Heterogeneous Microstructure Research Articles

On the cyclic deformation behavior of wire-based directed energy deposited Fe-Ni Invar alloy

Fe-basis alloys with Ni concentrations of approximately 36 wt.-% are widely used for components with high requirements on mechanical properties and dimensional accuracy. Due to the remarkably low coefficient of thermal expansion, these so-called Invar alloys provide for dimensional accuracy over a wide temperature range. Generally, additive manufacturing technologies have already been qualified for processing of Invar alloys in terms of maintaining mechanical as well as functional properties. However, the mechanical behavior under cyclic loading still is not comprehensively understood. Therefore, the present study focuses on the fatigue properties of the Fe-Ni36 Invar alloy manufactured by wire-based directed energy deposition (DED) employing either an electric arc (DED-Arc) or a laser beam (DED-LB) as energy source. Here, a wire-based DED-LB Invar condition is comprehensively studied for the first time in open literature. For initial assessment, specimens were analyzed using optical and scanning electron microscopy as well as computed tomography and tensile testing. Here, the DED-LB material is characterized by a heterogeneous grain size distribution, less pronounced crystallographic texture as well as increased tensile strength and ductility compared to the DED-Arc counterparts. Despite increased strength and ductility, the DED-LB Invar consistently shows inferior fatigue properties. This behavior is discussed based on microstructural heterogeneity, cyclic deformation response, process-induced porosity and the contribution of plastic strain. Furthermore, investigations on the thermal expansion behavior show that the DED Invar retains its outstanding thermal properties, respectively.

Development of carboxymethyl cellulose/starch films enriched with ZnO-NPs and anthocyanins for antimicrobial and pH-indicating food packaging

Active packaging, which can monitor food freshness and extend the shelf life, has gained significant attention in recent years. This study aims to develop a novel carboxymethyl cellulose (CMC)/starch/anthocyanins/ZnO active films with enhanced properties and specific functionalities. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) revealed that the addition of anthocyanins and nano-ZnO particles (ZnO-NPs) led to heterogeneous microstructures and a slight decrease in the crystallinity. Fourier transform infrared spectroscopy (FTIR) indicated that there were no chemical interactions among film components. Active films containing ZnO-NPs exhibited improved ductility, as well as enhanced light barrier and water resistance properties. Notably, a shift from hydrophilic to hydrophobic behavior of the films was observed with high ZnO-NP content, as evidenced by a significant increase in the water contact angle (from 63.44° to 114.22°). Furthermore, the presence of only 1 % ZnO-NPs resulted in efficient inhibition of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) growth. Moreover, active films containing both anthocyanins and ZnO-NPs were highly sensitive to pH changes in buffer solutions (pH 2–11). Based on the results, a recommended film formulation for future active packaging applications is a 80:20 CMC/starch blend with 3 % ZnO-NPs and 0.1 g anthocyanins.

Excellent combinations of strength-ductility and corrosion resistance in SAF 2205 duplex stainless steel with multi-scale grain distribution

In this work, a multi-scale grain structure was obtained in SAF 2205 duplex stainless steel (DSS) by severe deformation and short-term annealing process. The influence of th is structure on the mechanical properties and electrochemical behavior is systematically investigated. Experimental results indicate that the sample subjected to short-term annealing at 1000 °C (SA-1000 °C) exhibits the best comprehensive properties, with a yield strength (YS) of 652.6 MPa and an elongation (EL) of 39.9 %. Both strength and ductility surpass those of the original sample and long-term annealed (LA-1000 °C) samples. The strength-ductility product is increased by 32 % compared to the original sample and by 18 % compared to the LA-1000 °C sample. The increase in YS is predominantly attributed to dislocation strengthening and grain refinement strengthening, and the heterogeneous microstructure leads to good ductility. Moreover, the multi-scale distribution of the grain structure exhibits enhanced corrosion resistance due to the increased low-Σ grain boundaries and the promotion of stable passivation film formation by a limited number of defects, thereby mitigating the corrosion rate.

Metallurgical characteristics and mechanical properties of dissimilar friction stir welded DH36 steel and UNS G10080 steel joints

The present study expanded the scientific comprehension of the friction stir welding process for dissimilar steels, namely high-strength shipbuilding grade DH36 steel and UNS G10080 steel. The effect of tool traverse speed and plunge depth on temperature history, microstructure characteristics, and mechanical properties is investigated experimentally. The metallographic characterizations were examined through an optical microscope and field emission scanning electron microscopy equipped with an energy-dispersive X-ray system. Microhardness, impact, and tensile tests were carried out on the friction-stir-welded specimens. Increasing the plunge depth and reducing the traversal speed resulted in an augmentation of the peak temperature, primarily attributable to higher heat generation. Within the range of process parameters used, the tool produced complex material movement, resulting in swirl-like and vortex-intercalated features, particularly adjacent to the stir zone/workpiece interface. These vortex-like features exhibited dynamically recrystallized fine-grained microstructures. The grain size in the stir zone and the thermo-mechanically affected zone is reduced by increasing the plunge depth and decreasing the traverse speed due to enhanced dynamic recrystallization, subsequently improving the hardness and toughness values. In the stir zone, the microstructure revealed the acicular-shaped bainite ferrite in the DH36 steel and the Widmanstatten ferrite in the UNS G10080 steel. The microhardness contours revealed the uneven hardness distribution across the weld cross-section due to the microstructural heterogeneity in the dissimilar steels. The maximum welding efficiency of 106 % and toughness of 46 J are obtained at 40 mm/min traverse speed with a plunge depth of 0.2 mm, which is attributed to sufficient heat generation and grain refinement.

Correlation between heterogeneous micromechanical properties and fracture toughness in X80 girth weld

The safety accidents caused by the service of pipelines will cause huge economic losses due to the deterioration of the fracture toughness of the girth weld. To clarify the fracture behaviors of girth weld, X80 girth welds with heterogeneous microstructure consisted acicular ferrite (AF) and grain boundary ferrite (GBF) with different micromechanical properties were investigated. The results show that the typical microstructure of X80 girth weld is heterogeneous, which is consist of high strength AF and low strength GBF. With the decrease of the difference in micromechanical properties between AF and GBF, the crack tip opening displacement of three girth welds is 0.13 ± 0.04 mm, 0.18 ± 0.04 mm and 0.29 ± 0.05 mm. Meanwhile, the type of microcracks gradually changed from secondary cracks with GBF aggregation to micro-voids without obvious aggregation, which means the fracture mechanism transforms from cleavage fracture to micro-voids aggregation fracture. The finite element analysis indicates that the change of fracture mechanism is mainly due to the reduction of the gap between the micromechanical properties of AF and GBF, the stress and strain distribution are homogenized, resulting the fracture toughness is significantly improved.

Heterogeneous microstructure of γ-irradiated pre-oxidized PAN fiber revealed by microfocus SR-SAXS reconstruction and molecular simulation.

The microstructure plays a crucial role in the manufacturing and application of polyacrylonitrile fibers, which serve as precursors for carbon fibers. Synchrotron radiation small angle x-ray scattering (SR-SAXS) is a non-destructive and precise technique for analyzing fiber structures. This study employed one-dimensional SR-SAXS mapping to extract key structural parameters such as periodicity, lamellae thickness, and the extent of amorphous regions, as well as the directional orientation in γ-irradiated, pre-oxidized polyacrylonitrile fibers. The analysis revealed a three-layered structure comprising a surface skin, a transitional layer, and a central core. Notably, the lamellar thickness exhibits a "U"-shaped distribution, while the long-period structures, amorphous regions, and orientational properties demonstrate a "wave-like" pattern. Within this structure, the skin exhibits a higher level of orientation, with the orientation decreasing progressively from the skin toward the core layer. The structure of the layered crystal was further corroborated by the morphological analysis. In addition, molecular simulations were performed to propose the mechanisms underlying the formation of this layered structure. This comprehensive investigation using SR-SAXS and one-dimensional mapping provides detailed insights into the microstructural and morphological characteristics of polyacrylonitrile fibers, which can inform future advancements in material processing and refinement techniques for the production of advanced fibers.

Numerical investigation of new rete ridge formation in a multi-layer model of skin subjected to tissue expansion

The skin is a multilayered organ with microstructural and antomical heterogeneities that contribute to its unique mechanophysiology. Between the epidermis layer at the top and the dermis layer below, the basal keratinocytes form an interface with sinusoidal-like geometry termed rete ridges. In previous computational work we showed that the rete ridges contribute to lower delamination risk by increasing surface area and reducing the stress jump across the interface. Experimentally, we and others have shown that upon repeated tissue expansion and growth, physiological rete ridge frequency is preserved. Here we implement a 2D multilayered skin model where each layer is able to grow in response to applied loading toward recovering the layer-specific homeostatic stretch. Our simulations support the hypothesis that mechanics of growing tissue can explain secondary buckling and new rete ridge formation in tissue expansion. The process is robust with respect to parameters such as homeostatic stretch, layer thicknesses, and shear moduli of the different layers. Thicker epidermis suppresses higher frequency features, and so does a stiffer epidermis with respect to the basal layer. Interestingly, new rete ridge valleys are formed at locations that were originally peaks of the sine wave, whereas original valleys remain valleys. This pattern might have a connection to the localization of stem cell and transient amplifying cells in the epidermis. This study does not discard the role of cell–cell signaling dynamics, but rather emphasizes the possibility of achieving robust geometric patterns with simple rules of growing tissue, even in the absence of complex regulatory networks.

Effect of martensite hardness on mechanical properties and stress/strain-partitioning behavior in ferrite + martensite dual-phase steels

Low-carbon dual-phase (DP) steels composed of ferrite and martensite are widely used in automotive industries due to their good combination of strength and ductility and remarkable strain-hardening ability. Such exceptional mechanical properties of DP steels arise from microstructural deformation heterogeneity due to the different deformation roles of soft ferrite and hard martensite. The present study investigated mechanical properties and local deformation behavior of DP steels having different martensite hardness (strength) using digital image correlation (DIC) technique and in-situ X-ray diffraction (XRD) experiment during tensile deformation. The martensite hardness was reduced by tempering treatment, and its mechanical properties were compared with the as-quenched DP specimen by tensile testing. The tempered DP specimen exhibited reduced strength but enhanced ductility, especially in post-uniform elongation, compared to the as-quenched DP specimen. DIC-strain analysis indicated that the tempering effectively inhibited strain localization, allowing a greater contribution of martensite to plastic deformation. On the other hand, the in-situ XRD analysis revealed that the tempering significantly restricted the stress-increasing potential of martensite. Our results suggest that martensite, acting as a hard domain, plays a significant role in enhancing strain-hardening ability and achieving large uniform elongation due to its high stress-increasing potential. However, the presence of hard martensite also imposes restrictions on post-uniform elongation due to the strong strain localization induced in microstructures.

Synergistic interaction behavior of deformation mechanisms in a new metastable β-Ti alloy with high strain hardening rate

The present study successfully designed a novel metastable β titanium alloy, Ti–8.8Cr–1.9Sn, exhibiting exceptionally high strain-hardening rate (∼2.4 GPa) as well as significant total elongation (∼48.2 %). Detailed analysis of the microstructure revealed that the successive activation of primary mechanical twins and primary SIM α" within β phases, along with secondary mechanical twins and secondary SIM α" within twinning zones during deformation, which results in a highly intricate network of deformation microstructures. Meanwhile, during deformation the microstructure undergoes dynamic refinement through the continuous occurrence of mechanical twinning and phase transformations, leading to a reduction in dislocation mean free path, thereby enhancing the strain-hardening rate. Importantly, the microstructural heterogeneity is formed due to generation of numerous heterogeneous interfaces, including β/α'', β/twin, α''/twin as well as twin/twinnano, thereby inducing additional HDI hardening resulting from the formation of GNDs for accommodating strain gradients near these interfaces. Furthermore, the relative mobile dislocation density ρm/ρm0 is significantly high during deformation, due to a reduced exhaustion rate and a high nucleation rate of mobile dislocations. The values of activation volume Va and V∗ typically range from 1b3–100b3, indicating the occurrence of cross-slip and dislocation nucleation. The interaction and accumulation between GNDs and mobile dislocations further contribute to the improvement of strain hardening rate. Therefore, the synergistic interaction activities of multiple deformation mechanisms lead to enhanced strain-hardening rate along with the large plasticity.

Microscopic characterisation of brittle fracture initiation in irradiated and thermally aged low-alloy steel welds of a decommissioned reactor pressure vessel

Microstructure has a significant effect on material's integrity and in a heterogeneous weld microstructure the discontinuities affect the brittle fracture initiation and propagation and determine the fracture toughness. The knowledge of brittle fracture initiation mechanisms in high-Mn/high-Ni welds is limited. The brittle fracture initiation behaviour of the decommissioned Barsebäck Unit 2 reactor pressure vessel (RPV) welds of high-Mn/high-Ni weld metal from three different locations, the RPV head and the beltline regions, were investigated and compared with specimens from the surveillance program with high fluence. Systematic fractography has been performed on impact and fracture toughness specimens and the main features of the brittle fracture initiation in the component weld are presented and discussed. Two main types of initiators are identified as the weakest links to initiate the cleavage fracture and the initiation mechanism is found independent from the operation condition. The high-fluence surveillance specimens have a larger amount of intergranular cracking. The cleavage fracture initiation appears to be independent of the operation conditions but dependent on the welding process and metallurgical features. The findings aid in the development of improved material-property correlations which will result in better computational tools for predicting aging of welds based on microstructure.

A phase field crystal model for real-time grain boundary formation and motion in complex concentration alloy

The complex concentration alloys exhibit the excellent mechanical property due to the dual roles of their heterogeneous compositions and microstructures. However, the formation and motion of grain boundaries to significantly regulate the mechanical properties remain unknown at several microsecond and nanoscale in the complex concentration alloys. Here, we utilize a phase field crystal model to investigate the real-time grain boundary formation and motion in complex concentration alloys, specifically, in comparison to traditional alloys, using the example of AlCu alloys. Our findings reveal that the low concentration alloys exhibit segregation primarily at grain boundaries with minimal compositional fluctuations over a long period of grain growth, and the complex concentration alloys display a high concentration gradient within the grains to cause rapid grain rotation and merging in a short time frame. In the complex concentration alloys, the large component fluctuations not only cause the local lattice distortion to hinder dislocation movement during the deformation process, but also lead to the occurrence from dislocation locking to self-unlocking, which is beneficial to improve the strength and ductility. The present work provides a real-time atomic-scale understanding of grain boundary evolution using in situ atomic-resolution simulations.

Heterogeneous Morphologies and Hardness of Co-Sputtered Thin Films of Concentrated Cu-Mo-W Alloys.

Heterogeneous microstructures in Cu-Mo-W alloy thin films formed by magnetron co-sputtering immiscible elements with concentrated compositions are characterized using scanning transmission electron microscopy (STEM) and nanoindentation. In this work, we modified the phase separated structure of a Cu-Mo immiscible system by adding W, which impedes surface diffusion during film growth. The heterogeneous microstructures in the Cu-Mo-W ternary system exhibited bicontinuous matrices and agglomerates composed of Mo(W)-rich phase. This is unique, as these are the slower-diffusing species, contrasting past reports of binary Cu-Mo thin films that exhibited Cu-rich agglomerates. The bicontinuous matrices comprised of Cu-rich and Mo(W)-rich phases exhibited bilayer thicknesses of less than 5 nm. The hardness of these thin films measured using nanoindentation is reported and compared to similar multilayers and nanocomposites in binary systems.

Probing the Linear-to-Plastic Transition in Polymer Nanocomposites via Atomistic Simulations: The Role of Interphases.

Polymer nanocomposites have found ubiquitous use across diverse industries, attributable to their distinctive properties and enhanced mechanical performance compared to conventional materials. Elucidating the elastic-to-plastic transition in polymer nanocomposites under diverse mechanical loads is paramount for the bespoke design of materials with desired mechanical attributes. In the current work, the elastic-to-plastic transition is probed in model systems of polyethylene oxide (PEO) and silica, SiO2, nanoparticles, through detailed atomistic molecular dynamics simulations. This comprehensive, multi-scale analysis unveils pivotal markers of the elastic-to-plastic transition, highlighting the quintessential role of microstructural and regional heterogeneities in density, strain, and stress fields, featuring the polymer-nanoparticle interphase region. At the atomic level, the behavior of polymer chains interacting with nanoparticle surfaces is traced, differentiating between free and adsorbed chains, and identifying the microscopic origins of the linear-to-plastic transition. The mechanical behavior of subregions are characterized within the PEO/SiO2 nanocomposites, focusing on the interphase and bulk-like polymer areas, probing stress heterogeneities and their decomposition into various force contributions. At the inception of plasticity, a disruption is discerned in isotropy of the polymeric density field, the emergence of low-density regions, and microscopic voids/cavities within the polymer matrix concomitant with a transition of adsorbed chains to free. The yield strain also emerges as an inflection point in the local versus global strain diagram, demarcating the elastic limit, and the plastic regime shows pronounced strain heterogeneities. The decomposition of the atomic Virial stress into bonded and non-bonded interactions indicates that the rigidity of the material is primarily governed by non-bonded interactions, significantly influenced by the volume fraction of the nanoparticle. These findings emphasize the importance of the microstructural and micromechanical environment at the polymer-nanoparticle interface on the linear-to-plastic transition, which is of great importance in the design of nanocomposite materials with advanced mechanical properties.

Effect of heterogeneous microstructures on mechanical properties of thin gradient nanotwinned copper foils

Strengthening of metals generally weakens ductility, but gradient strengthening is an exception. Heterogeneous microstructures, characterized by variations in grain size and twin spacing (structural gradients), were incorporated into 20-μm thin nanotwinned copper (NT-Cu) foils to enhance their mechanical properties. Four distinct heterogeneous NT-Cu samples, including two-layer (2L) structures and three-layer (3L) structures, were produced through rotary electroplating. The mechanical properties, work hardening rates, and microstructural features of these samples were systematically analyzed and compared. The results showed that an increase of 9.5 %, 6.4 %, and 20.1 % in ultimate tensile strength, yield strength, and work hardening rate, respectively, could be achieved in 3L structures, and the elongation was maintained at 5.5 %. The significant structural gradients and two interfaces of 3L structures were confirmed to stimulate the production of geometrically necessary dislocations (GNDs), which increased strength and work hardening rate. Two interfaces of 3L structures also contributed to the efficient moderation of plastic strains and thus improved the ductility. This study significantly enhances the strength and ductility of thin electroplated NT-Cu foils by introducing heterogeneous microstructures, promoting the application of thin NT-Cu films in electric vehicles, batteries, and the semiconductor industry.

The combined effects of carbides and martensite blocks heterogeneity on the fatigue life scatter in bearing steel

With the mitigation of inclusion-related harm through purifying methods, the fatigue life scatter of bearing steels is now predominantly governed by microstructural heterogeneity. This work employs an integrated experimental-numerical approach to elucidate the microstructure-related localized plastic strain in 52100 bearing steel under cyclic loading. We observed significant localized plastic strain in the coarse grains adjacent to aggregated carbides after unloading, which may act as a precursor to fatigue cracks and contribute to fatigue life scatter. This strain arises from two primary factors: stress concentration due to aggregated carbides and the relatively lower strength of coarse matrix grains, which is usually overlooked. Enhancing the homogeneity of both carbide distribution and matrix grain size will be a strategy to reduce the fatigue life scatter of bearing steels.

Anisotropic corrosion behavior of laser-based direct energy deposited H13 steel in molten ADC12 aluminum alloy

The immersion corrosion experiment of laser-based direct energy deposited H13 steel in ADC12 aluminum alloy melt was conducted to evaluate the demands for corrosion resistance after mold repair. The effect of the anisotropic microstructure of the deposited H13 steel on corrosion behavior was elucidated by characterizing the microstructure before and after corrosion. The results show that the microstructural heterogeneity affects the initiation, development and stabilization process of corrosion. Corrosion tends to initiate near the heat-affected zone due to the heterogeneous distribution of grain size, carbides, and dislocation density. Planes with weak microstructural heterogeneity are less likely to develop cracks in the intermetallic compounds (IMCs) layer during the corrosion development stage, thus exhibiting a slower corrosion rate. The IMCs layer of plane with weak microstructural heterogeneity stabilizes more quickly during the corrosion stabilization stage. The plane perpendicular to the build direction exhibits the highest corrosion resistance due to fewer corrosion initiation locations and the weakest microstructural heterogeneity. Furthermore, the composite structure of carbide core + Si phase shell, generated during corrosion, contributes to improving localized corrosion resistance. This study provides insights into understanding the influence of the heterogeneous microstructure from additive manufacturing on the reaction-diffusion process and offers guidance on direction selection when preparing components for applications in contact with aluminum melts using additive manufacturing techniques.

Microstructure-Dependent Macroscopic Electro-Chemo- Mechanical Behaviors of Li-Ion Battery Composite Electrodes

The rapid development of the electric vehicle industry has created an urgent need for high-performance Li-ion batteries. Such demand not only requires the development of novel active materials but also requires optimized microstructure of composite electrodes. However, due to complicated heterogeneous electrode microstructure, there still lacks a relationship between the electrode microstructure and the macroscopic electro-chemo-mechanical performance of the battery. In this study, electrochemical and mechanical multi-scale models are developed in order to account for the influence of the heterogeneous microstructure on the macroscopic mechanical and electrochemical behavior of the battery. It is found that porosity and particle size are two important parameters to characterize the microstructure that can affect the macroscopic mechanical and electrochemical behavior. The models developed in this study can be served as designing guidelines for the optimization for the Li-ion battery composite electrodes.

Dynamic plastic deformation at liquid nitrogen temperature and aging effects on microstructure and properties of Cu-0.3Zr-0.15Cr alloys

The 9R phase is rarely formed in copper alloys because of its high stacking fault energy. In this study, we generated a large amount of 9R phase in Cu-0.3Zr-0.15Cr alloys by dynamic plastic deformation (DPD) at liquid nitrogen temperature for the first time, and revealed the transformation mechanism of 9R to nanotwins. The process of DPD + aging treatment enables the alloy to obtain excellent comprehensive properties (strength of 657 MPa, electrical conductivity of 75.2 % IACS, and elongation of 14.6 %), which are derived from the heterogeneous microstructure composed of ultrafine structures, coarse grains, and nano precipitated phases. In addition, we have evaluated the phase transformation kinetics after DPD at liquid nitrogen temperature. This work provides a new way to better understanding the microstructure transition and precipitation kinetics under dynamic plastic deformation with aging treatment at liquid nitrogen temperature.

Correlation structures of statistically isotropic stiffness and compliance TRFs through upscaling

This paper reports a procedure to develop random fields of material properties on a mesoscale level, coarser than the microscale level of heterogeneous material microstructure. Since the anisotropy of properties at the mesoscale level is unavoidable, tensor-valued random fields (TRFs) need to be constructed. The construction satisfies three criteria: (i) the passage from the micro to mesoscale must be conducted according to micromechanics, (ii) any anisotropic properties must be grasped, and (iii) full (spatial) correlation structure must be accounted for. The construction is illustrated in the example of a linear elastic microstructure modeling a planar interpenetrating phase composite (IPC), where each phase is interconnected throughout the microstructure. The most general representation of a statistically homogeneous and isotropic correlation structure of the TRFs of stiffness and compliance is employed. Four material functions of the representation are determined through scale-dependent homogenization, which grasps any auto- and cross-correlations within/among different components of TRFs. The Gaussianity of the TRFs is also assessed with a finding that, overall, the smaller is the mesoscale and the more pronounced is the microstructural randomness, the more non-Gaussian are the mesoscale TRFs.

The evaluation of the effects of different curing methods on concrete

This research explores the impact of various curing methods on the water absorption and compressive strength of concrete. To evaluate these properties, 45 cubes of mix ratio 1:2:4 were tested under different curing methods namely water ponding, sand, polythene sheet coverage, sawdust and water sprinkling. All the cubes were cured at an average laboratory temperature. Water absorption, compressive strength and SEM-EDX tests were conducted on the concrete cubes after curing for 7, 14 and 28 days. The study found that the water absorption and compressive strength values at 7, 14, and 28 curing days ranged from 1.53 to -1.12% and 21.30 to 31.40 N/mm2, respectively and varied depending on the curing method used. All the cementitious compositions of the concrete cubes of all the curing methods showed an increase with the increase in curing age and ranged between 0.01 and 81.67, while their SEM images revealed heterogeneous microstructures consisting of aggregates, cement paste, and voids.Sand and sawdust curing produced the highest and lowest compressive strength values, respectively. Based on these findings, sand curing was identified as the most favourablemethod. Further investigation should be carried out.