Sn5 precipitates and kinematic compatibility across the interfaces are analyzed theoretically and found to be consistent with the simulations. These simulation predictions demonstrate excellent agreements with experimental observations.

Microstructure and orientation evolution in unidirectional solidified Al–Zn alloys

Morphological instability and growth orientation evolution during unidirectional solidification of Al–Zn alloys with different pulling speeds were investigated by X-ray diffraction (XRD) and electron back-scatter diffraction (EBSD) in scanning electron microscope (SEM). The experimental results show that, as the pulling speed increases, the primary dendrite spacing becomes smaller gradually and dendrite trunks incline to the heat flow direction perfectly in unidirectional solidified Al–9.8wt%Zn and Al–89wt%Zn alloys. However, regardless of the pulling speed in unidirectional solidified Al–Zn alloys under fixed thermal gradient, the regular dendrites with <100> directions of primary trunks and secondary arms in 9.8wt% Zn composition are replaced by <110> dendrites of primary trunks and secondary arms in 89wt% Zn composition. In unidirectional solidified Al–32wt% Zn alloy, cellular, fractal seaweed, and stabilized seaweed structures were observed at high pulling speeds. At a high pulling speed of 1000µm/s, seaweed structures transform to the columnar dendrites with <110> trunks and <100> arms. The above orientation evolution can be attributed to low anisotropy of solid-liquid interface energy and the seaweed structure is responsible for isotropy of {111} planes.

Microstructure of L10 FePt thin films with anisotropic interfacial energy coefficients and anisotropic atomic mobilities

The microstructure evolutions of FePt-X (segregant) thin films were studied by employing a three-dimensional phase field model. Simulated results show that in the absence of substrate constraint related with elastic energy, the morphology of the FePt-X thin films significantly depends on the film thickness, anisotropic interfacial energy coefficients, and anisotropic atomic mobility. Small in-plane anisotropic interfacial energy coefficients or out-of-plane anisotropic mobility of atom diffusion are beneficial to induce columnar shape FePt grains, using any one of parameters of which can make the FePt grains keep columnar shape when FePt-X thin films thickness is 10 nm. Using segregant with both proper in-plane anisotropic interfacial energy coefficients and out-of-plane anisotropic mobility can make the FePt grains keep columnar shape when the FePt-X thin films thickness reaches to at least 15 nm.

Low artificial anisotropy cellular automaton model and its applications to the cell-to-dendrite transition in directional solidification

A low artificial anisotropy cellular automaton (CA) model is developed for the simulation of microstructure evolution in directional solidification. The CA model’s capture rule was modified by a limited neighbor solid fraction (LNSF) method. Various interface curvature calculation methods have been compared. A modified curvature calculation method based on the variation of the unit vector normal (VUVN) to the solid-liquid interface using volume of fluid (VOF) interpolation technique gets better results. The equilibrium shapes were simulated to quantify the artificial anisotropy, when the interface energy anisotropy coefficient is varied from ε=0.0 to ε=0.05. The low artificial anisotropy CA model is used in the numerical simulation of the cell-to-dendrite transition (CDT) in directional solidification. The influence of physical parameters (Г, Dl, k0, ml) on CDT has been investigated. The main finding in this paper is the discovery of the changing behavior of the Vcd when the solute partition coefficient k0 is larger than a critical value. When k0 is less than 0.125, the Vcd follows the Kurz and Fisher criterion Vc/k0; while when k0>0.125, the Vcd equals to 8Vc. The experimental data of succinonitrile-acetone (SCN-ace, k0=0.1) and SCN-camphor (k0=0.33) support the conclusions from CA simulations.