Balance Of Toughness Research Articles

Study on enhanced wear resistance of FeCoCrNi2MoSi high entropy alloy coatings induced by nano-layered eutectic and Laves phase

Wear is one of the primary challenges for high-temperature working components. In this paper, FeCoCrNi2MoSi eutectic high-entropy alloy coatings were successfully prepared by introducing MoNiSi ternary silicides with Laves structure into FeCoCrNi high-entropy alloys using laser cladding technology. The alloy achieves a balance of strength and toughness through the nanoscale eutectic structure with alternating FCC and Laves phases. Tribological experiments conducted over a wide temperature range from 25 °C to 800 °C show that the synergistic effect of the nano-lamellar structure and the alternating soft and hard structure makes the coating have excellent wear resistance at room temperature. The improvement of wear resistance at high temperatures (800 °C) is mainly due to the formation of the oxide film on the coatings.

Numerical analysis of fracture in core-shell particle reinforced composites

The fracture of core-shell particle reinforced composites was simulated using a crack phase field method. A pre-existing crack may grow along the interface between the core and shell (core debonding), may penetrate the shell and the core (trans-core fracture), or may grow in the matrix (brittle fracture in the matrix). Shell fracture behaviours resulting from the competition between the fracture resistance and fracture driving force are crucial to the crack growth mechanisms. It was found that moderate strength and low modulus of the shell favoured trans-core fracture, and low strength and high modulus of the shell favoured core debonding. The effects on the overall mechanical properties were also identified. The composite strength usually benefited from stiffer and tougher components, regardless of the crack growth mechanisms, while the composite toughness was more complex. The toughening effect was highly related to how the crack evolved, as core debonding improved the toughness at the significant cost of strength, and trans-core fracture contributed most to the toughness with a slight reduction in strength compared with the matrix. These findings indicate design strategies for epoxy composites which can achieve a balance of strength and toughness, enabling the design of safer lightweight composites for electric vehicles and transport applications.

Fracture behavior and constitutive relations of Sn-3.0Ag-0.5Cu solder alloy at cryogenic temperature

The mechanical behavior, underlying fracture mechanism and constitutive relations of Sn-3.0Ag-0.5Cu (SAC305) solder alloy at cryogenic temperature through in-situ uniaxial tensile experiments were investigated over a wide temperature range of 293 K–77 K. The activation of deformation twin in the solder alloy contributes to improve the tensile strength and quasi-static toughness, and maintains the fracture elongation at 35–45% with the decline of ambient temperature. SAC305 solder alloy with a mixed mode of ductile and brittle fracture achieves the optimal balance of strength, ductility and toughness at 123 K, and the large mismatch between twins thickening rate (∼13 μm/s) and dislocation slip velocity (∼3.6 μm/s) finally leads to the brittle fracture at 77 K. In addition, Anand model as a unified viscoplastic constitutive law has been employed to describe the constitutive relations of SAC305 solder alloy and fails in fitting the stress-strain curves at invalid temperature below 233 K. Hollomon equation is conducted to perfectly characterize the constitutive relations of the alloy at temperatures lower than 233 K. The method for controlling the formation of deformation twins in the solder alloy and solder joints is really beneficial to solve the reliability problems for cryogenic electronics.

Strength-toughness matching mechanisms of a boride-reinforced Ti6Al4V compositionally graded material fabricated using spark plasma sintering

In this work, an in-situ boride-reinforced Ti6Al4V compositionally graded material was prepared successfully by spark plasma sintering (SPS), and exhibited a good potential for military application because of its stepwise transition properties from a high-toughness end to a high-strength end along the thin-thickness direction. Microstructure characterization shows the chemical gradient ranged from 0 to 30 wt% TiB2 along the thickness direction, which was correlated with increasing micro-hardness values ranging from 565±5HV to 1270±5HV, respectively. The strength-toughness matching mechanisms of the gradient composites with different compositional gradients were discussed based on the static compression experiments and the dynamic Hopkinson compression rod experiments. The primary phases of the gradient composites included the α-Ti phase, β-Ti phase, remained TiB2 and in-situ formed whisker-like TiB phases. The composites with a compositional gradient of G84 (0, 0, 15, 30 wt%) exhibited the best impact resistance which achieved a good balance of high toughness and strength, and exhibited a compressive strength of 2148±5 MPa and a good elongation of 20 ± 0.5%. The strength-toughness matching mechanisms can be explained by the dispersion strengthening effect due to the presence of the in-situ formed TiB phase, and the consumption of more fracture energy by dislocation defects, which can prevent the crack propagation. Moreover, the pull-out of the whisker-like TiB phases toughened the gradient composites furthermore. This research demonstrated a reliable method for preparing TiB2/Ti–6Al-4Vgradient composites with high-impact resistance.

Principal Preferred Orientation Evaluation of Steel Materials Using Time-of-Flight Neutron Diffraction

Comprehensive information on in situ microstructural and crystallographic changes during the preparation/manufacturing processes of various materials is highly necessary to precisely control the microstructural morphology and the preferred orientation (or texture) characteristics for achieving an excellent strength–ductility–toughness balance in advanced engineering materials. In this study, in situ isothermal annealing experiments with cold-rolled 17Ni-0.2C (mass%) martensitic steel sheets were carried out by using the TAKUMI and ENGIN-X time-of-flight neutron diffractometers. The inverse pole figures based on full-profile refinement were extracted to roughly evaluate the preferred orientation features along three principal sample directions of the investigated steel sheets, using the General Structure Analysis System (GSAS) software with built-in generalized spherical harmonic functions. The consistent rolling direction (RD) inverse pole figures from TAKUMI and ENGIN-X confirmed that the time-of-flight neutron diffraction has high repeatability and statistical reliability, revealing that the principal preferred orientation evaluation of steel materials can be realized through 90° TD ➜ ND (transverse direction ➜ normal direction) rotation of the investigated specimen on the sample stage during two neutron diffraction experiments. Moreover, these RD, TD, and ND inverse pole figures before and after the in situ experiments were compared with the corresponding inverse pole figures recalculated from the MUSASI-L complete pole figure measurement and the HIPPO in situ microstructure evaluation, respectively. The similar orientation distribution characteristics suggested that the principal preferred orientation evaluation method can be applied to the in situ microstructural evolution of bulk orthorhombic materials and spatially resolved principal preferred orientation mappings of large engineering structure parts.

Elucidating the role of diverse mineralisation paradigms on bone biomechanics - a coarse-grained molecular dynamics investigation.

Bone as a hierarchical composite structure plays a myriad of roles in vertebrate skeletons including providing the structural stability of the body. Despite this critical role, the mechanical behaviour at the sub-micron levels of bone's hierarchy remains poorly understood. At this scale, bone is composed of Mineralised Collagen Fibrils (MCF) embedded within an extra-fibrillar matrix that consists of hydroxyapatite minerals and non-collagenous proteins. Recent experimental studies hint at the significance of the extra-fibrillar matrix in providing the bone with the stiffness and ductility needed to serve its structural roles. However, due to limited resolution of experimental tools, it is not clear how the arrangement of minerals, and in particular their relative distribution between the intra- and extra-fibrillar space contribute to bone's remarkable mechanical properties. In this study, a Coarse Grained Molecular Dynamics (CGMD) framework was developed to study the mechanical properties of MCFs embedded within an extra-fibrillar mineral matrix and the precise roles extra- and intra-fibrillar mineralisation on the load-deformation response was investigated. It was found that the presence of extra-fibrillar mineral resulted in the development of substantial residual stress in the system, by limiting MCF shortening that took place during intra-fibrillar mineralisation, resulting in substantial compressive residual stresses in the extra-fibrillar mineral phase. The simulation results also revealed the crucial role of extra-fibrillar mineralisation in determining the elastic response of the Extrafibrillar mineralised MCF (EFM-MCF) system up to the yield point, while the fibrillar collagen affected the post-yield behaviour. When physiological levels of mineralisation were considered, the mechanical response of the EFM-MCF systems was characterised by high ductility and toughness, with micro-cracks being distributed across the extra-fibrillar matrix, and MCFs effectively bridging these cracks leading to an excellent combination of strength and toughness. Together, these results provide novel insight into the deformation mechanisms of an EFM-MCF system and highlight that this universal building block, which forms the basis for lamellar bone, can provide an excellent balance of stiffness, strength and toughness, achieving mechanical properties that are far beyond the capabilities of the individual constituents acting alone.

Preparation of biodegradable PLA/PBAT blends with balanced toughness and strength by dynamic vulcanization process

Polylactic acid (PLA) has received significant attention in recent years as a commercially available biodegradable resin. However, its brittleness limits its broader adoption. Blending PLA with another biodegradable resin poly(butylene adipate-co-terephthalate) (PBAT) is a simple strategy to toughen PLA, however, the effect is limited and the tensile strength is generally harmed. In this work, PBAT, initiator BIBP and cross-linker TAIC were initially mixed and then extruded into pre-crosslinked PBAT pellets. Subsequently, the pre-crosslinked PBAT and PLA were melt-blended to create PLA/micro-crosslinked PBAT composites. The effects of different TAIC contents on the degree of PBAT gelation, properties and morphological structure of composites were explored. The results showed that TAIC could effectively improve the mechanical properties of PLA/PBAT, particularly the notched impact strength. When the TAIC content was 0.3 wt% of PBAT, the notched impact strength and elongation at break of PLA/PBAT (7/3) reached 42.26 kJ/m2 and 264.26 %, respectively, which are both far higher than those of the blend without cross-linking (9.37 kJ/m2 and 21.29 %). The composites still exhibited excellent tensile strength (46.23 MPa) and tensile modulus (1676.84 MPa), surpassing those of PLA/un-crosslinked PBAT composites. This enhanced toughening and tensile strength maintaining effect is attributed to the network structure in PBAT and the increased compatibility between PLA and PBAT caused by cross-linking within PBAT and at PLA/PBAT interface. The dynamic vulcanization strategy using an extruder provides a possibility for the large-scale preparation of stiff-tough PLA. Of course, further work, including pilot or industrial trials, is necessary to validate scalability.

Preparation of rigidity toughness balance and stable poly(glycolic acid) based on chain extension reaction

AbstractAs a completely degradable material, poly(glycolic acid) (PGA) has excellent rigidity, barrier properties, degradability and biocompatibility. However, the inherent brittleness and easily hydrolysable characteristics limit its widespread application. Herein, we introduced chain extension ADR to modify PGA to ameliorate these defects. The results showed that ADR could react with PGA to form “comb‐like” and “cross‐linked” chain extension structures resulting in the improvement of complex viscosity and formation of gel. Because of the chain entanglement of chain extension structures, the toughness of modified PGA was improved (which was verified by the impact strength value of 4.3 kJ/m2 that was about 126% of virgin PGA) without sacrificing its rigidity. Furthermore, the hydrolysis results indicated that the representative PGA/1.0ADR had better hydrolysis stability, it had more complete morphology and slower degradation rate than virgin PGA in the 17‐day hydrolysis experiment. Apart from better toughness and hydrolysis stability, there was also improvements in crystallization performance, the sample PGA/1.0ADR endowed with higher crystallization peak temperature (2.3°C higher than virgin PGA) and lower crystallization activation energy. In short, this work prepared a kind of modified PGA with rigidity toughness balance and better hydrolysis stability, which could expand the application range of PGA‐based materials.

High-strength and high-toughness austenitic stainless steels based on type 316LN at 4.2 K

High-strength and high-toughness prototype steel forgings, N2, N3, and N4, were developed based on type 316LN as a cryogenic structural material. By extrapolating the proportional relationship between carbon plus nitrogen content (C + N) and 0.2 % proof stress (σ0.2) in the range of approximately 0.10–0.25 mass% C + N content, the C + N content was successfully designed to achieve the target σ0.2. To suppress the degradation of the plane-strain fracture toughness (KIC(J)) while increasing the nitrogen solid-solution content, the alloying level was designed by considering the stability of the austenite phase and stacking fault energy. The steel forgings showed a good balance of strength and toughness at 4.2 K, i.e., N2 (σ0.2 = 1142 MPa, KIC(J) = 228 MPa√m), N3 (σ0.2 = 1226 MPa, KIC(J) = 219 MPa√m) and N4 (σ0.2 = 1296 MPa, KIC(J) = 215 MPa√m). The σ0.2 and KIC(J) of ITER-grade 316LN (316LNL, 316LNM, and 316LNH) thick plates with low interstitial (C + N) content of 0.13–0.17 mass% were also evaluated at 4.2 K. Then, the effects of nitrogen content on strength and fracture toughness were successfully clarified in the full range of interstitials concentrations. An alloy design with higher Ni, Mn, and Mo contents in the chemical composition range of 316LN would be desirable for improving the fracture toughness by increasing σ0.2 due to the higher nitrogen content.

The Evolution of a Microstructure during Tempering and Its Influence on the Mechanical Properties of AerMet 100 Steel

In order to provide guidance for furthering the balance of strength and toughness of AerMet 100 steel through tempering treatment, the effects of the tempering time on microstructure and mechanical properties are investigated. The microstructure evolution, especially M2C precipitates and austenite in AerMet 100 tempered at 482 °C for 1~20 h, was characterized, and its influences on the mechanical properties were studied. The tensile strength decreases gradually, the yield strength increases first and then decreases, and the fracture toughness KIC increases gradually with an increasing tempering time. The strength and toughness matching of AerMet 100 steel is achieved by tempering at 482 °C for 5~7 h. Without considering the martensitic size effect, the influence of the dislocation density on the tensile strength is more significant during tempering at 482 °C. The precipitation strengthening mechanism plays a dominant role in the yield strength when tempering for 5 h or less, and the combined influence of carbide coarsening and a sharp decrease in the dislocation density resulted in a significant decrease in tensile strength when tempering for 8 h or more. The fracture toughness KIC is primarily influenced by the reverted austenite, so that KIC increases gradually with the prolongation of the tempering time. However, a significant decrease in the dislocation density resulting from long-term tempering has a certain impact on KIC, giving rise to a decrease in the rising amplitude in KIC after tempering for 8 h or more.

(AlCrMoTiNi)1-XNX high entropy ceramic with high hardness and toughness prepared by co-filtered cathodic vacuum arc deposition

High entropy ceramic (HEC) coatings have become candidates for new protective coating due to their high strength. However, the hardness - toughness balance of the ceramic coatings is still a difficult problem to solve. The strong nitride forming elements Al, Ti, Cr, Mo, and the non-nitride forming element Ni were selected as the components of the (AlCrMoTiNi)1-XNX HEC coatings to regulate the hardness and toughness. When x = 0 and 0.29, the composite structure of amorphous and metallic Ni phase is observed in these coatings. When x = 0.37, 0.40, and 0.45, the phase structure changes to the composite structure of nanoscale face-centered cubic (FCC) phase, metallic Ni, and amorphous phase. When x = 0.45, the maximum hardness of the HEC coating is 31.25 GPa, and it also has high toughness with a KIC value of 3.25 MPa m1/2. The HEC coating with high hardness and toughness also presents excellent wear resistance and corrosion resistance. Therefore, the (AlCrMoTiNi)0.55N0.45 HEC coating has potential as a protective coating applied in various areas.

Enhanced strength and ductility in nanocarbon hybrid reinforced B4C/Al laminated composites fabricated by vacuum hot pressing

In order to develop B4C/Al composites with good strength-toughness balance, nanocarbon hybrid-reinforced B4C/Al composites with obvious laminar structure were fabricated by the flake powder metallurgy and vacuum hot pressing. Al-30B4C-0.75CNTs-0.25GNPs composites achieved the best balance of tensile strength and toughness, with average values of 324.37 MPa and 10.14%, respectively. The interconnected network structure created by CNTs and GNPs has a more effective load transfer effect than that of single CNTs, which is attributed to the synergistic strengthening effect of CNTs and GNPs. The geometrically necessary dislocation strengthening caused by the mismatch of thermal expansion coefficients between B4C and Al matrix is the main contribution of B4C to the composite. Additionally, the γ-Al2O3 uniformly distributed in the Al matrix is beneficial for the strength enhancement of the composite due to the Orowan mechanism, and sintering and pressing under vacuum prevents excessive formation of Al2O3.

Possible globularization mechanism in LPBF additively manufactured Ti-6Al-4 V alloys

Owing to the great technical significance such as better balance of strength and toughness, the conversion of lamellar to globular microstructure finds its great importance. In order to obtain globular microstructure, it is necessary to shed light on the globularization mechanisms. This paper attempts a systematic study on globularization of Laser Powder Bed Fusion (LPBF) processed Ti-6Al-4 V alloy including microstructure evolution and globularization mechanism. Annealin g was performed in the α + β field, in attempting to transform the acicular α’-martensite into globular microstructure. The required driving force for static globularization during annealing is achieved with the presence of defects such as dislocations and twins which is inherently present within α’-martensite. The static globularization fraction increases with increasing annealing temperature. During annealing, the static globularization involved the evolution of sub-boundaries, phase transformation, and coarsening of α-phase. Furthermore, the possible mechanism was explored through various characterization techniques such as SEM, EBSD and TEM analysis. Globularization mechanisms such as boundary splitting, direct cylinderization, edge spheroidization, termination migration, and Ostwald ripening have been observed. Most of the mechanism activates at higher annealing temperature close to β-transus temperature. The globularization has not taken place homogeneously in the microstructure causing globularization heterogeneity. The inability of achieving a full globularization in LPBF additively manufactured Ti-6Al-4 V alloy is due to its characteristic α-lamellae microstructure such as aspect ratio and morphology.

A comparative study of polylactic acid (PLA)-Based unidirectional green hybrid composites reinforced with natural fibers such as kenaf, bamboo and coir

Plant-based composites are considered as new generation materials that offer sustainability, eco-friendly and green chemistry, utilized by numerous sectors. Composites of green hybrid made from polylactic acid (PLA) and three different plant fibers were prepared using two symmetrical fibers stacking sequences; namely high modulus and brittle (HMB) fibers (kenaf-bamboo-coir/PLA) and low modulus and ductile (LMD) fibers (coir-bamboo-kenaf/PLA) in the outmost layers. The stacking sequence was prepared using a simple slot tool to properly align the fibers and it was dried at 50 °C before being compressed using hot press. It was found that the tensile strength and tensile modulus of both hybrid composites increased linearly up to 158 MPa and 7 GPa respectively. It shows that the stacking sequences had no significant effect on tensile strength, but fracture strain increased by 58% produced by LMD fibres. However, the use of HMB fibres in the outmost layers significantly improved flexural strength up to 49% higher than that of LMD fibres. In contrast, the impact strength of the green hybrid composites using LMD fibres in the outmost layers was 21% higher than that of its counterpart. Like other plant fiber composites, both composites had an increased water absorption capacity of up to 39% due to increased fibre content. These stacking sequence properties are essential to fabricate a specific application to ensure properties suitable for the job's features. High modulus and brittle composites require a high load to deform permanently, as well as low modulus and ductile composites. Both exhibit versatile mechanical characteristic, providing a balance of sealant toughness and the ability to retain overall shape, such as in sound absorption and vibration in transportation.

Extraordinary strength–ductility–toughness in Fe–0.08C plain low-carbon steel via introducing weblike martensite: Towards the third generation

The effect of large-strain cold rolling followed by intercritical annealing on the microstructure and mechanical properties of plain low-carbon steel was investigated. The grain size of ferrite reduced from 38 μm (for the as-received steel) to about 4 μm (for the dual-phase steel). The martensite phase was connected in 3D space in the dual-phase steel and a weblike structure of martensite was created. The stress-strain curve of dual-phase steel represents the continuous yield behavior without any distinct evidence of the yield point phenomenon. By introducing weblike martensite into the microstructure and refining α grains, the yield strength, ultimate tensile strength, and tensile toughness significantly increased to 740 MPa, 1062 MPa, and 193 J cm−3, respectively. The fracture surface of the DP sample had a dimpled appearance, indicating a ductile fracture mode. The results of the present work indicated that a low-cost plain low-carbon DP steel composed of weblike martensite and fine-grained ferrite exhibits extraordinary strength–ductility–toughness balance. Therefore, the dual-phase steel produced in the present work, by moving towards the third generation, can find wide applications in the automotive industry.

An experimental investigation of the tensile, fracture and microstructural characteristics of ABS/SBS reinforced with HNTs

AbstractThermoplastic composites are broadly used in many industries. Excellent mechanical characteristics are frequently required to achieve the appropriate properties for the applications. Therefore, the objectives of this study were (1) presenting an in‐depth analysis of the effects of styrene butadiene styrene (SBS) and halloysite nanotubes (HNTs) on the tensile and fracture properties of the thermoplastic composites based on acrylonitrile butadiene styrene (ABS), and (2) examining the relationship between the microstructure of the thermoplastic composites and their mechanical behavior. For these purposes, HNTs were utilized with four levels (0, 1, 3, and 5 wt%) and SBS was used with two levels (15 and 30 wt%). The tensile strength and modulus were reduced by adding SBS to 30 wt%; however, the elongation at break was increased by 30%. Furthermore, tensile strength and modulus, and elongation at break were improved by increasing HNTs up to 3 wt%. The results indicated the elastic work (we) and plastic work (βwp) rose by 29% and 10%, respectively, when 30 wt% of SBS was added to ABS. Furthermore, we and βwp rose by the addition of HNTs up to 3 wt%. To examine fracture mechanisms, field emission scanning electron microscope (FESEM) images of the fracture surface of the essential work of fracture (EWF) specimens were taken. Several voids are present in the compounds containing SBS, activating the plastic deformation mechanism. Based on the optimization process, the best strength, stiffness, and toughness balance was attained for the compound consisting of 15 wt% SBS and 1 wt% HNTs.Highlights The effects of HNTs and SBS on the fracture properties of polymer composites. The addition of SBS increased we, βwp, and the elongation at break. HNTs had a positive effect on the tensile strength and modulus, we, and βwp. The important mechanisms include plastic deformation and cavitation.

Enhanced Hardness-Toughness Balance Induced by Adaptive Adjustment of the Matrix Microstructure in In Situ Composites.

With the development of high-speed and heavy-haul railway transportation, the surface failure of rail turnouts has become increasingly severe due to insufficient high hardness-toughness combination. In this work, in situ bainite steel matrix composites with WC primary reinforcement were fabricated via direct laser deposition (DLD). With the increased primary reinforcement content, the adaptive adjustments of the matrix microstructure and in situ reinforcement were obtained at the same time. Furthermore, the dependence of the adaptive adjustment of the composite microstructure on the composites' balance of hardness and impact toughness was evaluated. During DLD, the laser induces an interaction among the primary composite powders, which leads to obvious changes in the phase composition and morphology of the composites. With the increased WC primary reinforcement content, the dominant sheaves of the lath-like bainite and the few island-like retained austenite are changed into needle-like lower bainite and plenty of block-like retained austenite in the matrix, and the final reinforcement of Fe3W3C and WC is obtained. In addition, with the increased primary reinforcement content, the microhardness of the bainite steel matrix composites increases remarkably, but the impact toughness decreases. However, compared with conventional metal matrix composites, the in situ bainite steel matrix composites manufactured via DLD possess a much better hardness-toughness balance, which can be attributed to the adaptive adjustment of the matrix microstructure. This work provides a new insight into obtaining new materials with a good combination of hardness and toughness.

Polylactide Films with Superhydrophobicity and Superior Mechanical Properties Constructed by Combining Annealing Stretching and Nonsolvent-Induced Phase Separation

Polylactide (PLA) is regarded as one of the most promising substitutes for petroleum-based thermoplastics, but the inherent nature of hydrophilicity and poor toughness severely limits its widespread application. In this work, PLA films with superhydrophobicity and outstanding mechanical properties are fabricated by combing “annealing stretching” and nonsolvent-induced phase separation (NIPS)-based surface modification. The prepared films have low surface adhesion characteristics, exhibiting water contact angles of up to 163.4° and extremely small sliding angles as low as 1°. Excellent self-cleaning function to solid contaminates and good repellency to several liquid contaminates are also demonstrated. In addition, the strong interfacial adhesion between the structured surface coating and the substrate enables the PLA films to resist various chemical and mechanical damages, exhibiting splendid durability under severe environmental conditions. Moreover, a superb stiffness–toughness balance is obtained. The tensile strength, Young’s modulus, and elongation at break of the fabricated PLA film are 98.9 MPa, 2.87 GPa, and 91.7%, respectively, which are dramatically improved compared to that of the unstructured casting PLA film (57.6 MPa, 1.57 GPa, and 21.0%, respectively). This work would provide significant guidance for the facile and potentially large-scale preparation of PLA films with mechanical robustness and superhydrophobicity and could broaden their application scope to self-cleaning and moisture-proof fields.

Outstanding fracture toughness combines gigapascal yield strength in an N-doped heterostructured medium-entropy alloy

The superior strength and toughness balance prevails in the face-centered-cubic-structured high/medium-entropy alloys (H/MEAs) of low strength, while a gain in yield strength is normally accompanied by a sacrifice in toughness, leading to the strength and toughness trade-off particularly when yield strength increases to the gigapascal levels. Here, we showed the superior fracture toughness by heterostructuring an N-doped CrCoNi MEA having different levels of yield strength higher than 1 GPa. The fracture toughness was 91 MPa·m1/2 at yield strength of 1.3 GPa in the heterogeneous lamella structure and 168 MPa·m1/2 at yield strength of 1.0 GPa in the heterogeneous grain structure. The fracture toughness was attributed to forest hardening plus an extra hetero-deformation induced hardening. The pile-ups of geometrically necessary dislocations were observed to be formed at the domain boundaries to accommodate strain gradient at the plastic zone of the crack tip. The chemical short-range orders were found for the enhanced strain hardening near the crack tip by the interaction with dislocations. Moreover, a new parameter was proposed to characterize the work hardening capacity at the crack tip by the integral of hardness increment in the plastic zone which shows a linear relation-ship with the J-integral value during the crack initiation.

Characterization of boundary precipitation in a heavy ion irradiated tungsten heavy alloy under the simulated fusion environment

In the concerted effort to identify materials capable of surviving the adverse environment of a fusion reactor interior, tungsten heavy alloys have been put forth as candidates. Experimental trials and behavioral studies have yielded positive results for their adoption by taking advantage of the alloy's unique balance of high fracture toughness and high tungsten content; yet due to their relative novelty in the fusion community, there remains a lack of understanding on the response of these materials to the extended high temperature irradiation environment of the reactor interior. To alleviate this issue and provide the necessary data on the behavior of tungsten heavy alloys to the simulated fusion environment, a 90W-7Ni-3Fe alloy has been subjected to elevated temperature sequential Ni+ and He+ ion irradiations to mimic the expected displacement damage and He gas production expected after five years of service as materials for plasma facing components. Atomic-scale structural analyses and nanoscale chemical mapping have identified the formation of two distinct precipitation structures, a surface localized η-carbide and a hexagonal W2C type tungsten carbide, both of which appear to originate at the bi-phase interface between W and the ductile phase. This irradiation enhanced and induced precipitate formation respectively is anticipated to adversely affect these materials by selective embrittlement at bi-phase interfaces leading to a reduction in the material's overall fracture toughness during prolonged high temperature irradiation. It is asserted that any potential exposure to C during fusion reactor operational service should be minimized as to prevent the formation of these phases.