Anisotropic Microstructure Research Articles

Preparation of robust and light‐weight anisotropic polyimide/graphene composite aerogels for strain sensors

AbstractThis study reports the fabrication of a serious of anisotropic composite aerogels comprising polyimide and graphene (PI/G) through a simple and eco‐friendly technique. Poly(amic acid) ammonium salt/graphene dispersion, which was produced through the mixture of poly(amic acid), graphene aqueous dispersion, triethylamine, and water, was utilized in the process of creating PI/G composite aerogels employing unidirectional freezing, freeze‐drying, and thermal imidization. The PI/G aerogels displayed anisotropic mechanical properties, thermal insulation, and electrical conductivity. The PI/G aerogels with an anisotropic microstructure demonstrated notable rigidity and a high compressive modulus (52.7 MPa) along the axial direction, while displaying favorable flexibility and a low modulus (18.2 MPa) along the radial direction. Furthermore, the PI/G aerogel exhibited anisotropic thermal conductivity (axial: 64 mW m−1 K−1; radical: 55 mW m−1 K−1). These properties enable the PI/G aerogel to have a wider range of potential applications, especially strain sensors. The research presented herein establishes a simple and eco‐friendly strategy for the production of anisotropic PI/G‐based composite aerogels.

Effect of single roll drive rolling on microstructure, texture, and mechanical property anisotropy of Al−5.6Zn−2.5Mg−1.4Cu aluminum alloy

The current research aimed to study the microstructure and texture development of the Al−5.6Zn−2.5Mg−1.4Cu alloy during single roll drive rolling process (SRDR). The relationship between mechanical property anisotropy and microstructure and texture was investigated in detail. The SRDR process was immediately performed on the solution-treated sample with four different thickness reductions of 10%, 20%, 40%, and 60%, respectively. Microstructural examination by optical and scanning electron microscopes, texture evaluation by X-ray diffraction, and mechanical properties evaluation by tensile tests were conducted. The results showed that the value of in-plane anisotropy was significantly reduced from 11.6% in solution-treated alloy to 2.4% and 3.1% in 40% and 60% reduction rolled samples, respectively. This was due to the formation of the Rotated Cube component, decreasing the overall texture intensity, and the formation of new grains through dynamic recrystallization. The yield strength in the rolling direction (RD) was higher than that in the transverse direction (TD), which was due to grain morphology and texture in these samples. The severity of Portevin−Le Chatelier (PLC) along the RD (0°) was much lower than that along the HD (45°) and TD (90°) owing to the presence of several strong orientations along the RD. By increasing the thickness reduction, the difference between strain hardening rates of RD, HD, and TD decreased due to the decrement of texture intensity. There was no difference between the fracture surfaces of 60% samples in the RD, HD, and TD, revealing nearly isotropic fracture behavior.

Rapid flapping and fibre-reinforced membrane wings are key to high-performance bat flight.

Bats fly using significantly different wing motions from other fliers, stemming from the complex interplay of their membrane wings' motion and structural properties. Biological studies show that many bats fly at Strouhal numbers, the ratio of flapping to flight speed, 50-150% above the range typically associated with optimal locomotion. We use high-resolution fluid-structure interaction simulations of a bat wing to independently study the role of kinematics and material/structural properties in aerodynamic performance and show that peak propulsive and lift efficiencies for a bat-like wing motion require flapping 66% faster than for a symmetric motion, agreeing with the increased flapping frequency observed in zoological studies. In addition, we find that reduced membrane stiffness is associated with improved propulsive efficiency until the membrane flutters, but that incorporating microstructural anisotropy arising from biological fibre reinforcement enables a tenfold reduction of the flutter energy while maintaining high aerodynamic efficiency. Our results indicate that animals with specialized flapping motions may have correspondingly specialized flapping speeds, in contrast to arguments for a universally efficient Strouhal range. Additionally, our study demonstrates the significant role that the microstructural constitutive properties of the membrane wing of a bat can have in its propulsive performance.

Synergetic strengthening through ultrafine-grained anisotropic microstructure and nanoscale heterogeneity in CoCuFeMnNi high entropy alloy wires

This study investigated the post-drawn annealed CoCuFeMnNi high-entropy wires with distinct dual phase structure and B2 nanoprecipitates. The exceptional hardness observed of the post-drawn annealed CoCuFeMnNi wires can be attributed to the synergetic effect of the ultrafine-grained (UFG) structure and nanoscale precipitation/heterogeneity in the post-drawn annealed CoCuFeMnNi wires.

Relationship among grain morphology, texture and sliding wear resistance of ZrB2/316L SS composites processed by SLM

Pure 316L stainless steel and 5 wt% ZrB2-reinforced 316L stainless steel were prepared by selective laser melting (SLM) in this paper. As compared to 316L SS, ZrB2/316L SS displays smaller columnar grains as well as less anisotropic microstructure. EBSD results show that the SLM 316L stainless steel exhibits a maximum texture index of 13 for the brass-like texture, while the ZrB2/316L SS composite exhibits a maximum texture index of 7 for the cubic texture. The hardness of the ZrB2/316L SS composite is significantly improved, reaching 415HV, which is much higher than the hardness of 316L SS (215HV). In addition, the wear resistance of ZrB2/316L SS increases, implying broadened applications of the stainless steel composites.

Anisotropy in 3D-printed (FeCoNi)86Al7Ti7 high entropy alloy

Anisotropic microstructure and mechanical properties are vital considerations in the practical applications of 3D-printed metallic materials; however, limited attention has been given to the anisotropy in 3D-printed high entropy alloys (HEAs). This study systematically explores the structural and mechanical anisotropy of (FeCoNi)86Al7Ti7 HEA with low porosity, fabricated via selective laser melting (SLM). The SLM process, characterized by directional and repeated thermal dissipation, creates an anisotropic structure consisting of epitaxial columnar grains aligned parallel to the building direction, subsequently influencing mechanical anisotropy. Tensile testing unveils slight anisotropy in strength but significant differences in ductility for the SLMed HEA. The 0° sample demonstrates the optimal combination of strength and plasticity, boasting a tensile elongation of 31.5 %, approximately 2.6 times higher than that of the 45° and 90° samples. The post-mortem microscopic analysis indicates that this ductility anisotropy primarily originates from the crack propagation behavior. In the 0° sample, crack propagation occurs through both intergranular and transgranular modes, with the robust interactions between the two modes providing an obstruction to crack propagation. On the contrary, crack widening occurs essentially along the continuous melting pool boundaries (MPBs) in 45° and 90° samples, leading to the straight crack propagation path along the MPBs and therefore reduced ductility.

Development of anisotropic Nd-Fe-B powder from isotropic gas atomized powder

This work presents an innovative approach to obtain anisotropic Nd-Fe-B powder from isotropic gas atomized powder. The new process was developed using a ternary Nd-Fe-B alloy, without the requirement for additional heavy rare earth or other critical raw materials. It comprises the following steps: (a) gas atomization to produce a polycrystalline isotropic powder; (b) annealing at high temperature to induce grain growth; (c) hydrogen decrepitation to obtain a monocrystalline powder; and (d) hydrogenation-disproportionation-desorption-recombination to obtain the final ultrafine anisotropic particles. The final particle shape is polygonal, which should improve the injection molding characteristics of current powder. The final powder exhibits both high remanence (0.97 T) and coercivity (1354 kA/m) for laboratory batch sizes, which is a result of its anisotropic ultrafine microstructure. Thus, gas atomization is considered a feasible alternative to casting methods as a first step to produce powders for anisotropic bonded magnet.

Influence of heat treatment on microstructure, mechanical and corrosion behavior of WE43 alloy fabricated by laser-beam powder bed fusion

Magnesium (Mg) alloys are considered to be a new generation of revolutionary medical metals. Laser-beam powder bed fusion (PBF-LB) is suitable for fabricating metal implants with personalized and complicated structures. However, the as-built part usually exhibits undesirable microstructure and unsatisfactory performance. In this work, WE43 parts were firstly fabricated by PBF-LB and then subjected to heat treatment. Although a high densification rate of 99.91% was achieved using suitable processes, the as-built parts exhibited anisotropic and layered microstructure with heterogeneously precipitated Nd-rich intermetallic. After heat treatment, fine and nano-scaled Mg24Y5 particles were precipitated. Meanwhile, the α-Mg grains underwent recrystallization and turned coarsened slightly, which effectively weakened the texture intensity and reduced the anisotropy. As a consequence, the yield strength and ultimate tensile strength were significantly improved to (250.2 ± 3.5) MPa and (312 ± 3.7) MPa, respectively, while the elongation was still maintained at a high level of 15.2%. Furthermore, the homogenized microstructure reduced the tendency of localized corrosion and favored the development of uniform passivation film. Thus, the degradation rate of WE43 parts was decreased by an order of magnitude. Besides, in-vitro cell experiments proved their favorable biocompatibility.

Nacre-like composites with a soft thermoplastic elastomer matrix

We produce nacre-like composites made of a soft thermoplastic elastomer matrix reinforced by alumina platelets, as a flexible and healable alternative to stiff brick-and-mortar materials while keeping their characteristic anisotropic microstructure. Materials are manufactured using step-wise deposition and subsequent controlled evaporation of a suspension containing the polymer and the platelets. Well-aligned microstructures loaded from 15 to 65 vol.% in alumina are obtained through hot-pressing densification of the resulting composites. Remarkably, by combining calorimetry and low-field nuclear magnetic resonance, we emphasize that the polymer mobility decreases significantly in spite of a lower degree of cristallinity when the platelet content is increased. This result sheds light on the origin of the limited quality of samples above 45 vol.%, opening new research directions to improve the design of organic nacre-like composites. Tensile, bending and indentation tests on samples containing up to 45 vol.% in platelets are then used to rationalize the mechanical properties variation related to the polymer state. Finally, we show that the addition of 3 vol.% of Fe nanoparticles into the composites is enough to enable their induction heating that we aim to utilize in the future as a contactless healing method.

Fatigue Crack Propagation on Uniaxial Loading of Biaxially Predeformed Pearlitic Rail Steel

Rolling contact fatigue loading causes the surface material in the top of the rail to severely deform. The microstructure aligns along the shear direction and the mechanical behavior in terms of plastic flow becomes anisotropic. Cracks are initiated in the severely deformed surface region and propagates along the direction of microstructure flow lines. However, the effect of large shear deformation on fatigue crack growth is not yet well understood. In this study, uniaxial fatigue crack propagation of undeformed and biaxially predeformed pearlitic rail steel R260 has been investigated. The material was predeformed to different deformation states using combined axial compression and shear deformation, corresponding to material found at different depths in the severely deformed surface layer of rails. Fatigue crack propagation rate was dependent on the material state and influenced by both work hardening and microstructural anisotropy. It was found that predeformed material exhibited lower crack growth rates than undeformed material under this loading condition. The influence of predeformation on the crack growth direction was limited.

The mechanism of anisotropic micro-milling properties in additively manufactured Ti-6Al-4V alloy

Selective Laser Melting (SLM) technology offers great flexibility in manufacturing complex parts. However, secondary processing of selective laser melted (SLMed) parts is essential due to the poor surface morphology. This study investigated the influence of microstructural anisotropy in SLMed Ti-6Al-4V alloy on micro-milling performance. The orthogonal milling tests were conducted on the top, front, and side surfaces of specimens. Parameters including cutting force, chip morphology, burr morphology, surface quality, microhardness, and microstructure were evaluated to represent the anisotropy. The experimental findings demonstrated that the tool's orientation relative to the columnar grains influenced the slip strain modes and the density of grain boundaries in the shear band, thereby determining the correlation of the cutting force with the machined surface and feed direction. The variations in grain boundary density in different machining regions caused plastic anisotropy, which affects chip morphology, burr formation, and machined surface roughness. This research could provide valuable insights for manufacturers in selecting appropriate machining methods for precision processing.

The effect of thickness on the defects and anisotropy of thin-wall Hastelloy X fabricated via laser powder-bed fusion

Laser powder-bed fusion (LPBF) possesses unique advantages in manufacturing complex structures. The mechanical properties of thin-wall structures are crucial for the application. This study investigates the influence of wall thickness on defects and mechanical properties of LPBF Hastelloy X (HX) superalloy. As the wall thickness decreases from 1.5 mm to 0.25 mm, the diameter of sub-grains inside the molten pool increases, exhibiting a progressively enhanced <100>//transverse direction (TD) texture in thin wall structure. Meanwhile, decreasing wall thickness leads to increased porosity and cracks due to unstable molten pool and higher residual stress. The crack initiates at columnar to equiaxed transition (CET) position and propagates along the building direction (BD). The tensile strength of the LPBF HX specimens in the horizontal and vertical directions is 773.5 MPa and 642.1 MPa with thickness of 1.5 mm, respectively, while the elongation of both specimens is close to 35%. When the wall thickness decreases to 0.25 mm, the elongation significantly decreases due to the evolution of microstructure anisotropy and the cracks along the BD, reaching 10.1% and 24.3% for horizontal and vertical specimens. This work illustrates the defects, microstructural evolution and the mechanical properties anisotropy in thin-wall structures of LPBF HX, providing a foundation for adjusting and optimizing the performance of subsequent thin-wall printing processes.

Fatigue behaviour analysis of thermal cyclic loading for through-silicon via structures based on backstress stored energy density

Through-silicon via (TSV) structures are widely used in microelectronic due to the ease of chip interconnection. Few research has focused on investigating the fatigue properties of TSV structures using crystal plasticity finite element (CPFE) simulations. As the process of TSV structures approaches the micron scale, the effects of anisotropic microstructure and grain size on their fatigue behaviours will become necessary. The present work aims to use CPFE simulations to investigate the fatigue properties of TSVs at the microscale. Finite element models with realistic EBSD morphology and orientation are established for four TSV structures with different grain sizes, and CPFE simulations with thermal cyclic loading from −55℃ to 125℃ are performed. From the energy point of view, the stored energy density based on backstress and strain gradient is proposed as a fatigue indicator parameter (FIP). CPFE simulation results show that low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs) dominate crack initiation at the top, bottom or edges of the TSV structure, which is still mainly due to the thermal mismatch effect caused by the difference in the coefficients of thermal expansion of the materials. The crack initiation in the internal structure is mainly caused by symmetric grains containing twin boundaries (TBs), or trigrain boundaries, and this is different from the macroscopic simulations. Combined with the local FIPs concentration distribution, the backstress stored energy density is able to capture more details than the cumulative plastic strain because of its direct correlation with the strain gradient, and validates its effectiveness as a FIP.

Programmable nanocomposites of cellulose nanocrystals and zwitterionic hydrogels for soft robotics

Stimuli-responsive hydrogels have garnered significant attention as a versatile class of soft actuators. Introducing anisotropic properties, and shape-change programmability to responsive hydrogels promises a host of opportunities in the development of soft robots. Herein we report the synthesis of pH-responsive hydrogel nanocomposites with predetermined microstructural anisotropy, shape-transformation, and self-healing. Our hydrogel nanocomposites are largely composed of zwitterionic monomers and asymmetric cellulose nanocrystals. While the zwitterionic nature of the network imparts both self-healing and cytocompatibility to our hydrogel nanocomposites, the shear-induced alignment of cellulose nanocrystals renders their anisotropic swelling and mechanical properties. Thanks to the self-healing properties, we utilized a cut-and-paste approach to program reversible, and complex deformation into our hydrogels. As a proof-of-concept, we demonstrated the transport of light cargo using tethered and untethered soft robots made from our hydrogels. We believe the proposed material system introduce a powerful toolbox for the development of future generations of biomedical soft robots.

Pioneering Enhanced Corrosion Resistance along the Normal Plane of an Ultra-Light Mg-Li Extruded Sheet.

The microstructure and corrosion anisotropy of the Mg-5Li extruded sheet were investigated in this work. Three distinct samples cut from the normal plane (A), longitudinal plane (B), and cross-sectional plane (C) of the as-extruded sheet were prepared. The microstructure was analyzed using optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The corrosion resistance and behaviors of the three samples in a 0.1 mol/L NaCl solution were evaluated by employing hydrogen evolution, mass loss testing, electrochemical assessments, and corrosion morphology analyses. The results revealed that sample A displayed a distinctive bimodal (0002) basal texture, along with clearly distinguishably larger grain sizes than the other samples. The effect of grain size and crystallographic orientation on the corrosion resistance was highlighted, indicating the pioneering corrosion resistance of sample A and the lowest corrosion resistance of sample C. Furthermore, all three samples exhibited the characteristic filiform corrosion during the initial stages of corrosion, progressing into the formation of corrosion pits, with sample C displaying pronounced susceptibility.

Creep of IN738LC manufactured with laser powder bed fusion: effect of build orientation and twinning

ABSTRACT The microstructural anisotropy caused by the highly oriented solidification of the metal Laser Powder Bed Fusion (PBF-LB/M) process affects mechanical short- and long-term properties. Component build orientation influences grain morphology and orientation, and thus, mechanical properties. While the creep behaviour of samples manufactured parallel and perpendicular to the build direction are studied intensively, the 45° build orientation remains uncharacterised. In this study, IN738LC creep samples are manufactured via PBF-LB/M in three build orientations (0°, 45° and 90°). While the results of 90° and 0° are as expected, where 90° achieves the longest time to rupture and largest rupture strain, the 45° specimen shows the least fracture time. Differences in microstructure and twinning behaviour are identified as one of the root causes for this unexpected behaviour. This study discusses the correlation between microstructure, twinning and build orientation and their effect on creep behaviour, with special focus on the 45° build orientation.

The synergic effects of heat treatment and building direction on the microstructure and anisotropic mechanical properties of laser powder bed fusion Corrax maraging stainless steel

An anisotropic microstructure and mechanical properties are big challenges in the development of various laser powder bed fusion (LPBF) alloys. This study investigated the roles of heat treatment and building direction (BD) on the microstructure and anisotropic mechanical properties of LPBF Corrax maraging stainless steel. The effects of solution treatment (ST) and integrated solution-aging treatment (SAT) were clarified. The results show that the grain size of martensite, amount of austenite, and features of grain boundaries were slightly varied with the building direction due to the thermal history. In the as-built state, the weak <111>α′||BD, <1 1‾ 0>α′||X, and <001>α′||BD textures could be found. After the SAT process, the <1 1‾ 0>α′||X texture was slightly intensified due to the coarsening of large columnar grains. However, the texture of the SAT sample was still weak.Furthermore, the building direction and heat treatment did not lead to obvious anisotropic tensile properties or change the ductile fracture mode. The weak texture and pores in LPBF Corrax did not dominate the tensile properties. Irrespective of sample state, the horizontally-built samples exhibited comparable strengths and slightly higher elongation than the vertically-built ones did. In the as-built condition, this phenomenon can be mainly attributed to the transformation-induced plasticity effect. In the ST and SAT conditions, smaller grain sizes of martensite and higher high-angle grain boundary ratios in the horizontally-built samples provided more resistance to crack propagation. LPBF Corrax maraging stainless steel exhibited superior tensile performances and low anisotropic tensile properties, which are very beneficial to the stability of the material during service.

Anisotropic particle multiphase equilibria in nonuniform fields.

We report a method to predict equilibrium concentration profiles of hard ellipses in nonuniform fields, including multiphase equilibria of fluid, nematic, and crystal phases. Our model is based on a balance of osmotic pressure and field mediated forces by employing the local density approximation. Implementation of this model requires development of accurate equations of state for each phase as a function of hard ellipse aspect ratio in the range k = 1-9. The predicted density profiles display overall good agreement with Monte Carlo simulations for hard ellipse aspect ratios k = 2, 4, and 6 in gravitational and electric fields with fluid-nematic, fluid-crystal, and fluid-nematic-crystal multiphase equilibria. The profiles of local order parameters for positional and orientational order display good agreement with values expected for bulk homogeneous hard ellipses in the same density ranges. Small discrepancies between predictions and simulations are observed at crystal-nematic and crystal-fluid interfaces due to limitations of the local density approximation, finite system sizes, and uniform periodic boundary conditions. The ability of the model to capture multiphase equilibria of hard ellipses in nonuniform fields as a function of particle aspect ratio provides a basis to control anisotropic particle microstructure on interfacial energy landscapes in diverse materials and applications.

Effect of heat treatment on microstructure and mechanical anisotropy of selective laser melted Al–Mn-Sc alloy

The anisotropy is an intrinsic property for selective laser melted (SLM) aluminum alloys, resulting from the directional growth of columnar crystal during layer-by-layer preparation and inherent heat treatment. Local remelting of the previous layer, competitive growth mechanism, and specific direction of heat flow result in the formation of columnar grains with <113>//BD preferential orientation along the building direction, while the transverse direction is dominated by equiaxed grains with <112>//TD and weak <212>//TD texture for as-built Al–Mn-Sc alloy, and mechanical anisotropy appeared. The effect of heat treatment on mechanical anisotropy was investigated by quasi-in-situ EBSD. The reason for the decrease of anisotropy was revealed by analyzing the strengthening mechanism after heat treatment. The results showed that the mechanical anisotropy was reduced after heat treatment at 300 °C for 5 h. The grain size and Taylor factor depending on texture were changed due to recrystallization. The reduction of Taylor factor difference was the main reason for the decrease of the mechanical anisotropy during post-heat treatment, while the grain refinement had little effect on mechanical anisotropy.

Generalized micromechanical formulation of fiber orientation tensor evolution equations

Fiber orientation tensors are widely used to efficiently describe the fiber orientation evolution of anisotropic fiber suspensions during mold-filling. Real viscous fiber suspensions show fiber-induced anisotropic behavior. It is an open question how to correctly account for the anisotropic microstructure of fiber suspensions in general in the evolution equation of the fiber orientation tensors. In this study, a generalized evolution equation for the fiber orientation tensor of arbitrary even order is formulated, which takes into account the microstructural anisotropy of the fiber suspension. This formulation is based on a linear homogenization approach which allows the application of arbitrary mean-field models. The derived interaction term, representing the anisotropic environment of a single fiber, is discussed as a micromechanical convergence criterion of the underlying integral operator. It is shown and discussed in detail how the special cases of the Jeffery equation and the Folgar–Tucker equation follow from this general formulation. In addition, the generalized evolution equation is specified for selected mean-field models. In this context, an equation is presented to describe the evolution of the fiber orientation tensor depending on the spatial distribution of the fibers. For simple shear flow, all models describing the orientation dynamics are numerically investigated and compared. The results for the second-order orientation tensor as well as for a single fiber show that the well-known periodic behavior is present and strongly depends on the volume fraction and the chosen mean-field model. Considering the spatial distribution of the fibers has a significant effect on the orientation evolution and strongly affects the periodic reorientation.