Microstructural Hierarchy Research Articles

Exceptional hardness in multiprincipal element alloys via hierarchical oxygen heterogeneities

Refractory multiprincipal element alloys (RMPEAs) are potential successors to incumbent high-temperature structural alloys, although efforts to improve oxidation resistance with large additions of passivating elements have led to embrittlement. RMPEAs containing group IV and V elements have a balance of properties including moderate ductility, low density, and the necessary formability. We find that oxidation of group IV-V RMPEAs induces hierarchical heterogeneities, ranging from nanoscale interstitial complexes to tertiary phases. This microstructural hierarchy considerably enhances hardness without indentation cracking, with values ranging between 12.1 and 22.6 GPa from the oxide-adjacent metal to the surface oxides, a 3.7 to 6.8× increase over the interstitial-free alloy. Our fundamental understanding of the oxygen influence on phase formation informs future alloy design to enhance oxidation resistance and obtain exceptional hardness while preserving plasticity.

Microstructural Hierarchy Descriptor Enabling Interpretative AI for Microelectronic Failure Analysis

Abstract This article proposes the MicroStructural Hierarchy Descriptor (µSHD) as a systematic and quantitative approach to spectra and image data in microelectronics failure analysis. It discusses concrete routes for employing µSHD directly as the quantitative descriptor for supervised and unsupervised machine learning. The authors propose that µSHD tools can be used to automate and improve characterization techniques and image processing and analysis protocols.

Enhanced fault tolerance in biomimetic hierarchical materials: A simulation study

Hierarchical microstructures are often invoked to explain the high resilience and fracture toughness of biological materials such as bone and nacre. Biomimetic material models inspired by such hierarchical biomaterials face the obvious challenge of capturing their inherent multiscale complexity, both in experiments and in simulations. To study the influence of hierarchical microstructure on fracture properties, we propose a large-scale three-dimensional hierarchical beam-element simulation framework, in which we generalize the constitutive framework of Timoshenko beam elasticity and maximum distortion energy theory failure criteria to the complex case of hierarchical networks of up to six self-similar hierarchical levels, consisting of approximately 5 million elements. We perform a statistical study of stress-strain relationships and fracture surface morphologies and conclude that hierarchical systems are capable of arresting crack propagation, an ability that reduces their sensitivity to preexisting damage and enhances their fault tolerance compared to reference fibrous materials without microstructural hierarchy.

MicroStructural Hierarchy Descriptor (μSHD)–property correlations of silicon carbide ceramics

A systematic and extensible framework for quantification of structural hierarchy is indispensable to understand the structure–property correlation of SiC ceramics. Microstructural hierarchy descriptor (μSHD) is constructed and used to quantify experimental microstructures reported in the literature. Despite a variation nature of microstructures and the scatterness in measured properties, μSHD–property correlations have been identified. The μSHD curves with less hierarchical levels suggest a high strength, while more hierarchical levels are related to a high toughness. The μSHD curve exhibiting a homogeneous distribution across all the scales and one exhibiting a gradual decrease from the smaller to larger scales can both stand for a high thermal conductivity. A quick drop from the smaller to larger scales on the μSHD curve is a characteristic of a high electrical resistivity, and a reverse trend, showing a gradual increase from smaller to larger scales and maintaining larger μSHDs at larger scales, characterizes a low resistivity.

Hierarchical structure in Al-Cu alloys to promote strength/ductility synergy

Experimental evidence demonstrated that equal channel angular pressing (ECAP) at cryogenic temperature, in comparison with ECAP at room temperature, led to promoted strength-ductility synergy in an Al-2.5wt.%Cu alloy. The simultaneous improvement is related to microstructural hierarchy of multimodal grains, low angle grain boundaries, and inter/intragranular precipitates, which was tuned by aging treatment in match with the low-temperature ECAP. The artificial aging could maintain multimodal grain size distribution, introduce a large number of low angle grain boundaries and produce intragranular precipitates to improve strength/ductility. A minor 0.3wt.% Sc addition was effective in optimizing the precipitations and further boosting the strength/ductility combination. The underlying mechanisms for higher strength and greater ductility were rationalized in terms of the low-temperature ECAP.

Extreme shear-deformation-induced modification of defect structures and hierarchical microstructure in an Al\u2013Si alloy

Extreme shear deformation is used for several material processing methods and is unavoidable in many engineering applications in which two surfaces are in relative motion against each other while in physical contact. The mechanistic understanding of the microstructural evolution of multi-phase metallic alloys under extreme shear deformation is still in its infancy. Here, we highlight the influence of shear deformation on the microstructural hierarchy and mechanical properties of a binary as-cast Al-4 at.% Si alloy. Shear-deformation-induced grain refinement, multiscale fragmentation of the eutectic Si-lamellae, and metastable solute saturated phases with distinctive defect structures led to a two-fold increase in the flow stresses determined by micropillar compression testing. These results highlight that shear deformation can achieve non-equilibrium microstructures with enhanced mechanical properties in Al–Si alloys. The experimental and computational insights obtained here are especially crucial for developing predictive models for microstructural evolution of metals under extreme shear deformation.

Multiscale structural characterization of yttria dispersed copper alloys fabricated by hot isostatic processing of mechanically alloyed powders

Multi-scale characterization techniques, including computed tomography (CT) based on serial sectioning by focused ion beam (FIB) and scanning transmission electron microscopy (STEM), have been employed to elucidate the microstructural hierarchy inside yttria (Y2O3) dispersed copper alloys designed for fusion reactors. The alloys were fabricated by the combination of mechanical alloying of metal and oxide powders and subsequent hot isostatic pressing. Optical microscopy, together with electron micro-probe analysis revealed bimodal size distribution of powders, where coarse Cu powders of several hundred micrometer are composed of metallic core surrounded by a crust of Cu oxides, while fine powders of several ten micrometer are mostly of oxides. Scanning electron microscopy (SEM) on the fabricated bulk alloys revealed heterogeneous inner structure, which is best described by an assembly of large Cu grain regions bordered by small Cu grain regions. Distribution of Y2O3 particles are visualized three dimensionally by serial sectioned FIB-SEM images and by CT-STEM, both of which revealed the particles are enriched in small Cu grain regions. These multi-scale characterization, together with mechanical bend testing, demonstrated that excess Y2O3 particles in grain boundaries leads to brittle fracture, while that large Cu grains are responsible for the ductility of the alloy.

Data-Driven Computational Simulation in Bone Mechanics.

The data-driven approach was formally introduced in the field of computational mechanics just a few years ago, but it has gained increasing interest and application as disruptive technology in many other fields of physics and engineering. Although the fundamental bases of the method have been already settled, there are still many challenges to solve, which are often inherently linked to the problem at hand. In this paper, the data-driven methodology is applied to a particular problem in tissue biomechanics, a context where this approach is particularly suitable due to the difficulty in establishing accurate and general constitutive models, due to the intrinsic intra and inter-individual variability of the microstructure and associated mechanical properties of biological tissues. The problem addressed here corresponds to the characterization and mechanical simulation of a piece of cortical bone tissue. Cortical horse bone tissue was mechanically tested using a biaxial machine. The displacement field was obtained by means of digital image correlation and then transformed into strains by approximating the displacement derivatives in the bone virtual geometric image. These results, together with the approximated stress state, assumed as uniform in the small pieces tested, were used as input in the flowchart of the data-driven methodology to solve several numerical examples, which were compared with the corresponding classical model-based fitted solution. From these results, we conclude that the data-driven methodology is a useful tool to directly simulate problems of biomechanical interest without the imposition (model-free) of complex spatial and individually-varying constitutive laws. The presented data-driven approach recovers the natural spatial variation of the solution, resulting from the complex structure of bone tissue, i.e. heterogeneity, microstructural hierarchy and multifactorial architecture, making it possible to add the intrinsic stochasticity of biological tissues into the data set and into the numerical approach.

Effect of cyclic rapid thermal loadings on the microstructural evolution of a CrMnFeCoNi high-entropy alloy manufactured by selective laser melting

Metallic materials produced by additive manufacturing experience complex stress and thermal gyrations along the build direction. This has the potential to produce complicated heterogeneous microstructures that may exhibit a wide variety of mechanical properties. There remains a paucity of studies on the nature and the formation mechanisms of the microstructural heterogeneity and this limits our capability for microstructural design in additively manufactured metallic materials. Here, we present an electron microscopy-based investigation of a CrMnFeCoNi high-entropy alloy produced by selective laser melting. We have focussed on a systematic investigation of the microstructural evolution along the build direction. Our results reveal a remarkable hierarchy of microstructures, including the formation of nanocrystalline grains, elemental segregation and precipitation, cellular dislocation structures, deformation twinning, and deformation-induced phase transformation. Our research clarifies the relationships amongst different features, and provides guidance for future structural manipulation of materials produced by additive manufacturing.

Hierarchical phase separation behavior in a Ni-Si-Fe alloy

Improving the properties of durable high-temperature alloys is based on the fundamental understanding of the link between microstructure and three-dimensional (3D) nanochemistry. Here we utilize a complementary approach of transmission electron microscopy and atom probe tomography to link microstructure and 3D nanochemistry of a ternary single crystal Ni83.9Si13Fe3.1 (at.%) model alloy. The formation of a γ/γ' microstructure is revealed, containing primary and secondary γ' precipitates analogous to Ni-based superalloys. Subsequently, microstructural hierarchy is created by the formation of γ particles inside primary γ' precipitates. The correlated supersaturation with γ forming elements (Ni, Fe) of primary γ' precipitates was identified as driving force for the formation of γ particles. The influence of aging on the mechanical properties is reported and peak hardness is achieved after 24 h of aging at 923 K. Thermo-Calc equilibrium phase concentrations based on the TTNi8 database where found to be closer to the APT data than the TCNi8 based values. Our results suggest that improved stability of γ particles can be achieved by tailoring the phase chemistry and the lattice misfit.

On the strengthening and embrittlement mechanisms of an additively manufactured Nickel-base superalloy

The γ′ phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ′ precipitated with various size and fraction depending on heat treatment process. For room-temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built sample while dislocations bypassing the precipitates via Orowan looping in the γ′ strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ′ strengthened samples have higher work hardening rate than as-built sample but encounter premature failure. Experimental evidence shows that the embrittlement mechanism in the γ′ strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strength and ductility of Nickel-base superalloy is discussed in detail.

On the Strengthening and Embrittlement Mechanisms of an Additively Manufactured Nickel-Base Superalloy

The γ' phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ' precipitated with various size and fraction depending on heat treatment process. For room temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built samples while dislocations bypassing the precipitates via Orowan looping in the γ' strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ' strengthened samples have higher work hardening rate than as-built sample but encountered premature failure. Experimental evidence shows that the embrittlement mechanism in the γ' strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strengthening and ductility of Nickel-base superalloy is discussed in detail.

Chemically modified graphene films with tunable negative Poisson\u2019s ratios

Graphene-derived macroscopic assemblies feature hierarchical nano- and microstructures that provide numerous routes for surface and interfacial functionalization achieving unconventional material properties. We report that the microstructural hierarchy of pristine chemically modified graphene films, featuring wrinkles, delamination of close-packed laminates, their ordered and disordered stacks, renders remarkable negative Poisson’s ratios ranging from −0.25 to −0.55. The mechanism proposed is validated by the experimental characterization and theoretical analysis. Based on the understanding of microstructural origins, pre-strech is applied to endow chemically modified graphene films with controlled negative Poisson’s ratios. Modulating the wavy textures of the inter-connected network of close-packed laminates in the chemically modified graphene films also yields finely-tuned negative Poisson’s ratios. These findings offer the key insights into rational design of films constructed from two-dimensional materials with negative Poisson’s ratios and mechanomutable performance.

Resolving fine electromechanical structure of collagen fibrils via sequential excitation piezoresponse force microscopy

Collagen is the main protein in extracellular matrix that is found in many connective tissues, and it exhibits piezoelectricity that is expected to correlate with its hierarchical microstructure. Resolving fine electromechanical structure of collagen, however, is challenging, due to its weak piezoresponse, rough topography, and microstructural hierarchy. Here we adopt the newly developed sequential excitation strategy in combination with piezoresponse force microscopy to overcome these difficulties. It excites the local electromechanical response of collagen via a sequence of distinct frequencies, minimizing crosstalk with topography, followed by principal component analysis to remove the background noise and simple harmonic oscillator model for physical analysis and data reconstruction. These enable us to acquire high fidelity mappings of fine electromechanical response at the nanoscale that correlate with the gap and overlap domains of collagen fibrils, which show substantial improvement over conventional piezoresponse force microscopy techniques. It also embodies the spirit of big data atomic force microscopy that can be readily extended into other applications with targeted data acquisition.

Hierarchical microstructure for improved fatigue properties in a eutectic high entropy alloy

Microstructural hierarchy can enable enhanced properties in high entropy alloys. A unique microstructure was observed in 50% cold-rolled and heat-treated eutectic high-entropy alloy (EHEA). Formation of recrystallized FCC grains and B2 precipitates in FCC lamellae with different microstructural length scale constitute the hierarchy in cold-rolled and heat-treated EHEA. Monotonic loading and high cycle fatigue properties of both microstructures were compared. In as-cast microstructure, the fatigue crack originated in FCC lamellae with the formation of persistent slip bands. The B2 precipitates in cold-rolled and heat-treated alloy delayed the crack initiation and improved fatigue properties.

Multi-scale electron microscopy of overnitrided Sm-Fe-Mn-N powder

Multi-scale electron microscopy technique that combines transmission electron microscopy (TEM) and focused ion beam - scanning electron microscopy (FIB-SEM) was utilized to investigate the influences of the change in microstructure of Sm2(Fe0.95Mn0.05)17Nx by overnitridation on the magnetic properties. The recent high-contrast backscatter electron imaging technique in SEM machines enabled us to reveal the formation events of cell-like microstructure from a quite large field of view. In addition, detailed TEM observations revealed the crystallographic orientations of the cells as well as the local chemical fluctuation. The combination of these techniques allows us to understand the microstructural hierarchy in this material, verifying that the original orientations of the individual particles are inherited to the nanocrystalline cells formed by overnitridation.

Sharp volumetric billboard based characterization and modeling of complex 3D Ni/Al high energy ball milled composites

We present an innovative image-based modeling technique, based on Google Earth like algorithms, to effectively resolve intricate material morphology and address the computational complexity associated with heterogeneous materials. This sharp volumetric billboard algorithm stems from a volumetric billboard method, a multi-resolution modeling strategy in computer graphics. In this work, we enhance volumetric billboards through a sharpening filter to reconstruct the statistical information of heterogeneous systems. A hierarchy of microstructures is created for high energy ball milled Ni/Al composites. We analyze the first- and second-order statistics of microstructures, and characterize both macro- and micro-mechanical material responses. Furthermore, we conduct a convergence study of the associated computational results. The statistical and mechanical robustness of data compression is demonstrated through the corresponding error analysis.

A cascade continuum micromechanics model for the effective elastic properties of porous materials

The elastic properties of porous materials with a disordered pore structure are estimated using the mean-field Eshelby homogenization scheme together with the principle of recurrence to generate a cascade of effective microstructures as a function of the porosity and the cascade level n. Starting with the Hashin–Shtrikman upper bound for porous materials, the proposed cascade micromechanics model generates a hierarchy of micro-structures which evolve from an initial configuration of a porous material with spherical pores embedded within an elastic solid phase consistent with the Mori–Tanaka matrix inclusion morphology to a porous material characterized by a hierarchic distribution of spherical elastic grains. The model is explicit and allows for an easy computational implementation. It predicts physically consistent threshold porosities, characteristic for the specific morphology of the porous material under consideration, beyond which the material loses its stiffness. The validity of the cascade micromechanics model is evaluated against experimental data for various materials ranging from foam to ceramics with different pore structures.

Research on Hydrophobic and Superhydrophobic Properties of Patterned Structure with Controllable Nano to Microstructural Hierarchy

Presented here is a scalable self‐assembly route for fabricating a hierarchically structured material, possessing the property of superhydrophobicity, based on the breath figure (BF) method. The composite‐structured elastomers with dynamically controllable micro and nanoscale roughness are fabricated by stamping close‐packed monolayer of monodispersed silica particles, with different sizes, onto the surface of polydimethylsiloxane (PDMS) protrusion arrays, prepared by copying various molding BF films. The result of water contact angles (WCAs) measurement reveals that the variations in size features of either microscale or nanoscale make little difference in the WCAs of micropatterned or composite‐structured elastomers. It has been found out that the ratio of prominence pitch to protrusion diameter of structured PDMS samples keeps to be constant when the micro or nanoscale roughness varied a lot. The computation based on Cassie's law indicates that such geometrical feature contributes to the constant WCAs upon the varied roughness both in micro and nanoscale. image

Polymorphism of CaCO3 and microstructure of the shell of a Brazilian invasive mollusc (Limnoperna fortunei)

Applying the theories of Materials Science and Engineering to describe the composition and hierarchy of microstructures that comprise biological systems could help the search for new materials and results in a deeper insight into evolutionary processes. The layered microstructure that makes up the freshwater bivalve Limnoperna fortunei shell, an invasive specie in Brazil, was investigated utilizing SEM and AFM for the determination of the morphology and organization of the layers; and XRD was used to determine the crystalline phases of the calcium carbonate (CaCO3) present in the shell. The presence of the polymorphs calcite and aragonite were confirmed and the calcite is present only on the external side of the shell. The shell of L. fortunei is composed of two layers of aragonite with distinct microstructures (the aragonite prismatic layer and the aragonite sheet nacreous layer) and the periostracum (a protein layer that covers and protects the ceramic part of the shell). A new morphology of the calcite layer was found, below the periostracum, without defined form, albeit crystalline.