Extrinsic Toughening Mechanisms Research Articles

High fracture toughness of an ultra-strong medium Mn steel with fibrous ferrite in a martensitic matrix

Developing ultrahigh strength steels that are fracture resistant and economical would be appealing for structural applications. In this work, an ultra strong-and-tough steel was fabricated by introducing fibrous ferrite into martensite matrix. This steel exhibited an ultrahigh yield strength and tensile strength, 1560 and 2103 MPa respectively, along with a 7.3 % uniform elongation. The fracture toughness and smoothly rising R-curve were measured through the elastic-plastic unloading compliance method using miniaturized side-grooved compact tension (CT) specimens. A high fracture toughness (KIC≈105 MPa m1/2) that is superior to many steels at the same strength level was achieved. The heterogeneous microstructure of fibrous ferrite embedded in a martensitic matrix allows for high strain hardening rate and adequate micro-void formation, consequently leading to higher fracture strain and the occurrence of localized necking. The toughening mechanisms of CT samples are discussed according to the energy criterion for fracture. The expanded plastic deformation area (intrinsic toughening mechanism), deflecting and bridging the primary crack propagation as well as interface delamination between fibrous ferrite and martensite (extrinsic toughening mechanisms) significantly improve the overall fracture resistance. The current findings promote the development of high-strength and high-toughness medium-Mn steels for structural applications.

Tuning chemical short-range order for simultaneous strength and toughness enhancement in NiCoCr medium-entropy alloys

The pursuit of enhancing strength and toughness remains a critical endeavor in the field of structural materials. This study explores two distinct strategies to overcome the traditional strength-toughness trade-off. Specifically, we manipulate the chemical composition and short-range order (SRO) of the NiCoCr medium-entropy alloy, which has shown remarkable fracture toughness in recent experiments. Utilizing molecular dynamics simulations, we uncover nano-scale deformation mechanisms during crack propagation. Our findings highlight that optimizing the SRO degree leads to improvements in both atomic scale strength and toughness defined as the area underneath stress-strain curves from MD simulations. In contrast, a trade-off between strength and toughness persists when only manipulating the Ni content in the NiCoCr alloy. Based on the simulation results, we establish a strong correlation between toughness, strength, surface energies, and unstable stacking fault energies. These factors are influenced by the chemical composition and SROs in NiCoCr, with SROs acting as strong obstacles to dislocations, thereby contributing to additional strength. The exceptional toughness of NiCoCr with SRO arises from a synergy of intrinsic and extrinsic mechanisms, including dislocation glide, nanobridging during nanovoid coalescence and zigzag crack path. It is found that, in the presence of SRO, intrinsic toughening mechanisms usually associated with crack tip blunting and dissipation can also facilitate the onset of extrinsic toughening mechanisms of nanobridging and zig-zag crack path associated with nanovoid formation and coalescence. This study emphasizes the importance of tailoring SRO in designing materials with enhanced strength and toughness.

A damage-tolerant hybrid aluminum composite reinforced with carbon nanotube and zirconia: study of strengthening mechanisms by combined experiments and molecular dynamics simulations

Aluminum composites, reinforced with CNT and ZrO2 particles, were synthesized via powder metallurgy, undergoing mechanical testing and molecular dynamics (MD) simulations. Hybrid reinforcement significantly increased strength and hardness while preserving toughness through extrinsic toughening mechanisms, preventing a substantial drop in energy absorption. MD simulations revealed dislocation nucleation at reinforcement interfaces, enhancing material strength during propagation. Various reinforcements activated specific strengthening mechanisms, quantified by models. The hybrid composite exhibited excellent strength enhancement and maintained toughness up to a specific reinforcement content. Despite a reduction in energy absorption, the remarkable strength improvement suggests synergistic reinforcement effects, suitable for high-performance, lightweight materials.

Tungsten fiber-reinforced tungsten composites and their thermal stability

Tungsten will be used as armor material for plasma-facing components in future fusion reactors, but its propensity to embrittlement by microstructural restoration at high temperatures poses a challenge for its use. Tungsten fiber-reinforced tungsten composites (Wf/W) with drawn tungsten wires embedded in a polycrystalline tungsten matrix remedy the inherent brittleness of tungsten and achieve pseudo-ductile behavior via extrinsic toughening mechanisms. As plasma-facing materials experience high heat fluxes during operation, their thermal stability is important. In Wf/W composites, the restoration processes at high temperatures differ significantly between wires and matrix: initially recrystallization dominates in the wires, as they were plastically deformed during wire drawing, whereas abnormal grain growth occurs in the matrix. Growing grains may obstruct the interface between wire and matrix and deteriorate the otherwise improved fracture properties of Wf/W. An yttria interlayer is introduced to separate wire from matrix, to hinder an interplay between the restoration processes and to impede grains from the wire from growing into the matrix and vice versa. Cylindrical model systems containing a single wire in a chemically vapor-deposited matrix are investigated without any interlayer and with an yttria interlayer of either 1 μm or 3 μm thickness. Isothermal annealing at 1450°C for different times up to 2 weeks, followed by microstructural characterization by means of EBSD are carried out to characterize the microstructural evolution. The role of the interlayer on the microstructural evolution is elucidated to establish if decoupling of the restoration processes is actually achieved.

Characterization of bamboo extrinsic toughening mechanism in bending by X-ray micro-computed tomography(micro-CT)

Bamboo as a natural biomaterial with excellent strength and toughness is widely used in construction, furniture, decoration, handicrafts and other fields. However, the internal cracks that lead to the bending-fracture behavior of bamboo during its application are not yet fully understood. Herein, we used micro-computed tomography (micro-CT) to investigate the external toughening mechanism of bamboo during bending aiming to broaden the application areas of bamboo. These studies demonstrated that bamboo's outstanding strength and toughness were attributed to the synergistic extrinsic toughening mechanism dominated by fibers. Moreover, the fiber delamination and interfacial debonding affected crack deflection in the surrounding parenchyma and vessels. Additionally, the fiber content was positively correlated with the energy release rate of fiber pullout. When the fiber content was close, the fibers closed to the tension side had a better inhibition effect on the crack propagation. Furthermore, the effects of different microstructure arrangements on crack deflection, crack deflection inside fibers, crack deflection in parenchyma, and the toughening mechanism of fiber pullout were also discussed. Therefore, this work further advances the mechanism strategy for enhancing the flexibility of bamboo, which indicates the great potential in bending applications.

Deformation modes and fracture behaviors of peak-aged Mg-Gd-Y alloys with different grain structures

The ductility and toughness of peak-aged (PA) Mg-RE alloys are significantly influenced by their grain structure characteristics. To investigate this issue, we examined PA Mg-8.24Gd-2.68Y (wt.%) alloys with two distinct grain structures: an extruded-PA sample with dynamic recrystallized (DRXed) fine grains and coarse hot-worked grains, and an extrusion-solution treated and PA sample with grown large equiaxed grains. The results showed that the extruded-PA sample demonstrated a favorable combination of tensile strength (426 MPa) and ductility (7.0 %). Although intergranular microcracks nucleated in the DRXed region due to strain incompatibility, crack propagation was impeded by the DRXed fine grains, inducing intrinsic and extrinsic toughening mechanisms. On the other hand, the hot-worked grains in the extruded-PA sample initiated transgranular cracks after a relatively high strain, attributed to the strain partitioning effect, ultimately leading to failure. In comparison, the solution-treated-PA sample exhibited lower tensile strength and ductility (338 MPa and 3.7 %, respectively). Intergranular cracks nucleated in the CG sample before necking, and the readily formed critical crack, facilitated by the large grain size, exhibited unstable crack growth, resulting in premature failure. This work offers valuable insights for designing high-performance PA Mg-RE alloys and preventing premature failure in practical applications.

Fracture behavior of PH15-5 stainless steel manufactured via directed energy deposition

The tensile properties, fracture toughness, and fatigue crack growth (FCG) characteristics of a directed energy deposited precipitation hardened stainless steel (grade: PH15-5; age hardening heat treatment condition: H900) were examined. In the as-fabricated condition, the alloy contains microfissures that are oriented parallel to the build direction, whose appearance was difficult to be detected using optical microscopy. Due to their relative orientation w.r.t. the loading direction, significant anisotropy in tensile and FCG behavior was noted, with the properties being particularly lower when the loading direction is perpendicular to the crack orientation. Despite the presence of microfissures, the alloy’s fracture initiation toughness is comparable to (or in some cases exceeds) those manufactured using either conventionally techniques or laser powder bed fusion. Activation of the extrinsic toughening mechanisms, such as crack deflection when its mode I direction is perpendicular to the microfissures and a combination of crack bridging and deflection when it is parallel, are the micromechanical reasons for the observed high toughness. The efficacy of such mechanisms is observed to depend on the plastic zone size relative to the microfissure spacing. The understanding developed in this study enables the development of strategies for enhancing the damage tolerance of additively manufactured alloys.

Role of extrinsic and intrinsic toughening mechanisms in graphene nanosheets reinforced magnesium matrix layered composites

Constructing a layered structure is an effective approach to achieve a balance between strength and toughness. The toughening mechanisms of layered composites are generally considered to include reinforcement bridging, pullout, and crack deflection. However, the above toughening mechanisms are more applicable to ceramic matrix composites with predominantly cleavage fracture. For metal matrix composites with a larger fracture process zone, the energy consumption in crack propagation may be dominated by the plastic deformation at the crack tip. Therefore, elucidating the dislocation emission behavior and the interaction between graphene nanosheets (GNSs) and crack is the key to revealing the toughening mechanisms. In this work, the extrinsic and intrinsic toughening mechanisms for the layered GNS/Mg composites were investigated systematically. The three-dimensional reconstruction of the fracture morphology, combined with statistical analysis and molecular dynamic (MD) simulations, elucidates that macroscopic crack deflection in layered composites does not always result in a microscopically longer crack propagation path. The mitigation and redistribution of stress concentration and the promotion of dislocation emission at the crack tip induced by the GNS layers are proposed to be critical toughening mechanisms in the GNS/Mg layered composites.

Effect of bimodal microstructure on the tensile properties and fracture toughness of extruded Mg-9Gd-4Y-0.5Zr alloy

The effects of bimodal microstructure on the tensile properties and fracture toughness of Mg-9Gd-4Y-0.5Zr alloys were investigated. The Mg-9Gd-4Y-0.5Zr alloy was extruded at different temperatures to produce samples with bimodal and fully dynamic recrystallized (DRXed) grain structures. The results show that the bimodal grain structure exhibits a better combination of yield strength (YS of 277 MPa) and plane strain fracture toughness (KIc = 21.5 MPa m1/2), compared to the fully DRXed microstructure (YS of 207 MPa, KIc = 16.3 MPa m1/2). The fracture mechanisms for the bimodal and fully DRXed microstructures are the ductile fracture mode and quasi-cleavage fracture mode, respectively. Both hetero-deformation induced (HDI) strengthening effect and strong basal texture can contribute to high tensile strength in the bimodal grain structured sample. As compared to the sample with fully DRXed microstructure, the stronger strain hardening capacity and larger hardened region around the crack tip in the bimodal grain structured sample can bring about the improvement in the fracture toughness. The tortuous crack propagation path and the hindrance of propagating crack by the un-DRXed grains result in larger fracture resistance and more energy dissipation. The increase of geometrically necessary dislocation (GND) density and the activation of non-basal dislocations also have positive effects on the fracture toughness of the samples with bimodal grain structure. The fracture toughness is effectively improved via intrinsic and extrinsic toughening mechanisms associated with the bimodal microstructure.

Evolution of Tungsten Fiber‐Reinforced Tungsten‐Remarks on Production and Joining

Material issues pose a significant challenge for the design of future fusion reactors. These issues require new advanced materials to be developed. W‐fiber‐reinforced W‐composite material (W f/W) incorporates extrinsic toughening mechanisms increasing the resistance against failure and thus granting steps toward application in a future fusion reactor. W f/W can be produced based on chemical vapor deposition or powder metallurgical routes. In this contribution, the efforts of upscaling the production of W f/W will be reviewed based on recent results. In addition, the activities related to enabling large‐scale production for new fusion applications are being studied. Herein, two main achievements are to be highlighted. First, an upscaled production is established to produce flat tile samples for joining tests on copper and steel, and second, a new method of joining W f/W on copper is established and tested under high heat‐flux conditions.

Bioinspired ceramic-polymer composites with hierarchical nacre-mimetic structure via accumulative rolling technique

Constructing nacre-mimetic architecture in ceramic-polymer composites is an effective approach to improve fracture toughness. Here, ceramic-polymer composites with nacre-mimetic brick-and-mortar structure were fabricated using an accumulative rolling technique. Non-isometric and flaky Ti3AlC2 powders with ultrafine dimensions along with Al2O3 and h-BN powders with micron-sized dimensions were employed as constituents and the effect of their size on mechanical properties was investigated. The results showed that the flaky ceramic powders were preferentially aligned along the rolling direction under the shear force exerted by the rollers, and formed staggered stacking layers. Significant differences in the characteristic dimensions of flaky ceramic powders contributed to a hierarchical structure of the composites. Compared with the composites containing merely ultrafine-sized Ti3AlC2 flakes, the strength of composites decreased when Ti3AlC2 flakes were replaced by larger Al2O3 and h-BN flakes. However, such substitution inhibited the crack propagation, leading to a stable crack growth behavior featured by rising R-curves owing to the activation of various extrinsic toughening mechanisms, including crack deflection, uncracked-ligament bridging and pull-out of ceramic platelets. The accumulative rolling technique offers a simple but effective means for toughening ceramic materials and optimizing their functional properties along specific directions.

Improved ballistic performance of a continuous-gradient B4C/Al composite inspired by nacre

Functionally gradient materials (FGMs) primarily composed of a discretely layered structure with relatively sharp macroscopic interfaces are highly susceptible to interlayer failure under impact loading. Nacre is regarded as a model system for lightweight armor with high impact-resistant performance owing to its multiple length-scale and gradient structures. However, the simultaneous translation of both structural features into artificial assemblies remains challenging. Herein, we constructed new B4C/2024Al FGMs with eliminated sharp interfaces, which reproduced the hierarchical and gradient architectures of nacre. A combined experimental and numerical approach was employed to investigate the protective mechanisms of nacre-like FGMs subjected to 7.62 mm armor-piercing incendiary bullet, and two homogeneous composites were also tested under the same conditions for comparison purposes. The FGMs exhibited improved performance to resist ballistic attacks and maintain integrity compared with the uniform structures. As the ceramic contents decreased along the thickness, the energy dissipation mechanisms transitioned from eroding projectile to plastic deformation. The toughness gradient and varying energy dissipation modes prevented the formation of crushed cones at the ceramic-rich layers. The nacre-like structure created several extrinsic toughening mechanisms, which led to the increased energy-absorbing capability of the target. Meanwhile, the eliminated abrupt interfaces avoided the occurrence of interlayer tearing by diminishing the tensile wave strength and the sudden change of wave impedance, which prolonged the projectile-target interaction period and strengthened the eroding effect on the projectile. The present study opens new opportunities to design advanced lightweight FGMs armors with high impact-resistant protective performance.

Preparation of laminated Al/B4C composites with gradient structures and properties through centrifugal freezing and pressure infiltration

Lightweight Al/B4C composites with a gradient lamellar structure were fabricated through centrifugal freezing and pressure infiltration. Their ceramic volume fraction showed a continuous increase from the central region to the surface region, the increase being from approximately 23 vol% to 44 vol%, and the spatial variation of their composition and microstructure led to a gradient change in their mechanical properties. The bending strength and hardness increased with the B4C content, and they attained the highest values of 424 ± 16 MPa and 130.6 ± 4.2 HV0.3 at the surface region of the composite, respectively. By contrast, the work of fracture reached the maximum value (3763 ± 268 J/m2) at the central region of the composite because of the high metal content and the existence of multiple extrinsic toughening mechanisms. The proposed method is versatile and economical and is suitable for the preparation of composites with heterogeneous structures and gradient properties.

Synergistic toughening on CFRP via in-depth stitched CNTs

Efficient toughening of the interlaminar fracture toughness for CFRP composites without sacrificing in-plane mechanical performance remained an unresolved challenge. Here, we present a synergistic toughening strategy by construction of hierarchical architecture, within which carbon nanotubes (CNTs) in-depth stitched into the nano-channel between neighboring carbon fibers at the interlaminar region. Mode I fracture test revealed that, even at low concentration of CNTs (0.3 wt%), a considerable improvement (50%) on mode I fracture energy GIc of composites can be realized, from 1098.6 to 1647.8 J/m2 which is nearly two times greater than that of most aerospace CFRP laminates (∼500 J/m2). The excellent fracture toughness is predominantly attributed to the desired hierarchical architecture, which simultaneously triggered the intrinsic toughening by CNTs nano-bridging and extrinsic toughening mechanisms due to carbon fiber bridging, as was demonstrated by SEM images of fracture surfaces and further verified by finite element simulations. These findings offer significant guidelines for designing CFRP composites with high fracture toughness by application of low content CNTs using cost-effective resin mixing process.

Improved interlaminar fracture toughness of carbon fiber/epoxy composites by a combination of extrinsic and intrinsic multiscale toughening mechanisms

Carbon fiber reinforced polymer (CFRP) composites with hierarchical structure interleave composed of nanoscale core-shell rubber (CSR) and microscale short carbon fiber (SCF) were fabricated. The hierarchical structure interleave exhibits superior interlaminar toughening efficiency than either CSR or SCF alone. Triggering intrinsic and extrinsic toughening mechanisms at different scales by CSR and SCF, respectively, the mode-I critical energy release rate (GIC) and mode-II critical energy release rate (GIIC) of multi-phase interleaved CFRP composite are 127% and 154% higher than those of unmodified sample. While the tensile properties and glass transition temperature (Tg) are not compromised. The fracture toughness of epoxy/CSR system was also investigated to better understand the effect of matrix toughness on interlayer toughness of CFRP. Numerical analysis and fracture surface observations revealed the synergistic toughening mechanisms in SCF/CSR interleaved CFRP composites. The synergy between nanoscale intrinsic toughening mechanism and microscale extrinsic toughening mechanism, evolving with crack propagation, contributes to a greater energy absorption. The usefulness of the present study for preparation of high-performance CFRP is discussed.

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.