Synergistic Strengthening Effect Research Articles

Mechanics of gradient nanostructured metals

The emergence of heterogeneous nanostructured materials (HNMs) offers exciting opportunities to achieve outstanding mechanical properties. Among these materials, gradient nanotwinned (GNT) Cu is a prominent class of HNMs, demonstrating superior strengths by gaining extra strengths compared to non-gradient counterparts. Its layered gradient structure provides a simplified quasi-one-dimensional model system for understanding the extra strengthening effects of structural gradients and resulting plastic strain gradients. This paper presents a comprehensive report for recent experimental and modeling studies on the mechanics of GNT Cu, covering advances in controlled material processing, back stress measurement, deformation field characterization, dislocation microstructure analysis, and strain gradient plasticity modeling. These studies unveil the spatiotemporal evolution of both plastic strain gradients and extra back stresses originating from structural gradients. Direct connections are established between the sample-level extra strength of GNT Cu and the synergistic strengthening effects induced by local nanotwin structures and their gradients. We emphasize the critical role of the size of the representative volume element in assessing the effects of plastic strain gradient and extra back stress. Moreover, lower-order strain gradient plasticity models are validated through experimental characterizations of GNT Cu, paving the way for future investigation into the mechanics of gradient nanostructured metals. Finally, we provide an outlook on research needs for understanding the mechanics of gradient nanostructured metals and, more broadly, HNMs, towards achieving exceptional mechanical properties.

Synergistic strengthening effect achieved good ductility in ultrahigh strength Fe–22Mn–8Al-0.8C austenitic lightweight steels

We present an ultra-high tensile strength of 1.86 GPa with an elongation of 11 % achieved in a Fe–22Mn–8Al-0.8C austenitic lightweight steels in this study. A partial recrystallized microstructure and κ carbides were obtained in the investigated steels by utilizing a simple thermomechanical processing. Interestingly, the existence of κ carbides not only increased the tensile strength of the investigated specimens but a dramatical increasement of ductility (from 4 % increase to 11 % in this work) was also achieved. The formation of intragranular κ carbides in the matrix can reduce the incompatibility within the material, which can further enhance tensile strength while providing good elongations.

Surface integrity and corrosion resistance of 18CrNiMo7-6 gear steel subjected to combined carburized treatment and wet shot peening

Typically, a cost-effective and versatile carburizing process is used to improve the surface integrity of high-value-added gear components. In this study, carburized treatment and wet shot peening (WSP) were applied to increase the corrosion resistance of 18CrNiMo7-6 gear steel. The surface integrity and corrosion resistance after carburizing and combined treatment (carburized treatment + WSP) were analyzed. The results indicated that the combined treatment converted the retained austenite surface layer within approximately 45 μm from the surface into the martensitic phase, with a 13.9 % reduction in equivalent grain size and a 23.6 % increase in dislocation density compared with carburizing. Furthermore, electrochemical experiments in a 3.5 wt% NaCl solution indicated that the combined treatment process increased the corrosion resistance compared with carburizing (55.4 % lower corrosion current density and a higher Fe2+/Fe3+ ratio). Despite the high surface roughness obtained with combined treatment, the improved corrosion resistance is primarily ascribed to the high compressive residual stress and dislocation density. This study provides insights into the surface strengthening design of 18CrNiMo7-6 gear steel for corrosive environments—particularly the synergistic strengthening effect of carburizing and WSP.

Ultralight, shapeable, and superhydrophobic polyacrylonitrile/polybenzoxazine aerogel for high oil-water separation capacity

Environmental and human health are significantly threatened by oily wastewater emission and oil spills. Polymeric porous aerogels are ideal materials to absorb oils or organic solvents selectively, thereby mitigating environmental pollution and protecting human health. In this study, a shapeable, super-hydrophobic, polyacrylonitrile fiber/polybenzoxazine (PAN/PBOZ) aerogel with high oil-water separation ability were successful designed. The preparation process of PAN/PBOZ aerogel is as follows: Firstly, PAN micro-nanofibers were prepared by electrospinning, and then benzoxazine was uniformly dispersed into PAN fibers. After freeze-drying and thermal curing, the aerogel was obtained. For comparative analysis, a pure polyacrylonitrile (PAN) fiber aerogel is also presented. The resulting PAN/PBOZ aerogel exhibits ultralight (0.017 g/cm3), remarkable hydrophobicity, as evidenced by a water contact angle of 156°, high adsorption capacity up to 157.7 g/g for oil or organic solvents, and a high water-in-oil emulsion separation flux (9718 ± 45 L m−2 h−1) efficiently. Notably, the rigid cross-linking structure of PBOZ and the toughness structure of PAN make the aerogel exhibit synergistic strengthening and toughening effects. When the PAN/PBOZ fiber aerogel is compressed to 60 %, an outstanding recovery stability of its original size is displayed. While the springback of pure PAN fiber aerogel is poor under the same compression test condition. In conclusion, this study provides a preparation method and design concept of aerogel that can meet the requirements of low density, high toughness and high oil adsorption capacity. The rationally designed PAN/PBOZ aerogel shows significant potential for practical applications in treating oily and organic wastewater.

The bifunctional low mismatch nano strengthening phase leads to a high strength-ductility W-Ta-Ni-Fe-Cu alloy

As a typical tungsten-based composite consisting of tungsten (W) particles and matrix phase, tungsten heavy alloys (WHAs) are extensively utilized in the military due to their exceptional strength and high density. However, the large grain size (>30 μm) of powder metallurgy liquid phase sintered (LPS) WHAs and the low strength of the matrix phase limit the further improvement of the alloy. In this work, high-density ultrafine WHAs were fabricated by two-step low-temperature sintering, and the density of the alloy reached 17 g/cm3 with an average W particle size of 7.81 μm. Additionally, the eutectic reaction between Ni and Ta was controlled to generate dispersed nano-Ni3Ta phases in situ in the matrix phase, further improving the strength of the alloy. Under the synergistic strengthening effect of fine-grain strengthening, dispersion strengthening, and solid solution strengthening, the average ultimate tensile strength of the alloy reached 1190.39 MPa. At the same time, the alloy maintained good elongation with a total elongation of 20.8 % due to the good co-grid interface orientation between the Ni3Ta phase and the matrix phase. This study provides a new idea for developing high-strength WHAs and has a guiding significance for developing Ni-based alloys.

The extrusion ratios modification of Mg-2Gd-xMn alloys to exploration of the strengthening mechanisms

Enhancing the strength of magnesium alloys, a lightweight material, is crucial in promoting its widespread adoption and utilization. In this comprehensive study, the effects of various extrusion ratios and manganese contents on the properties of Mg-2Gd-xMn alloys have been rigorously examined using state-of-the-art techniques such as Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). Observations reveal that the microstructure of as-extruded Mg-2Gd-xMn alloys typically exhibit a bimodal structure, characterized by a mix of equiaxed recrystallized grains and elongated unrecrystallized grains. The reduction of the extrusion ratio reduces the deformation energy that promotes the nucleation and growth of recrystallization, which effectively reduces the alloy recrystallization grain size and increases the volume fraction of the unrecrystallized microstructure. The uncrystallized microstructure have a significant fiber texture that poses challenges in activating basal slip and is accompanied by a more pronounced work-hardening trend. Additionally, the investigation reveals the vital role of α-Mn particles in obstructing dislocation movement and refining the grains, thereby contributing to enhanced alloys strengthening. Therefore, by harnessing the synergistic strengthening effects of the aforementioned mechanisms, the Mg-2Gd-2Mn alloy demonstrates significantly improved yield strength and tensile strength, reaching an impressive 240 MPa and 281 MPa, respectively.

Enhanced dynamic-mechanical property and microstructural mechanism of a FeCoNiCrMo0.2 high-entropy alloy fabricated using powder bed fusion

Powder bed fusion (PBF), along with subsequent heat treatment, plays a crucial role in enhancing the impact toughness of FeCoNiCrMo0.2 high-entropy alloys (HEAs) and expanding their potential applications in field for high-speed deformation. In this study, the dynamic-mechanical properties and microstructure of the as-built PBF–FeCoNiCrMo0.2 HEAs and quenched PBF–FeCoNiCrMo0.2 HEAs heated at 600–750 °C for 8 h were investigated. As the heating temperature increases, the columnar grains and cellular structures undergo coarsening and the dislocation density gradually decreases. Moreover, higher heating temperatures facilitate the precipitation of Mo-rich second phases. This occurrence can be primarily attributed to the segregation of Mo at the boundaries of the as-built specimens. Consequently, in the quenched specimens, the μ phases are predominantly distributed along the boundaries of the cellular structures. In the quenched Q700 specimen (heated at 700 °C for 8h), the size and volume fraction of the μ phases are measured to be 63.4 nm and 3.59%, respectively. Additionally, under the same condition, the Q700 specimens also exhibited a relatively slight increase in the size of the cellular structures. The impact energy absorption, dynamic yield strength, and dynamic compressive strength of the Q700 quenched specimens were found to be 210.1 MJ/m3, 1066 MPa, and 1649 MPa at a strain rate of 1840 s−1. These values represent a dramatic improvement of 38.1%, 52.7%, and 14.7% higher in comparison to those of the as-built specimens. Under impact deformation, the presence of the μ phases plays a significant role in impeding the movement of dislocations by acting as a pinning agent for the boundaries of the cellular structures, thereby enhancing the strength. Simultaneously, the cellular structures were significantly elongated to form the deformation bands and to coordinate the impact deformation, leading to good impact energy absorption. In combination, the synergistic strengthening and toughening effect of the μ phases and the elongated cellular structures contribute to a remarkable improvement in the impact toughness of the PBF-HEAs.

Microstructural evolution and high-temperature strengthening mechanisms of the IN 738LC superalloy prepared by selective laser melting

Defect-free SLM-IN 738LC superalloy samples were fabricated using selective laser melting (SLM). The as-built samples exhibit a cellular structure with fine carbides, γ′ phases, and high-density dislocations at the cell boundary and nano-size γ′ phases in the interior, resulting in excellent deformation resistance and favorable mechanical properties at room temperature, with a yield strength (YS) of 970 MPa, an ultimate tensile strength (UTS) of 1263 MPa, and an elongation (EL) of 33%. This study investigates the evolution behavior of the γ′ phase at different solid solution treatment (SHT) temperatures and its impact on alloy strength and toughness. The 1020 + AHT sample exhibits a uniform distribution of coarse γ′ phases. The samples subjected to 1070 + AHT and 1120 + AHT display a bimodal distribution of γ′ phases. When the SHT temperature was further increased, fine spherical γ′ phases are evenly distributed in the 1160 + AHT and 1200 + AHT samples. Moreover, the γ/γ′ eutectics at the grain boundaries gradually dissolve with increasing SHT temperature, whereas the size and fraction of carbides increase. The room temperature tensile test of the 1070 + AHT sample demonstrated a favorable balance between strength and plasticity (YS = 1217 MPa, UTS = 1472 MPa, EL = 9.25%). This is attributed to the synergistic strengthening effect of the bimodal distribution of the γ′ phase. The temperature-dependent deformation behavior of the 1070 + AHT sample was analyzed at 23 °C, 800 °C, 900 °C, and 1000 °C. At 23 °C, numerous dislocations accumulated around the γ/γ′ interfaces. When the temperature increased to 800–900 °C, the dislocations shearing γ′ phases is activated and the alloy remained high strength. At 1000 °C, both the dislocation shearing and dislocation bypassing mechanisms coexist, resulting in a decrease in the strength of the alloy and an increase in its plasticity. This work provides scientific and theoretical support for SLM-IN 738LC parts with favorable properties.

Enhanced carburization efficiency and performance of vacuum carburized layers by La2O3-doped supersonic fine particle bombardment

In this work, the fabrication of supersonic fine particle bombardment (SFPB)-doped rare-earth pretreatment modified vacuum carburized layers were applied. The SFPB-doped La2O3 pretreated vacuum carburized reinforced layer was explored by common testing methods such as XRD, XPS, SEM, and TEM, focusing on the effects of La2O3-doped SFPB pretreatment on the decomposition, adsorption and diffusion of active carbon atoms and the diffusion mechanism. The composite carburizing behavior of La2O3-doped SFPB pretreatment modified layer under the action of thermal coupling is explored, and the influence law of rare earth modified carburizing and nano-modified carburizing on the thickness, residual stress, impact toughness, tribological properties of the carburized layer were clarified. The mechanism of mass transfer under the coupling role of pre-set surface defects and rare earth pinning was revealed. The case depth of the LSPC layer was increased by 14.3%, and the residual stress was significantly higher. The carbon diffusion coefficients of LSPC were 1.31 times superior to that of the vacuum carburizing layer, which is mainly attributed to the formation of a large number of nanocrystals with high-angle grain boundaries (HAGBs) by the rare-earth doped nanosized pretreatment, and the multiple synergistic strengthening effects of rare-earth oxides and nanocrystals.

Microstructure and mechanical properties of SiCp/CF/Al hybrid composites with heterostructure constructed by macroscopic “core-shell” structure

Designing heterostructure is an effective way to comprehensively improve the strength-ductility of composites. However, the current methods of heterostructure integration consumes a large amount of energy. In this study, a macroscopic “core-shell” structure is designed with low cost and low energy consumption. The SiC particles (SiCp) and carbon fiber (CF) reinforced aluminum-based hybrid composite (SiCp/CF/Al) with heterostructure was fabricated utilizing the spark plasma sintering (SPS) method. The microstructure, interfacial structure, and mechanical properties of the SiCp/CF/Al before and after hot rolling were comprehensively investigated. The results revealed that the Vickers hardness values of the rich particle zone with high SiCp content were 54.32% and 48.10% higher than those of the poor particle zone before and after hot rolling, respectively. The synergistic strengthening effect of SiCp and CF significantly enhanced the yield strength and plasticity of the SiCp/CF/Al. Before and after hot rolling, the yield strength of SiCp/CF/Al was 242.68% and 173.16% of single CF-reinforced Al matrix composites, respectively. The extrinsic toughening was achieved through crack passivation and deflection at the interface of the rich particle zone and poor particle zone. Furthermore, the hindering and storing effects of the SiCp and CF on dislocations represented the primary intrinsic toughening mechanisms.

Microstructure Evolution and Frictional Wear Behavior of Al–18Si–4Mg–2Mn–0.5Fe–xCu Alloys Fabricated by Laser Additive Manufacturing

The microstructure evolution, corrosion resistance, and wear resistance of Al–18Si–4 Mg–2Mn–0.5Fe–xCu (x = 0,1,2,3,4,5 wt%) alloys prepared by laser additive manufacturing (LAM) process are studied. It is found that the addition of Cu not only produces Cu‐rich strengthened phase, but also promotes the precipitation of the second phase. With the addition of Cu, the corrosion resistance of the alloy decreases, which is due to the increase of potential difference caused by the increase of the strengthening phase. The addition of Cu can effectively inhibit corrosion cracking. The hardness and wear resistance of the alloy increase with the increase of Cu content under the synergistic strengthening of the Q‐Al5Cu2Mg8Si6 phase and Al2Cu phase. When the Cu content is 3 wt%, the hardness and wear resistance of the alloy are the best. However, with the increase of Cu content, part of the Q phase is transformed into the Al2Cu phase; this synergistic strengthening effect is weakened, and the hardness and wear resistance of the alloy are reduced, but still higher than that of the alloy without Cu addition, which indicates that the addition of Cu can effectively improve the hardness and wear resistance of the alloy.

Non-negligible role of gradient porous structure in superelasticity deterioration and improvement of NiTi shape memory alloys

Bone-mimicking gradient porous NiTi shape memory alloys (SMAs) are promising for orthopedic implants due to their distinctive superelastic functional properties. However, premature plastic deformation in weak areas such as thinner struts, nodes, and sharp corners severely deteriorates the superelasticity of gradient porous NiTi SMAs. In this work, we prepared gradient porous NiTi SMAs with a porosity of 50% by additive manufacturing (AM) and achieved a remarkable improvement of superelasticity by a simple solution treatment regime. After solution treatment, phase transformation temperatures dropped significantly, the dislocation density decreased, and partial intergranular Ti-rich precipitates were transferred into the grain. Compared to as-built samples, the strain recovery rate of solution-treated samples was nearly doubled at a pre-strain of 6% (up to 90%), and all obtained a stable recoverable strain of more than 4%. The remarkable superelasticity improvement was attributed to lower phase transformation temperatures, fewer dislocations, and the synergistic strengthening effect of intragranular multi-scale Ti-Ni precipitates. Notably, the gradient porous structure played a non-negligible role in both superelasticity deterioration and improvement. The microstructure evolution of the solution-treated central strut after constant 10 cycles and the origin of the stable superelastic response of gradient porous NiTi SMAs were revealed. This work provides an accessible strategy for improving the superelastic performance of gradient porous NiTi SMAs and proposes a key strategy for achieving such high-performance architectured materials.

Preparation, Mechanical Properties and Strengthening Mechanism of W-Re Alloys: A Review.

W-Re alloys are one of the most important refractory materials with excellent high-temperature performance that were developed to improve the brittleness of tungsten. In the present work, we firstly summarized the research progress on the preparation and strengthening methods of a W-Re alloy. Then, the strengthening mechanisms of the W-Re alloy were discussed, including the influence of Re, solid solution strengthening, second-phase reinforcement and fine-grain strengthening. The results showed that the softening effect of Re was mainly related to the transformation of the preferred slip plane and the introduction of additional d-valence electrons. Some transition elements and refractory metal elements effectively strengthened the W-Re alloy. Carbides can significantly enhance the high-temperature mechanical properties of W-Re alloys, and the reasons are twofold: one is the interaction between carbides and dislocations, and the other is the synergistic strengthening effect between carbides and Re. The objective of this work was to enhance the comprehension on W-Re alloys and provide future research directions for W-Re alloys.

Grain-level mechanism of plastic deformation in harmonic structure materials revealed by high resolution X-ray diffraction

Materials with heterogeneous microstructures have been reported to have an attractive combination of strength and ductility. This is attributed to synergistic strengthening effects from the difference in strength of fine- and coarse-grained regions. Understanding the interaction of the regions is crucial for further optimization of the microstructures. In this work, we fabricated nickel of harmonic structure (HS) and a reference with homogenous coarse grains. The HS constitutes of an interconnected fine-grained network that surrounds regions of coarse grains. The interplay of the regions was studied by monitoring Bragg reflections from individual grains in situ during tensile deformation until approximately 2 % strain through synchrotron X-ray diffraction. The technique allows grain-level assessment of the degree of plastic deformation. Two grains were followed in the reference and two small grains (fine-grained region) and two large grains (coarse-grained region) in the HS. Three deformation regimes were identified: elastic deformation, onset of plastic deformation and significant plastic deformation. Our results reveal that the large grains in the harmonic structure onset plastic deformation during the macroscopic elastic stage. With increasing applied stress, the small grains yield plastically also and once a large fraction of the fine-grained network deforms plastically the large grains undergo significant plastic deformation. Notably, the onset of significant plastic deformation of large grains in the HS occurs at approximately 100 MPa higher applied stress than in the grains in the reference. This shows that fine grains constrain the large grains from deforming plastically in the HS.

Stimulating adequate strengthening effect through alloying regulation for powder metallurgy CNTs reinforced aluminum matrix composites

Carbon nanotubes (CNTs) reinforced aluminum matrix composites (AMCs) have light weight and high specific strength, showing great potentials for making key structural components in aerospace and transportation industries. Aiming to address the bottleneck problem of unsatisfactory strengthening effect caused by the weak intrinsic interface and adverse interface reaction between CNTs and Al matrix, Si alloying element was added into Al-CNTs system via powder metallurgy method in this study. It was found that the introduction of Si element led to an inhibitory effect on Al-CNTs interface reaction. The discontinuous Al/Si/CNTs composite interface formed by the precipitation of Si significantly improved the wettability of Al/CNTs interface and enhanced its interface bonding through the pinning effect of Si particles, effectively promoting the full play of the load-bearing effect of CNTs. Furthermore, Si particles were effectively refined and passivated through appropriate heat treatment, which facilitated its dispersion strengthening and dislocation strengthening effect, and significantly reduced cracks of coarse Si particles. As a result, the synergistic strengthening effect of CNTs and micro-nano Si particles was significantly exerted in AMCs, remarkably improving the performance of Al–Si-CNTs composites.

A novel Ti/Al interpenetrating phase composite with enhanced mechanical properties

Lightweight, energy-absorbing materials with excellent mechanical properties are highly desired in practical engineering applications. A novel Ti-Al interpenetrating phase composite (Ti/Al IPC) was successfully fabricated by filling an Al alloy (ZL102) in the pores of an additively manufactured triply periodic minimal surface (TPMS) structure composed of Ti alloy (TC4). This IPC demonstrates a remarkable combination of attributes, including a high yield stress of 305 MPa, a Young's modulus of 64 GPa, and an ultimate tensile strength of 420 MPa. These mechanical properties are 1.5 to 2.9 times stronger than those of a single TC4 TPMS structure of the same size. The enhanced performance can be attributed to the synergistic strengthening and toughening effect resulting from the internal interpenetration of the ZL102 phase within the structure.

Microstructural effects in rate-dependent responses of smooth and notched magnesium bars

We perform three-dimensional crystal plasticity simulations of smooth and notched bar geometries made of polycrystalline hexagonal close-packed material representing a magnesium alloy. The polycrystalline microstructure is explicitly resolved to investigate the combined effect of initial texture and grain size on the rate-dependent macroscopic responses and their micromechanical underpinnings under uniaxial and multiaxial stress states. The simulations reveal that in addition to the textural effect recently investigated by Ravaji et al. (2021), grain size plays an important role in the anisotropy of macroscopic responses. For a given texture, the lateral deformation anisotropy increases with grain size refinement for all strain rates considered here. The load-deformation responses exhibit a synergistic strengthening effect in microstructures with stronger initial textures and finer grain sizes, which is enhanced with increasing notch acuity. A transition from a conventional power-law type load-deformation response to a sigmoidal load-deformation response may occur, which depends on the imposed strain rate. It is a result of the interaction between textural weakening and grain size refinement that influence extension twinning together with an equitable landscape of the different slip mechanisms. We discuss possible implications of the net material plastic anisotropy due to texture and grain size on macroscopic failure using a micromechanical basis.

Pulsed laser cladding on IN718 alloy using pre-coated CrCoNi-TiC/SiC powders for enhancing wear resistance

By employing a pulsed laser, laser cladding was performed on IN718 alloy pre-coated with CrCoNi-TiC/SiC powders and three defect-free coatings were prepared successfully. Microstructures of these coatings were subjected to dedicated characterization and well correlated with their surface performances (hardness/wear resistance). The CrCoNi coating was found to be predominantly composed of coarse grains. The addition of TiC and SiC powders will not only generate fine carbides dispersed in the coatings but also lead to the changes of grain and substructure morphologies through modifying the solidification thermodynamics in the cladding zone. The CrCoNi-TiC and the CrCoNi-SiC coatings have average grain sizes of 8.1 ± 7.9 μm and 6.6 ± 5.6 μm, respectively, much smaller than that of the CrCoNi coating (19.3 ± 19.2 μm), indicating effective grain refinement after incorporating such carbides. The average hardness values of CrCoNi-TiC and the CrCoNi-SiC coatings are measured as 315.9 ± 5.3 HV and 321.8 ± 8.6 HV, respectively, considerably higher than those of the CrCoNi coating (265.3 ± 6.9 HV) and the substrate (255.3 ± 4.5 HV). Both the composite coatings also have lower wear rates (6.6 × 10−4 mm3·N−1·m−1 for the CrCoNi-TiC coating and 2.9 × 10−4 mm3·N−1·m−1 for the CrCoNi-SiC coating) than the CrCoNi coating (9.3 × 10−4 mm3·N−1·m−1) and the IN718 substrate (11.1 × 10−4 mm3·N−1·m−1). Based on dedicated microstructural analyses, the synergistic strengthening effects due to grain refinement, second phase particles, and solid solution of alloying elements are found to have contributed to the superior surface performances of both the composite coatings, with the SiC demonstrated to be a more effective additive.

Abnormal strain-hardening in Co-rich CoNiV medium-entropy alloys

Two new nonequiatomic Co70Ni20V10 and Co65Ni20V15 medium entropy alloys were designed to investigate the synergistic strengthening effects of the dislocation slip, hexagonal close-packed (HCP) martensitic transformation, chemical short-range order (CSRO), and chemical medium-range order (CMRO), using X-ray diffraction, backscattered electron, electron channeling contrast imaging, electron backscattered diffraction, and transmission electron microscope. The results showed the Co65Ni20V15 alloy presented an abnormally higher strain-hardening rate than the Co70Ni20V10 alloy in the range of 5∼30 % strain, although the Co70Ni20V10 alloy possessed higher dislocation density with extra HCP martensitic transformation strengthening. This abnormality could be attributed to the profound CSRO and CMRO strengthening in the Co65Ni20V15 alloy with more large-sized V atoms, which was evidently observed by a high-resolution transmission electron microscope. It is expected that the strength of the CoNiV medium entropy alloys could be further enhanced via synthetically tuning the CSRO and CMRO effects, together with other strengthening mechanisms.

Constitutive equation and microstructural evolution of one distinctive Al–based hybrid composite reinforced by nano–AlN and micro–TiC particles during hot compression

In this study, an (8.2AlN + 4TiC)/Al–0.3Fe–0.1Mn composite was successfully fabricated for elevated– temperature applications by the liquid–solid reaction method, and the high–temperature performance (hot workability) of the composite was evaluated by unidirectional hot deformation experimentation. The tested temperatures and strain rates were 350–500 °C and 0.001–1 s−1, respectively. The results indicated that the peak stress of the composite was as high as 226 MPa at 350 °C and 1.0 s−1, which was significantly higher than the reported strength values of commercial deformed aluminum alloys or other particle–reinforced Al matrix composites under the same experimental conditions. Although the flow stress of the composite decreased with the increase of deformation temperature and the decrease of strain rate, the peak stress remained at 82 MPa under the experimental conditions of 500 °C and 0.001 s−1. This reflected the excellent high–temperature synergistic strengthening effect of the in–situ nanosized AlN and submicron TiC particles. Based on the true stress–true strain curve, the hot deformation constitutive equation of the composite was established and fitted. The relative microstructural evolution of the composite during hot deformation was characterized by electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). It was found that the Orowan strengthening and load–transfer strengthening effect of the nanosized AlN and submicron TiC particles were the main reasons for their high–temperature strengthening, and the softening effect of the composite could be attributed to dynamic recovery (DRV) and continuous dynamic recrystallization (CDRX), accompanied by partial discontinuous dynamic recrystallization (DDRX). This study provided a new strategy for the design of heat–resistant Al alloys and their high–temperature strengthening and softening mechanism. This study may provide new insights for designing and achieving good workability in Al–based composites for elevated temperature applications.