Extrinsic Toughening Mechanisms Research Articles

Powder Metallurgy Produced Aligned Long Tungsten Fiber Reinforced Tungsten Composites

For the future fusion reactor, tungsten is the main candidate material as the plasma-facing material. However, considering the high thermal stress during operation, the intrinsic brittleness of tungsten is one of the issues. To overcome the brittleness, tungsten fiber reinforces tungsten composites (Wf/W) developed using extrinsic toughening mechanisms. The powder metallurgy process and chemical vapor deposition process are the two production routes for preparing Wf/W. For the powder metallurgy route, due to technical limitations, previous studies focused on short random distributed fiber-reinforced composites. However, for short random fiber composites, the strength and reinforcement effect are considerably limited compared to aligned continuous fiber composites. In this work, aligned long tungsten fiber reinforced tungsten composites have been first time realized based on powder metallurgy processes, by alternately placing tungsten weaves and tungsten powder layers. The produced Wf/W shows significantly improved mechanical properties compared to pure W and conventional short fiber Wf/W.

Interface and mechanical properties of the single-layer long fiber reinforced Wf/W composites fabricated via field assisted sintering technology

Tungsten fiber reinforced tungsten composites (Wf/W) show a pseudo-ductile behavior because of extrinsic toughening mechanisms such as interface de-bonding, pull-out and plastic deformation of the fiber. In the present work, single-layer long fiber Wf/W composites with and without yttrium oxide (Y2O3) interface were fabricated by a field assisted sintering technology (FAST) process. The microstructure and mechanical properties of the prepared Wf/W composites were characterized. The fracture behavior and toughening mechanisms were analyzed in detail combining the results of experiments and finite element modelling. Wf/W composites with Y2O3 interface (weak interfacial strength) show a typical pseudo-ductile fracture behavior and a higher flexural strength than the composites without Y2O3 interface (strong interfacial strength). The fracture energy dissipation is mainly driven by plastic deformation of the fibers, but interface de-bonding is a necessary factor to ensure any extrinsic toughening mechanisms.

Fabrication and characterisation of alumina/aluminium composite materials with a nacre-like micro-layered architecture

Many natural materials demonstrate ideal design inspirations for the development of lightweight composite materials with excellent damage tolerance. One notable example is the layered architecture of nacre, which possesses toughness an order of magnitude higher than its constituent parts. Man-made nacre-like ceramic/polymer composites obtained through direct infiltration of polymer in ceramic scaffolds have been shown to produce improved mechanical properties over other composite architectures. Replacing the polymer phase with metal could provide higher damage tolerance but the infiltration of metal into complex ceramic scaffolds is difficult due to the surface tension of molten metal. To address this, bioinspired nacre-like micro-layered (µL) alumina scaffolds with different ceramic fractions from 18 to 85% were infiltrated with aluminium alloy 5083 via pressureless and squeeze casting infiltrations techniques. The scaffolds were created using a bi-directional freeze-casting and one-step densification method. As a result, the µL alumina/aluminium composites displayed significant extrinsic toughening mechanisms with both high strength and toughness. The mechanical performance was highly dependent on the interface, microstructure, and composition. The nacre-like composites with 18% alumina and AlN interface displayed a maximum resistance‐curve toughness up to around 70 MPa.m½ (35 MPa.m½ at the ASTM limit) and a flexural strength around 600 MPa.

Investigations on the interface-dominated deformation mechanisms of two-dimensional MAX-phase Ti3Al(Cu)C2 nanoflakes reinforced copper matrix composites

Two-dimensional (2D) materials showing the giant advantage over their bulk counterparts are believed to be the ideal reinforcement for metal matrix composites, whereas the deformation mechanism of which is currently unknown. Herein, we investigated the interface effect of 2D MAX phase Ti3Al(Cu)C2 nanoflakes (MAX NFs) obtained by a Cu-atoms-diffusion assisted exfoliation method on the plastic deformation process of Cu matrix. Tensile tests demonstrated a simultaneous enhancement in strength and fracture elongation with the increase of MAX NFs content. The composites exhibited a four-stage deformation process involving an extended “yield plateau” before obvious strain hardening stage. Microstructural examination revealed that the 2D nature of MAX NFs and the coherent interface structure facilitated the nucleation of partial dislocations, which coordinated the initial plastic deformation by gliding and interacting to form Lommer-Cottrel (LC) locks. At high strain levels, both LC locks and MAX NFs served as obstacles for dislocation cross-slip and thus achieved significant forest hardening (or latent hardening) effect, validated by EBSD texture from the in-situ tensile tests and visco-plastic self-consistent simulation. This sort of interface-induced microstructure evolution promoted tortuous micro-cracks along discontinuous slip bands and significant crack bridging and deflection extrinsic toughening mechanisms. The current findings provide a possible strategy of simultaneously enhancing strength and ductility through tailoring 2D geometric shape of reinforcements and their interface with the metal matrix.

Phase field models of interface failure for bone application - evaluation of open-source implementations

Bone is a composite material where tubular osteon structures surrounded by thin interfaces reinforce the tissue at the microscale. These interfaces provide alternative weak paths for crack growth and give rise to potent extrinsic toughening mechanisms. Numerical models can help to gain a deeper understanding of fracture resistance in bone, however, capturing the interface mechanics is key. For this purpose, the phase field method for fracture is appealing due to its capability of capturing complex cracking phenomena. Still, it is far from well-established in the field of biomechanics. In this study, we evaluated a selection of recent open-source implementations of the phase field method to find the best approach for simulating crack growth in bone tissue. We also proposed a new method for correcting the interface fracture toughness in a phase field model. The selected implementations were compared using benchmark tests, including single edge notched tensile specimens with and without interfaces, as well as models representing typical bone geometries. We found the quasi-Newton monolithic solvers to be superior both in terms of computational speed and robustness compared to the evaluated staggered solvers. We also showed that correcting the fracture toughness of the interface is key for simulating crack patterns that are consistent with analytical predictions from linear elastic fracture mechanics.

On the damage tolerance of 3-D printed Mg-Ti interpenetrating-phase composites with bioinspired architectures

Bioinspired architectures are effective in enhancing the mechanical properties of materials, yet are difficult to construct in metallic systems. The structure-property relationships of bioinspired metallic composites also remain unclear. Here, Mg-Ti composites were fabricated by pressureless infiltrating pure Mg melt into three-dimensional (3-D) printed Ti-6Al-4V scaffolds. The result was composite materials where the constituents are continuous, mutually interpenetrated in 3-D space and exhibit specific spatial arrangements with bioinspired brick-and-mortar, Bouligand, and crossed-lamellar architectures. These architectures promote effective stress transfer, delocalize damage and arrest cracking, thereby bestowing improved strength and ductility than composites with discrete reinforcements. Additionally, they activate a series of extrinsic toughening mechanisms, including crack deflection/twist and uncracked-ligament bridging, which enable crack-tip shielding from the applied stress and lead to “Γ”-shaped rising fracture resistance R-curves. Quantitative relationships were established for the stiffness and strengths of the composites by adapting classical laminate theory to incorporate their architectural characteristics.

On mode-I and mode-II interlaminar crack migration and R-curves in carbon/epoxy laminates with hybrid toughening via core-shell rubber particles and thermoplastic micro-fibre veils

This study investigates the influence of hybrid toughening—via core-shell rubber (CSR) particles and non-woven thermoplastic veils—on the delamination resistance, crack migration and R-curve behaviour in carbon fibre/epoxy laminates under mode-I and mode-II conditions. Core-shell rubber particles, varying in size from 100 nm to 3 μm, with 0–10 wt% content, are dispersed within the epoxy resin, and thermoplastic micro-fibre veils with polyphenylene sulfide (PPS) fibres, with 5–20 g/m 2 areal weight, are introduced at the interlaminar region to achieve hybrid toughening. Carbon fibre/epoxy laminates are manufactured with a two-part resin using vacuum infusion and out-of-autoclave curing. Double cantilever beam (DCB) and four-point end-notch-flexure (4ENF) specimens are used to obtain mode-I and mode-II fracture energies and R-curves. Damage mechanisms and crack paths are characterised using fractography that provide understanding of energy dissipation. The results show that the hybrid toughening significantly improves fracture initiation and propagation energies ( i.e . mode I initiation by ∼245% and propagation by ∼275%, and mode-II initiation by ∼64% and propagation ∼215%) by extrinsic and intrinsic toughening mechanisms. Moreover, it is shown that rising R-curves can be achieved with hybrid toughening when compared with falling R-curves obtained with just thermoplastic veil toughening. Fractography revealed that the hybrid toughening constrained the crack predominantly within the veil region, making it harder to grow and absorb more energy.

Micro/nano-reinforcements in bimodal-grained matrix: A heterostructure strategy for toughening particulate reinforced metal matrix composites

To overcome strength-ductility trade-off in particulate metal matrix composites (PRMMCs), we developed a novel and controllable strategy, for the first time, via a combination of micro/nano-reinforcements and ultrafine/coarse-grained (UFG/CG) matrices. A powder assembly was developed to fabricate micro B4Cp/6061Al composites, which features UFG/CG matrices containing ex/in-situ nano dispersoids and exhibits superior combination of strength and ductility. It was found that such a novel architecture possesses both intrinsic and extrinsic toughening. As the intrinsic toughening mechanisms, enhanced dislocation storage capability of UFG regions, twinning, and hetero-deformation induced toughening were detected. Moreover, crack-deflection induced by nano dispersoids as well as crack-blunting at UFG/CG regions were considered as the main extrinsic toughening mechanisms. The proposed strategy can be applied to design the architecture of other PRMMCs with both high strength and high ductility. Aluminum alloys; Powder processing; Heterostructures; Particulate reinforced composites; Microstructural toughening

Developing nacre-inspired laminate-reticular 2024Al/B4C composites with high damage resistance by adjusting compositional wettability

To address the issue that B4C ceramics are difficult to be wetted by aluminum metals in the composites, TiB2 was introduced via an in-situ reaction between TiH2 and B4C to regulate their wettability and interfacial bonding. By pressure infiltration of the molten alloy into the freeze-cast porous ceramic skeleton, the 2024Al/B4C–TiB2 composites with a laminate-reticular hierarchical structure were produced. Compared with 2024Al/B4C composite, adding initial TiH2 improved the flexural strength and valid fracture toughness from (484±27) to (665±30) MPa and (19.3±1.5) to (32.7±1.8) MPa·m1/2, respectively. This exceptional damage resistance ability was derived from multiple extrinsic toughening mechanisms including uncracked-ligament bridging, crack branching, crack propagation and crack blunting, and more importantly, the fracture model transition from single to multiple crack propagation. This strategy opens a pathway for improving the wettability and interfacial bonding of Al/B4C composites, and thus produces nacre-inspired materials with optimized damage tolerance.

Extrinsic toughening of recycled carbon fibers in polypropylene composites in the absence of plasticity penalty

Advanced composite materials used in high-tech fields are widely reinforced with carbon fibers. One of the growing application areas for carbon fibers is their reinforced composites which are used to replace metallic automotive parts. This reduces carbon footprint through weight reduction, which is a strategy pursued globally to reduce the environmental impacts of passenger vehicles. In this study, we assess the reinforcement potential of recycled carbon fibers in a polypropylene (PP) homopolymer with high strength and flowability. The highly crystalline PP homopolymer with low impact properties was used to minimize intrinsic plasticity penalty associated with fiber reinforcement and ascribe the impact strength enhancement solely to extrinsic toughening mechanisms. The reinforced composites are manufactured through extrusion compounding followed by injection molding. Modification of the transition phase connecting the bulk matrix with the bulk carbon fibers led to 78% enhancement in the strength of the composites, compared to the unmodified composites, without any loss in other properties. Compared to a commercial steel bonnet, the compatibilized composites reinforced with recycled carbon fibers exhibited superior specific strength accompanied by ∼87% weight reduction. Morphological analysis showed that all the extrinsic toughening mechanisms are effectively used by the recycled fibers in the reinforced composites.

Artificial Nacre with High Toughness Amplification Factor: Residual Stress-Engineering Sparks Enhanced Extrinsic Toughening Mechanisms.

The high fracture toughness of mollusk nacre is predominantly attributed to the structure-associated extrinsic mechanisms such as platelet sliding and crack deflection. While the nacre-mimetic structures are widely adopted in artificial ceramics, the extrinsic mechanisms are often weakened by the relatively low tensile strength of the platelets with a large aspect ratio, which makes the fracture toughness of these materials much lower than expected. Here, it is demonstrated that the fracture toughness of artificial nacre materials with high inorganic contents can be improved by residual stress-induced platelet strengthening, which can catalyze more effective extrinsic toughening mechanisms that are specific to the nacre-mimetic structures. Thereby, while the absolute fracture toughness of the materials is not comparable with advanced ceramic-based composites, the toughness amplification factor of the material reaches 16.1± 1.1, outperforming the state-of-the-art biomimetic ceramics. The results reveal that, with the merit of nacre-mimetic structural designs, the overall fracture toughness of the artificial nacre can be improved by the platelet strengthening through extrinsic toughening mechanisms, although the intrinsic fracture toughness may decrease at platelet level due to the strengthening. It is anticipated that advanced structural ceramics with exceeding performance can be fabricated through these unconventional strategies.

Tungsten fiber reinforced tungsten (Wf/W) using yarn based textile preforms

Material related limitations are one of the main challenges for the design of future fusion reactors. Tungsten (W) as the primary material choice is considered resilient against erosion, has the highest melting point of any metal and shows low activation after neutron irradiation. However, W is intrinsically brittle and faces operational embrittlement. To overcome these issues, W-based composites have been in development. W fiber-reinforced W composite materials (W f /W) incorporate extrinsic toughening mechanisms allowing the redistribution of stress peaks and thus overcoming the intrinsic brittleness problem. In this contribution recent results on the incorporation of new textile preformes into W f /W production will be given with a focus on the production via chemical vapor deposition of tungsten-based materials. The use of tungsten yarns, instead of single wires for the textile production is elaborated.

Bioinspired Stochastic Design: Tough and Stiff Ceramic Systems

AbstractCeramics possess desirable stiffness, compressive strength, and thermal properties compared to alternative material classes. Despite this, the adoption of ceramics into advanced industries has been hindered by their inherent brittleness. Using a state‐of‐the‐art manufacturing platform, the authors incorporate disordered microstructural features inspired by those found in natural, impact‐resistant organisms using tessellated ceramic cells. To precisely mimic these natural patterns, a disorder parameter is introduced to modulate the stochasticity of the ceramic architectures. By modifying simple geometrical features such as cut depth and the disorder parameter, the energy absorption is dramatically improved, and the stiffness of the system can be tailored. It is found that the stochastic designs exhibit elevated damage tolerance, denoted by higher dynamic energy absorption (up to 330% for the 3rd impact) and stiffness (up to 200% for the 3rd impact) than both monolithic and perfectly hexagonal architectures. The results show a superior multi‐hit resistance owing to optimal cut depth and stochasticity which give access to extrinsic toughening mechanisms that can be influenced through design parameters. This highly‐scalable, digital manufacturing platform for creating numerically programmable architectures propels the automated production of intelligent, high‐performance, and tailorable ceramic systems for industrial applications in aerospace, protective devices, and medicine.

Towards the strength-ductility synergy of Al2O3/Al composite through the design of roughened interface

In the study, a roughened interface is designed to achieve the strength-ductility synergy of Al2O3/Al composites fabricated by flake powder metallurgy. The roughened SiO2@Al2O3 platelets are fabricated by high-shear dispersion process and subsequent sintering at the high temperature. The results show that different morphologies of asperities on the surface of SiO2@Al2O3 platelets can be obtained via controlling the sintering time. As the roughness increases, the strength and ductility of SiO2@Al2O3/Al composites are simultaneously improved. Load transfer efficiency is enhanced by platelet-matrix interlocking at the roughened SiO2@Al2O3/Al interface, compared to Al2O3/Al composite with smooth Al2O3–Al interface. The improved tensile ductility in SiO2@Al2O3/Al composite is attributed to the extrinsic toughening mechanism. This design strategy offers a simple and promising approach to improve the mechanical properties of metal matrix composites.

Computational homogenisation based extraction of transverse tensile cohesive responses of cortical bone tissue.

The numerical assessment of fracture properties of cortical bone is important in providing suggestions on patient-specific clinical treatments. We present a generic finite element modelling framework incorporating computational fracture approaches and computational homogenisation techniques. Finite element computations for statistical volume elements (SVEs) at the microscale are performed for different sizes with random osteon packing with a fixed volume fraction. These SVEs are loaded in the transverse direction under tension. The minimal SVE size in terms of ensuring a representative effective cohesive law is suggested to be 0.6 mm. Since cement lines as weak interfaces play a key role in bone fracture, the effects of their fracture properties on the effective fracture strength and toughness are investigated. The extracted effective fracture properties can be used as homogenised inputs to a discrete crack simulation at macroscopic or structural scale. The extrinsic toughening mechanisms observed in the SVE models are discussed with a comparison against experimental observations from the literature, giving beneficial insights to cortical bone failure.

Design of tungsten fiber-reinforced tungsten composites with porous matrix

To overcome the brittleness of tungsten, tungsten fiber-reinforced tungsten composites (Wf/W) have been developed using an extrinsic toughening mechanism. In this work, a novel type of Wf/W with porous matrix produced by field assisted sintering technology (FAST) is studied. The material is optimized regarding mechanical behavior, standing on the adjusting of matrix porosity and fiber mass fraction. Two series of samples with different matrix density and fiber mass fraction are prepared. Based on the mechanical testing, porous matrix Wf/W can represent a promising pseudo ductile behavior. Relatively lower matrix density is helpful to avoid a sudden load-drop during crack opening. The different fracture behavior is attributed to the different fiber/matrix interface bonding condition. By increasing fiber mass fraction from 20% to 50%, porous matrix Wf/W can facilitate improved mechanical properties regarding fracture toughness and strength. However, by further increasing the fiber mass fraction from 50% to 60%, a deterioration of mechanical properties is observed. The high porosity of porous matrix Wf/W causes a degradation of the thermal conductivity compared to conventional bulk tungsten. No significant change regarding thermal expansion coefficient is observed when decreasing the matrix density.

Deuterium retention in tungsten fiber-reinforced tungsten composites

• Preparation of purpose-built samples of tungsten fibre reinforced tungsten. • Laterally resolved measurements of surface deuterium retention in W fibre reinforced W. • Quantitative comparison of deuterium retention of W fibre reinforced and power metallurgical W. • Identification of D retention mechanisms. In future fusion reactors, plasma-facing materials (PFMs) have to withstand unique conditions such as high temperatures, ion and neutron irradiation. Tungsten (W) has been established as main candidate material due to its favorable properties regarding the fusion environment but brings one major challenge: Its brittleness at moderate temperatures can lead to failure of tungsten components. Tungsten fiber-reinforced tungsten (W f /W), a tungsten matrix containing drawn tungsten fibers, was developed to mitigate this problem by using extrinsic toughening mechanisms to achieve pseudo-ductility. The deuterium (D) retention in W f /W manufactured by chemical vapor deposition (CVD) has been investigated using W f /W single layered model systems consisting of a single plane of unidirectional tungsten fibers embedded in a tungsten matrix produced by CVD. Various parameters with potential influence on the D retention, such as the choice of an erbium oxide interface and potassium doping, have been included in the investigation. The samples have been ground to varying distances between surface and fiber plane - exposing distinct details of the W f /W microstructures at the surface. The samples were exposed to a low temperature D plasma at 370 K for 72 h resulting in a total fluence of 10 25 D/m 2 . The D retention of all samples was measured by nuclear reaction analysis (NRA) and thermal desorption spectroscopy (TDS). The D retention in W f /W composites is higher than in reference samples made from hot-rolled W by factors between 2 and 5. In addition, a comparison of NRA and TDS data indicates that D penetrates faster into the depth of W f /W material than into hot-rolled tungsten.

Microcantilever Fracture Tests on Eutectic NiAl–Cr(Mo) In Situ Composites

Lamellar eutectic NiAl–Cr(Mo) alloys show an increased fracture toughness due to different toughening mechanisms. These mechanisms result from the fibrous or lamellar microstructure of the two constituting phases α‐Cr(Mo) and β‐NiAl. However, the fracture toughness of the individual phases and the evolution from early crack growth to the toughening mechanisms have not yet been systematically studied. Herein, bending tests on focused ion beam (FIB)‐notched microcantilever beams are used to characterize the small‐scale fracture properties. The micromechanical investigations reveal that the fracture toughness of the α‐Cr(Mo) phase () is much higher than the fracture toughness of β‐NiAl (). Larger cantilevers in the crack arresting orientation show an enhanced fracture toughness with up to , which is still lower than the one of macroscopic experiments. This is attributed to the small interaction volume of the crack, which hinders the full exploitation of potential extrinsic toughening mechanisms.

Microstructural Effects on the Interfacial Adhesion of Nanometer-Thick Cu Films on Glass Substrates: Implications for Microelectronic Devices.

Improving the interface stability for nanosized thin films on brittle substrates is crucial for technological applications such as microelectronics because the so-called brittle–ductile interfaces limit their overall reliability. By tuning the thin film properties, interface adhesion can be improved because of extrinsic toughening mechanisms during delamination. In this work, the influence of the film microstructure on interface adhesion was studied on a model brittle–ductile interface consisting of nanosized Cu films on brittle glass substrates. Therefore, 110 nm thin Cu films were deposited on glass substrates using magnetron sputtering. While film thickness, residual stresses, and texture of the Cu films were maintained comparable in the sputtering processes, the film microstructure was varied during deposition and via isothermal annealing, resulting in four different Cu films with bimodal grain size distributions. The interface adhesion of each Cu film was then determined using stressed Mo overlayers, which triggered Cu film delaminations in the shape of straight, spontaneous buckles. The mixed-mode adhesion energy for each film ranged from 2.35 J/m2 for the films with larger grains to 4.90 J/m2 for the films with the highest amount of nanosized grains. This surprising result could be clarified using an additional study of the buckles using focused ion beam cutting and quantification via confocal laser scanning microscopy to decouple and quantify the amount of elastic and plastic deformation stored in the buckled thin film. It could be shown that the films with smaller grains exhibit the possibility of absorbing a higher amount of energy during delamination, which explains their higher adhesion energy.

Engineering toughening mechanisms in architectured ceramic-based bioinspired materials

Ceramics offer many attractive properties including low-density, high compressive strength, remarkable thermal stability, and high oxidation/corrosion resistance. However, these materials suffer from brittleness, which substantially limits the range of their applications, where high toughness is required. This investigation draws inspiration from a concept of architectures with three-dimensional (3D) networks of weak interfaces targeting high toughness ceramics. In this study, a comprehensive method combining an advanced computational model with 3D digital image correlation (DIC) was developed to engineer bioinspired multilayered architectured ceramics and assesses their toughening and deformation mechanisms when subjected to a low-velocity impact load regime. A complete finite element (FE) analysis was conducted to precisely evaluate the crack growth and displacement field of the architectured ceramics and is compared to those of plain ceramics. The damage and displacement evolution results from FE analysis and experimental testing revealed that the primary source of toughening of the architectured ceramic systems is extrinsic, resulting from extensive crack deflection and delamination. Crack propagation along an irregular long path at the weak interfaces of architectured layers increased the toughness of the plain ceramics by two orders of magnitude. Based on the DIC data, both extrinsic and intrinsic toughening mechanisms were captured: sliding of the tiles in the architectured ceramics and channel plastic deformation in adhesive interlayers, respectively.