Heterogeneous Microstructure Research Articles

Two-stage rolling and partial annealing enables heterogeneous structure and strength-ductility enhancement for austenitic stainless steel

Austenitic stainless steels are widely used in civil and defense industries. However, its low yield strength severely limits its fields of application. Under conventional processing routes, uniform grain refinement improves strength but unfortunately sacrifices ductility. In order to achieve the balance of strength and ductility, a new processing route of cold rolling and cryogenic rolling combined with partial annealing was proposed in this work. The results showed that the two-stage rolling and annealing produced dual-phase heterogeneous microstructure containing micron-sized grains (>1 μm), submicron-sized grains (500–1000 nm), and ultra-fine grains (100–500 nm) arranged in lamellar structures. Compared with the single-stage cold rolled and annealed steel, the tensile strength increased to 1.1 GPa, which was slightly higher than the single-stage cryogenic rolled and annealed steel. Furthermore, the elongation was improved to 25 %. The improvement of tensile properties was mainly due to the contribution of persistent hetero-deformation induced (HDI) hardening and twining induced plasticity (TWIP) effect to work hardening. This study not only proposed a simple and effective way to improve the mechanical properties of austenitic stainless steels, but also provided guidance for the design and development of high performance heterostructured materials.

Low cycle fatigue properties of CoCrFeNiMn high-entropy alloy with heterogeneous microstructure

Heterogeneous microstructure is a hot topic in materials science which facilitates tackling the dilemma of strength-ductility trade-off in engineering materials including high-entropy alloys. The present investigation was conducted on the impact of different microstructures including heterogeneous microstructure, constructed by thermomechanical treatment including cold rolling followed by annealing, on the monotonic and cyclic behavior of a CoCrFeNiMn high-entropy alloy. The tailored microstructure contained three features: non-recrystallized as a hard domain, ultrafine-recrystallized, and fine-recrystallized regions. A desirable combination of strength and ductility including UTS of ∼1.1 GPa together with an elongation of ∼15 % was achieved due to benefiting from hetero-deformation-induced hardening and activation of deformation twins in the fine recrystallized grains. Regarding low-cycle fatigue behavior, at the strain amplitude of 0.4 %, heterogeneous microstructure exhibits remarkable fatigue properties, including high maximum stress and extraordinary lifetime (more than 2 × 105). The compelling reasons for these desirable properties are triggering deformation twins in fine recrystallized grain, the absence of brittle sigma phase as a good location of crack initiation, and the non-recrystallized area as a hard domain for suppressing crack propagation. The results suggested that the heterogeneous microstructure has significant benefits during applying low strain amplitude, however, at high strain amplitude, grain refinement is not recommended, and coarse-grained microstructure provides better properties. The present investigation is a step forward to understand the significance of heterogeneous microstructures to improve fatigue properties of high- or medium-entropy alloys.

Neural networks for emergent behavior in biological microstructures

PurposeThis paper develops a neural network surrogate model based on a discrete lattice approach to investigate the influence of complex microstructures on the emergent behavior of biological networks.Design/methodology/approachThe adaptability of network-forming organisms, such as, slime molds, relies on fluid-to-solid state transitions and dynamic behaviors at the level of the discrete microstructure, which continuum modeling methods struggle to capture effectively. To address this challenge, we present an optimized approach that combines lattice spring modeling with machine learning to capture dynamic behavior and develop nonlinear constitutive relationships.FindingsThis integrated approach allows us to predict the dynamic response of biological materials with heterogeneous microstructures, overcoming the limitations of conventional trial-and-error lattice design. The study investigates the microstructural behavior of biological materials using a neural network-based surrogate model. The results indicate that our surrogate model is effective in capturing the behavior of discrete lattice microstructures in biological materials.Research limitations/implicationsThe combination of numerical simulations and machine learning endows simulations of the slime mold Physarum polycephalum with a more accurate description of its emergent behavior and offers a pathway for the development of more effective lattice structures across a wide range of applications.Originality/valueThe novelty of this research lies in integrating lattice spring modeling and machine learning to explore the dynamic behavior of biological materials. This combined approach surpasses conventional methods, providing a more holistic and accurate representation of emergent behaviors in organisms.

Scanning Acoustic Microscopy Characterization of Cold-Sprayed Coatings Deposited on Grooved Substrates

AbstractThe effect of non-planar substrate surface on homogeneity and quality of cold-sprayed (CS) deposits was studied by scanning acoustic microscopy (SAM). Fe coatings were cold-sprayed onto Al substrates containing artificially introduced grooves of square- and trapezoid-shaped geometries, with flat or cylindrical bottoms. The Al substrates were either wrought or cold-sprayed, to comprehend their prospective influence on the Fe coatings buildup. SAM was then used to assess morphological properties of the materials from the cross-view and top-view directions. The microstructure below the surface of the studied samples was visualized by measuring the amplitudes of the reflection echoes and the velocity of the ultrasonic waves. The SAM analysis revealed that the regions of coating imperfections around the grooves are larger than what is suggested by standard scanning electron microscopy (SEM) observations. Furthermore, we found that the seemingly non-influenced coating regions that appear perfectly homogeneous and dense in SEM do, in fact, possess heterogeneous microstructure associated with the individual CS nozzle passes.

Inorganic–organic polymer networks derived from a cyclic siloxane tetrafunctional glycidyl ether resin

AbstractA tetrafunctional glycidyl ether cyclic siloxane epoxy resin (TGTS) has been synthesized, characterized and cured with four aromatic amine hardeners: 1,3‐phenylenediamine (PDA), diethyltoluenediamine (DETDA), 4,4‐diaminodiphenylmethane (DDM) and 1,3‐bis(4‐aminophenoxy)benzene (APB). Each of the cured networks produces transparent and homogeneous networks, although when TGTS is cured with DETDA, reduced compatibility led to lower epoxide consumption, a more heterogenous microstructure and deleterious effects upon properties. Reduced miscibility of DETDA significantly impacts the chemical structure and microstructure of the network, resulting in significant reductions in thermal and mechanical properties but higher UV‐A transmission. The PDA‐, APB‐ and DDM‐cured networks conversely were more miscible and display properties typical of organic–inorganic hybrid networks, such as good mechanical properties at ambient and sub‐ambient temperatures, comparatively high glass transition temperatures, improved resistance to oxidation and lower UV‐A transmission. © 2024 The Author(s). Polymer International published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Friction Stir Welding of AZ31 Mg Alloy: Microstructure, Micro hardness and Electrochemical Behavior

In this investigation, rolled plates of AZ31 Mg alloy were successfully friction-stir welded. It was found that the process leads to the microstructural refinement in the NZ due to the dynamic recrystallization with a global microstructural heterogeneity through the weld. The maximum microhardness values were attributed to the finest microstructure. The obtained results revealed that the cathodic behavior of the grain boundaries improves the corrosion resistance of the NZ. Thus, the corrosion resistance enhancement was ascribed to the most homogeneous microstructure and the finest grains.

Enhancement of Flow Boiling Heat Transfer Characteristics on Diamond/Cu Heterogeneous Surface

Surface treatment is an effective method of enhancing the heat transfer performance of flow boiling. Diamond/Cu, as a particle‐reinforced composite, exhibits heterogeneous surface properties and surface microstructures that play distinct roles in heat transfer. To elucidate the synergistic effect of heterogeneous material surfaces and microstructures, pure copper surface (PC), untreated diamond/Cu surface (DC), and surface‐treated diamond/Cu surface (SDC) are prepared for this experiment. These surfaces demonstrate various combinations of diamond, copper, and their corresponding surface microstructures. DC, characterized by heterogeneous wetting properties, demonstrates a heat transfer coefficient (HTC) that is 9%–24% higher than that of PC. The surface treatment of SDC results in the formation of a superhydrophilic CuO nanosheet structure, significantly enhancing both surface roughness and energy. SDC develops microcracks on the surface of diamond. The increased roughness provides numerous nucleation areas for the hydrophobic diamond, enhancing the overall wettability of the surface when combined with the superhydrophilic CuO region. SDC demonstrates the advantage of enhanced flow boiling on the surface of heterogeneous materials. At the identical heat flux, SDC exhibits a reduction of 5–10 °C in wall superheat and an increase of 38%–47% in HTC.

A Comprehensive Study of Friction Stir Processing Technique

Friction Stir Processing technique is a surface modification method that transforms heterogeneous microstructures into homogeneous ones, enhancing the mechanical properties of metal sheets. This single-step process, introduced by The Welding Institute (TWI) in 1991, offers advantages such as superior strength and formability, especially in aluminum alloys. FSP's simplicity, cost-effectiveness, and environmental friendliness make it preferable over traditional methods. Despite challenges in aerospace and transportation applications, FSP shows promise for forming lightweight alloys at room temperature. However, producing ultrafine grain sheet metals remains a challenge. Overall, FSP presents a viable solution for improving material properties and processing efficiency in various industrial applications. This study includes the literature review of research publications from different researchers to develop the understanding regarding friction stir processing technique.

Development of novel protein-rich foods: Studies of composite hydrogels formed from potato proteins and gellan gums with different degrees of acylation

The textural attributes of mixed hydrogels formed from potato protein (PP) and gellan gum (GG) were examined for their potential application as novel protein-rich foods (NPFs). Initially, the properties of gels formed by the individual biopolymers were characterized using dynamic shear rheology, compression testing, and confocal fluorescence microscopy. Rheological analysis revealed that PP dispersions formed irreversible heat-set gels, whereas GG formed reversible cold-set gels at lower temperatures. The effects of adding PP to high acyl gellan gum (HA-GG) or low acyl gellan gum (LA-GG) solutions on the microstructure and rheology of the mixed gels formed were then examined. The PP/LA-GG composite gels had a highly heterogeneous microstructure consisting of protein-rich and polysaccharide-rich regions. In contrast, the PP/HA-GG composite gels had a more uniform microstructure. Interestingly, the incorporation of the PP weakened the PP/LA-GG composite gels (antagonism) but strengthened the PP/HA-GG composite gels (synergism), which was mainly attributed to their different impacts on gel microstructure. This study shows that semi-solid plant protein-dietary fiber composites can be formed with a variety of textural attributes by altering the nature and concentration of biopolymers used to formulate them. These mixed gels may be useful for the creation of NPFs. This study also highlights the complexity of the interactions between different food biopolymers and their impacts on food properties, highlighting the need for further research on understanding structure-function relationships in multicomponent biopolymer materials.

Microstructural Evolution and Strengthening of Dual-Phase Stainless Steel S32750 during Heavily Cold Drawing

S32750 dual-phase stainless steel (DSS) wires were prepared by cold drawing with a strain of ε = 0~3.6. The mechanical behavior and microstructural evolution of these DSS wires at different strains were investigated. Specifically, the yield strength and ultimate tensile strength of a S32750 DSS wire at a strain of ε = 3.6 reached 1771 MPa and 1952 MPa, respectively. The microstructure of the wire was transformed into a heterogeneous microstructure, which consisted of ferrite fiber grains and a nanofibrous grain structure consisting of austenite and strain-induced martensite nanofiber grains. A sub-grain structure was observed inside the ferrite fiber. The austenitic phase followed the evolutionary steps of stacking faults, twinning, ε-martensite, α-martensite, and, finally, austenite, before transitioning into a nanofibrous grain structure. This nanofibrous grain structure significantly contributed to the strength compared with the relatively coarse ferrite phase.

Extension of a phase-field KKS model to predict the microstructure evolution in LPBF AlSi10Mg alloy submitted to non isothermal processes

The out-of-equilibrium heterogeneous microstructure typical of AlSi10Mg processed by Laser Powder Bed Fusion (LPBF) is often modified by further heat treatment to improve its ductility. According to literature, extensive experimental investigations are generally required in order to optimize these heat treatments. In the present work, a phase-field approach is developed based on an extended Kim–Kim–Suzuki (KKS) model to guide and accelerate the post-treatment optimization. Combined with CALculation of PHAse Diagrams (CALPHAD) data, this extended KKS model predicts microstructural changes under anisothermal conditions. To ensure a more physical approach, it takes into account the enhanced diffusion by quenched-in excess vacancies as well as the elastic energy due to matrix/precipitate lattice mismatch. As the developed model includes the computation of the evolution of the thermo-physical properties, its results are validated through comparison with experimental DSC curves measured during the non-isothermal loading of as-built LPBF AlSi10Mg. The computed microstructure evolution reproduces the microstructural observation and successfully explains the peaks in the DSC heat flow curve. It thus elucidates the detailed microstructural evolution inside the eutectic silicon phase by considering the growth and coalescence of silicon precipitates and the matrix desaturation.

Mechanism and improvement of the rolling contact fatigue of the surface layer with heterogeneous microstructures of the rail steel treated by laminar plasma jet

Surface layer with heterogeneous microstructures (SLHM), characterized by hardened spots discretely embedded in a soft substrate, can be prepared by laminar plasma quenching (LPQ) to improve the wear resistance of the railway rail. However, severe rolling contact fatigue (RCF) cracks and spalling would occur in the hardened spots that only consist of quenched microstructure. This study analyzed the RCF damage mechanism of the LPQ prepared hardened spots by contact simulation. To improve the RCF resistance while maintaining the high wear resistance of the treated rail, a novel SLHM, whose discrete hardened spots are surrounded by transitional zone with the hardness lower than that of the quenched zone but higher than that of the substrate, was proposed and prepared by laminar plasma quenching-tempering (LPQT). Then, twin-disc tests and contact simulations were conducted to investigate the wear and RCF resistances of the LPQT treated rail steel. Results showed that the cause of the severe RCF cracks and spalling at the entry side of the LPQ prepared hardened spots was that the cyclic tensile stress at the entry side was always higher than that at the exit side under the cyclic rolling contact loading. The high tensile stress at the entry side of the LPQ prepared hardened spots resulted from the elastic deformation induced by tensile plastic deformation of the neighboring substrate. When the thickness of the transitional zone was about 0.1 mm, the generation of severe RCF cracks and spalling was significantly inhibited in the LPQT prepared hardened spots because the transitional zone decreases the cyclic tensile stress at the entry side of the quenched zone. Besides, the wear loss of such LPQT treated rail steel was only increased by 18.1 % compared with that of the LPQ treated rail steel, indicating that the wear resistance of the LPQT treated rail steel was not significantly reduced. These findings are promising for designing the microstructure of the surface layer to obtain wear-resistant rail with high RCF resistance.

Exploring the effects of austenitizing conditions on the heterogeneous microstructure and mechanical properties of 11Cr–11Ni–Ti–Mo maraging stainless steel: Breakthrough in the strength-ductility trade-off

Based on the microstructure analysis, the present research investigated the influence of various austenitizing conditions on the transformation behavior of intermetallic compound particles, i.e. η-Ni3Ti, and the reversed austenite during ageing treatment in Ti–Mo maraging stainless steel, as well as the related mechanical properties. The results revealed numerous tiny rod-like intermetallic compound particles, i.e. η-Ni3Ti, having a Burgers orientation relationship, i.e., {101}α′//{0001}η ; <111‾>α′//<112‾0>η, with the martensite matrix, as well as blocky, lath, and granular reversed austenite of various sizes distributed within the tempered martensite matrix. Regardless of the austenitizing treatment condition, more reversed austenite was found in the samples aged at 520 °C than in those aged at 640 °C. Although this finding was not consistent with the Thermo-Calc predictions, it should be associated with the thermal stability of reversed austenite at different ageing temperatures. In other words, the greater amount of reversed austenite that formed at 640 °C did not remain at room temperature due to secondary martensite transformation during cooling, as was verified by EBSD analysis.

Simultaneously enhancing mechanical and anti-corrosion properties of aluminum through electrodeposition of supersaturated Al(Fe) solid solutions

The heterogeneous microstructure in bulk aluminum (Al) alloys or coatings, comprising an α-Al solid solution matrix and coarse intermetallic compounds (IMCs), poses a challenge in balancing mechanical and anti-corrosion properties. This challenge is particularly pronounced in Al alloys with high iron (Fe) content. Here, supersaturated Al(Fe) solid solutions with a single phase have been electrodeposited using imidazolium chloroaluminate ionic liquids (ILs). Ferrous chloride (FeCl2) dissolved in Lewis-acidic ILs is assumed to form Cl-bridged [Fe(AlCl4)4]2−, which, together with the well-known Al2Cl7− species, supports the reduction and deposition of Al(Fe) alloy coatings. The supersaturated Fe solutes significantly strengthens the Al matrix, with the hardness of Al(Fe, 6.6 at.%) reaching up to 3.43 GPa, surpassing that of any commercial Al alloys and other binary Al-based solid solutions. Moreover, the Al(Fe) deposits demonstrate superior anti-corrosion performance compared to pure Al. The homogeneous microstructure and extended Fe solubility within Al(Fe) solid solutions inherently contributes to a simultaneous improvements in mechanical and anti-corrosion properties. The nanograined and compact structure also benefit these properties. Accordingly, the combination of electrodeposition techniques and Al(Fe) alloys offers a promising approach for providing effective mechanical and chemical protection on metal surfaces.

Effect of rolling on the microstructure and mechanical properties of (FeCrNi)94Ti3Al3 medium entropy alloy fabricated via selective laser melting

The microstructure evolution and mechanical properties of the (FeCrNi)94Ti3Al3 medium entropy alloy, fabricated through selective laser melting, were investigated after warm-rolling. The as-built (FeCrNi)94Ti3Al3 alloy exhibited a heterogeneous microstructure with alternating coarse face-centered cubic (FCC) phase grains (∼20 μm) and fine body-centered cubic (BCC) phase grains (∼1 μm). However, after warm-rolling and subsequent recrystallization, the alloy transformed into a dual-phase equiaxed grains structure consisting of FCC and BCC phases with an average grain size of approximately 2.8 μm. Additionally, the previously banded distribution of the BCC phase changed to a random distribution. Notably, during the deformation process, a twinning-induced plasticity (TWIP) effect was observed in the warm-rolled and recrystallized (FeCrNi)94Ti3Al3 alloy, resulting in excellent ductility and a wide work-hardening platform.

Hierarchically heterogeneous microstructure and mechanical properties in laser-directed energy deposition of γ-TiAl alloy through intrinsic cyclic heat treatment

The complex thermal effects of laser direct energy deposition (L-DED) provide significant advantages for production of high-performance γ-TiAl alloy, whereas challenges still exist in microstructure tailoring and mechanical properties. This work focuses on the influence of intrinsic cyclic heat treatment (ICHT) on the evolution of microstructures and mechanical properties in different building height regions of an L-DED TiAl alloy thin-wall sample. The results indicate that ICHT induces discontinuous coarsening (DC) effects by consuming the lamellar interface energy of the initial lamellae, resulting in grain refinement. With the increase in building height, the thermal accumulation leads to an increase in the initial lamellar spacing, which weakens the DC effect. At the top, insufficient thermal cycles prevent fine grain growth and erode coarse grains, hindering overall grain refinement. Ultimately, the hierarchically heterogeneous microstructure forms in the L-DED TiAl alloy, including refinement of hierarchical lamellar colony structure decorated with fine (α2+γ) lamellar structure and (α2+6H-LPS) lamellar structure (6H-LPS refers to 6H long-period structure phase). Specifically, the bottom region exhibits the best combination of ultimate tensile strength (706 ± 33 MPa) and strain (0.68 ± 0.05 %). The high tensile strength can be attributed to the hindrance of dislocation movement by the fine lamellae and the 6H-LPS phase ,caused by the high cooling rate of L-DED, while the elongation is attributed to the reduction in crack propagation energy due to the presence of fine grains.

Obtaining Heterogeneous Microstructure and Enhanced Mechanical Properties in ECAP-Processed AZ61 Alloys via Single-Pass Rolling with Increased Rolling Reduction

Material design and preparation based on constructing heterogeneous microstructures can break the conventional performance limitations of fine-grained magnesium alloys. In this study, AZ61 alloys processed via multi-pass equal channel angular pressing (ECAP) were subjected to single-pass rolling (SPR) with increased rolling reductions. The effect of rolling reduction on the formation of heterogeneous microstructure and the mechanical properties of the alloy was investigated. Microstructural examinations revealed that a heterogeneous microstructure was formed in the alloy at varied rolling reductions, but the desired heterostructure with higher fine grain contents could only be achieved at increased rolling reduction. This was mainly due to the fact that the alloy underwent partial dynamic recrystallization (PDRX) under SPR, and PDRX more easily occurred with higher rolling reduction. The tensile test results showed that with increased rolling reduction, the strength of the alloy first increased and then decreased slightly, with the ductility steadily increasing. Improved mechanical properties were achieved in the alloy rolled at increased rolling reductions owing to the heterogeneous microstructure with a greater content of fine grains.

Geometry smoothing and local enrichment of the finite cell method with application to cemented granular materials

AbstractIn recent times, immersed methods such as the finite cell method have been increasingly employed in structural mechanics to address complex-shaped problems. However, when dealing with heterogeneous microstructures, the FCM faces several challenges. Weak discontinuities occur at the interfaces between the different materials, resulting in kinks in the displacements and jumps in the strain and stress fields. Furthermore, the morphology of such composites is often described by 3D images, such as ones derived from X-ray computed tomography. These images lead to a non-smooth geometry description and thus, singularities in the stresses arise. In order to overcome these problems, several strategies are presented in this work. To capture the weak discontinuities at the material interfaces, the FCM is combined with local enrichment. Moreover, the L$$^2$$ 2 -projection is extended and applied to heterogeneous microstructures, transforming the 3D images into smooth level-set functions. All of the proposed approaches are applied to numerical examples. Finally, an application of cemented granular material is investigated using three versions of the FCM and is verified against the finite element method. The results show that the proposed methods are suitable for simulating heterogeneous materials starting from CT scans.

Synergistic mechanism of hetero-structured Al–Mg–Si–Cu–Zn–Fe–Mn alloys with greatly enhanced strength, ductility and formability

Al–Mg–Si alloy sheets applied in automobile industry are required to possess good formability, high strength and ductility. Herein, our results show that a heterogeneous cellular microstructure, i.e., soft domains embedded in hard domains, can be readily attained via a short-flow processing route with concomitant improvement in the strength (YS: 177–184 MPa, UTS: 317–328 MPa), elongation (25.7%–27.3 %) and formability (r‾= 0.771, Δr = 0.047) of pre-aged alloy. The distribution of recrystallization grains, texture and coupled soft/hard domains in the Al–Mg–Si–Cu–Zn–Fe–Mn alloys and the effect on mechanical properties have been investigated. Moreover, the formation and synergistic action mechanisms of coupled soft/hard domains were put forward in this work.

Hydroxypropyl methylcellulose phthalate films reinforced with nanocrystalline cassava starch and intended its applications for colonic drug delivery

Controlled-release drug delivery systems are crucial for improving therapeutic efficacy and patient compliance. Hydroxypropyl methylcellulose phthalate (HPMCP) films are promising candidates due to their film-forming properties and tunable release profiles. However, their mechanical properties can limit their applicability. This study aimed to assess the effect of addition of nanocrystalline cassava (NCS) starches to HPMCP films on the physicochemical and mechanical properties and the ability of films to control drug release rate. Reinforced HPMCP films with 2, 4 and 6 % w/w of NCS were characterized as a function of the microstructure, X-ray diffraction pattern, mechanical properties, and dissolution property. It was found that increasing starch amount induced an increase in heterogeneity of the film microstructure and crystallinity. The tensile strength of HPMCP films increased with addition of starches, while the elongation at break decreased. The lowest dissolution in pH 6.8 phosphate buffer solution was found for films containing NCS at 2 and 4 % w/w. For the application purposes, HPMCP containing different amounts of NCS was used to coat over diclofenac sodium tablets. The tablets coated with HPMCP containing 4 % w/w NCS showed the lowest drug release of 73.5 % in pH 6.8 phosphate buffer solution at 240 min. According to our results, the addition of NCS at adequate amount allowed to improve the mechanical properties of HPMCP films and to decrease the drug release rate which is important for the site-specific drug delivery process.