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A novel precipitate-strengthened Co–Cr–Ni medium-entropy alloy with a pure “FCC + L12” dual-phase structure is developed by adding a high concentration (6 at.%) of Al and Ti elements. After aging heat treatment at 700 °C for 4 h, the hierarchical L12 phases, composed of primary large-sized L12 phases (∼70 nm) and secondary L12 particles (∼14 nm), are achieved. The designed alloy featuring hierarchical L12 phases exhibits excellent strength-ductility combinations and strain-hardening ability, properties that are much improved over their counterparts with homogeneous dispersed L12 phases (∼13 nm) and without L12 phases. Especially, the yield strength (1320 MPa), ultimate tensile strength (1886 MPa), and average strain-hardening rate (5.15 GPa) are obtained in the alloy with hierarchical L12 precipitates deformed at cryogenic temperature. The Orowan bypass mechanism of the primary L12 phases, along with the shearing mechanism of the secondary L12 phases, collectively contributes to the exceptional strength of the alloy with hierarchical L12 precipitates. The penetration of dislocations and stacking faults (SFs) in the primary L12 phases, combined with hetero-deformation-induced (HDI) hardening attributed to these hierarchical L12 precipitates, may be a key factor in improving strain-hardening properties of the alloy.

Single-phase CrCoNi-based medium-entropy alloys (MEAs) have garnered considerable attention as a promising class of metallic materials. However, despite this growing interest, the dynamic mechanical response of CrCoNi-based MEAs at high strain rate remains controversial. The dynamic shear mechanical behavior of CoCrNiSi0.3 was characterized through Hopkinson-bar experiment utilizing hat-shaped specimen. In this study, a damage model based on the maximum shear strain of each slip system was chosen to describe the damage initiation and evolution. Combined with this micro-scale damage and dislocation density-based crystal plasticity theory, two constitutive models were proposed to perform predictions of the dynamic shear behavior of CoCrNiSi0.3. The strain rate effect, adiabatic temperature rising, damage evolution, and twin volume fraction during dynamic shear deformation were tracked by investigating the influence of resolved shear stress, dislocation density, texture evolution, and twinning systems shear strain. The presented model effectively predicts the deformation state of specimen, the formation and evolution of Adiabatic Shear Bands (ASBs), and the variations in strain rate and temperature at different locations within the shear region. Adiabatic Shear Bands and texture evolution are caused by the local dislocation density rising and misorientation distributions.

A Co-free non-equiatomic Ni2·5CrFeAl0·25Ti0.25 medium-entropy alloy (MEA) with an excellent strength-ductility synergy was fabricated, which shows a multiphase structure composed of face-centered cubic (FCC), L12 (ordered FCC), and Cr-rich body-centered cubic (BCC) phase by thermomechanical processing. Specifically, the aged sample displays the outstanding yield tensile strength (YTS, ∼1188 MPa), ultimate tensile strength (UTS, ∼1560 MPa) and work-hardening rate (WHR, ∼4.5 GPa) values as well as an acceptable plasticity of ∼16.6%. Theoretical calculations suggest that precipitation strengthening significantly contributes to achieving the fascinating tensile strength among various strengthening contributors. Further analyses reveal that multiple nanoscale stacking-fault (SF) networks are activated during plastic deformation in the aged alloy. Accordingly, the dual effects consisting of the hierarchical precipitation structure and SF networks lead to the combination of excellent tensile strength and strain-hardening capacity.

The CrCoNiSi0.3 MEA exhibits excellent cryogenic mechanical properties upon both quasi-static and dynamic tension. Under quasi-static tension at cryogenic temperature, the engineering yield and ultimate tensile strengths (UTS) reach 980 MPa and 1800 MPa, respectively, with notable ductility (62 %). The product of UTS and total elongation (TE) is 111.6 GPa %, surpassing most cryogenic high strength-ductility alloys. The significant mechanical strength enhancement is attributed to the denser deformation twins (DTs), multiple twinning, and extensive face-centered-cubic to hexagonal-close-packed (HCP) phase transitions, resulting in a high work hardening capacity. Upon dynamic tension at cryogenic temperature, the strength and strain hardening are further improved, which originates from the thickening DTs and HCP sequence and localized plastic deformation. The effects of temperature and strain rate on phase transition are studied. It is proposed that there is a competing relationship between high strain rate and increased stacking fault energy (SFE) due to temperature rise. The coupling effect of cryogenic temperature and high strain rate inhibits phase transition due to the deformation inhomogeneity in CrCoNiSi0.3 MEA. The findings make a valuable contribution to understand the influence of temperature and strain rate on the mechanism of FCC-to-HCP phase transition.

Ultrathin foils are used in multiple fields, including aerospace, and precision instrument design. In this paper, CoCrNi medium-entropy alloy (MEA) ultrathin foils with terminal thicknesses of 77 μm were prepared by accumulative cold rolling at room temperature. The quasi-static tensile properties of the rolled ultrathin foils were tested. The influence of the reduction ratio and thickness values on their mechanical properties was studied through EBSD and TEM. The results showed that the CoCrNi MEA ultrathin foils exhibited excellent tensile performance when the thickness reached 160 μm, in which the tensile strength reached 1.69 GPa at a tensile elongation of 7 %. Microstructure characterization revealed that the dominant strengthening mechanism of the CoCrNi ultrathin foils were composed of grain boundary and dislocation strengthening. Specifically, when the thickness exceeded 160 μm, enhanced dislocation and grain size strengthening cause ' work hardening ', and the strength gradually increased with decreasing thickness. When the thickness was 160 μm, the strengthening effect produced by dislocation banding, twinning, and multistage twinning delivery significantly influenced the mechanical properties of the foils. When the thickness was less than 160 μm, this process mainly occurred due to the interactions of dislocation proliferation and annihilation in the grain, causing the strength to decrease gradually with decreasing thickness, thereby resulting in ' work softening '.

To enhance the strength and toughness of metallic materials simultaneously over a wide range of temperatures and strain rates, a eutectic high-entropy FCC/Laves composite with a heterogeneous initial microstructure was fabricated by powder plasma arc additive manufacturing. The mechanical behavior of the composite over a wide range of temperatures and strain rates was tested with the aid of an electronic universal testing machine and an improved Split Hopkinson pressure bar. The high-entropy FCC/Laves composite possesses a unique combination of strength and ductility over the selected temperature and strain rate ranges due to the in situ composite nature with both soft high-entropy FCC phase and hard high-entropy Laves phase. Complicated thermal viscoplastic behavior is presented. To reveal the mechanisms of the complicated thermal viscoplastic behavior, microstructure evolution was characterized. The high-entropy Laves phase, as a kind of multi-component intermetallic, defies convention by displaying plastic deformation at room temperature and different strain rates. Superior damage tolerance of the high-entropy FCC/Laves composite can be achieved over the selected temperature and strain rate ranges with the contribution of deformation twin in FCC phase, as well as dynamic recrystallization over high temperature range. Finally, a constitutive description was developed, which is shown to be able to accurately describe the complicated plastic behavior over a wide range of temperatures and strain rates. These findings suggest promising prospects for advanced material design, opening a new avenue to achieve a fine balance between strength and ductility across different conditions.

A novel Ni-Co-Cr-based medium-entropy superalloy with a high Si content (7.5 at%) strengthened by an L12 phase was developed. The pure L12 phase, characterized by an average size of 50 nm and a volume fraction of 46%, was precipitated within the FCC matrix. This alloy exhibits excellent mechanical properties over a wide range of temperatures from 77 K to 1073 K. A yield strength of 1005 MPa, an ultimate tensile strength of 1620 MPa, and a tensile elongation of 36% were achieved at 77 K, with a maximum value of 4.8 GPa at the second stage of the work-hardening rate. The alloy maintains a basically consistent yield strength of approximately 800 MPa from 298 K to 973 K, showcasing significant strain-hardening capabilities, with values of 2.5 GPa, 3.7 GPa, and 4.8 GPa at 873 K, 298 K, and 77 K, respectively. Microscopic analysis revealed that at room and cryogenic temperatures, multilayer stacking faults (SFs), SF bands, and SF networks, rather than twins, effectively stored a large number of dislocations and impeded dislocation movement, thereby enhancing the work-hardening ability of the alloy. Furthermore, at 773 K, the primary deformation mechanism involved high-density dislocation walls (HDDWs) consisting of dislocation tangles and SF lines. As the temperature rose to 973 K, the work-hardening process was influenced by the APB shearing mechanism (in the form of dislocation pairs), SF lines, and microtwins generated through atomic rearrangement. This study not only provides valuable insights for the development of new oxidation-resistant superalloys but also enhances our understanding of high-temperature deformation mechanisms.

The effects of Si addition on the microstructures and properties of CoCrNi medium-entropy alloy (MEA) were systematically investigated. The CrCoNiSix MEA possesses a single face-centered cubic (FCC) phase when x is less than 0.3 and promotes solution strengthening, while the crystal structure shows a transition to the FCC+σ phase structure when x = 0.4 and the volume fraction of the σ phase increases with a microstructure evolution as the Si content increases. The Orowan mechanism from σ precipitation effectively enhances the strength, hardness, and stain hardening of CrCoNiSix MEA, which also exhibits superior hardness at high temperatures. Furthermore, a large amount of σ phase decreases the wear resistance because of the transformation of the main wear mechanism from abrasion wear for σ-free CrCoNiSix MEA to adhesion wear for σ-contained CrCoNiSix MEA. This work contributes to the understanding of the effect of Si addition on FCC structured alloys and provides guidance for the development of novel Si-doped alloys.

The three levels of precipitation with the primary Ni16Ti6Si7 intermetallic, the secondary large-sized L12 phase and the tertiary diffused distributed nanoscale L12 particles is achieved in an aged Ni2CoCr0.5Si0.3Al0.2Ti0.2 medium entropy alloy (MEA). The yield strength of 1222 MPa and ultimate strength of 1626 MPa as well as the uniform elongation of 16% and excellent work hardening rate exceeding 4.5 GPa are obtained. The precipitation strengthening model is employed to understand the comprehensive effect of the shearing and Orowan strengthening on yield strength. The synergistic strengthening mechanisms, including dislocation Orowan mechanisms for the Ni16Ti6Si7 intermetallic, the shearing and Orowan mechanisms of large-sized L12 precipitation, the shearing mechanism and pining effect from the small-sized L12 precipitation, together with Lomer-Cottrell (LC) locks and nano-spacing stacking fault (SF) networks, result in the ultra-high work hardening during plastic deformation, which provides a novel strategy for the design of excellent precipitation-strengthened alloys.

Nanoindentation of FeCoCrNiCu high entropy alloys (HEAs) was simulated by molecular dynamics (MD) simulation to investigate the mechanical properties of HEAs in terms of Young’s modulus, dislocation, and shear strain. And the effects of loading rates and temperatures on the deformation of alloy matrix were studied. The young's modulus obtained by fitting was approximately consistent with the experimental results. In nanoindentation, HEAs underwent elastic deformation first, and then began plastic deformation. Dislocations continuously nucleated and expanded under the indenter and eventually formed dislocation loops. Owing to the complex element composition and the strain gradient effect, the distribution of shear strain in the alloy was not uniform. The loading rate had an effect on the growth of dislocation, and the critical plastic depth also increased with the increase of loading rate. The deformation of HEAs was significantly affected by temperature. With the increase of indentation temperature, atoms moved more violently, and the matrix was easily deformed. The indentation force increased obviously at low temperature, because the low temperature itself reduced the atomic mobility and facilitated the formation of twins, which further strengthened the matrix.

The dynamic loading of CoCrNiSi0.3 medium-entropy alloys was performed by using Split Hopkinson Pressure Bar. The compression properties at different strain rates were investigated, and found to be sensitive to strain rates and have obvious strain rate strengthening effect. Under quasi-static compression (strain rate 10-4 s-1), the strain rate sensitivity m is 0.13, while under dynamic compression (strain rate 5 150 s-1) m 0.25. The shear properties of the alloy were studied through hat shaped shear test. The results show that the quasi-static shear yield strength is 330 MPa and the dynamic shear yield strength is 630 MPa. With the increase of the width of the reserved shear region, the peak stress decreases, but the shear failure strain increases obviously. When the width of the reserved shear region is 50 μm, the shear strain is as high as 12.8. Through the shear “freezing” experiment, the effect of increasing shear deformation on the grain microstructure and texture evolution of the material was studied. The results show that the geometric dislocation density in the grain increases with the increase of deformation, and the deformation texture evolves to A and P type texture, while the Cu type texture decreases gradually. Before the formation of adiabatic shear band, the temperature rise calculated by power-heat conversion lacks accuracy, which proves that the formation of adiabatic shear band is not caused by thermal softening.

Macroscopic mechanical response and microstructure evolution of 7075-T6 aluminum alloy during dynamic compression were investigated by experiments and crystal plastic finite element method (CPFEM). The dynamic compression experiment of 7075-T6 aluminum alloy with a strain rate of 2 000 s-1 was carried out with a split Hopkinson pressure bar (SHPB). The microstructure of aluminum before and after dynamic compression was characterized by electron backscattering diffraction technique (EBSD). The internal state variables of dislocation density were introduced into the CPFEM model by modifying the strengthening model and the flow criterion. The effects of friction coefficient, dislocation multiplication coefficient, and dislocation annihilation coefficient on the macro mechanical properties of the specimens were investigated. The results show that the increase of dislocation multiplication coefficient aggravates the hardening behavior and increases the ultimate strength of the material, and the increase of dislocation annihilation coefficient reduces the ultimate strength of the material while weakens its hardening behavior. Through the comparison of experimental and simulation results, it could be seen that CPFEM accurately predicts the variation trend of the texture during dynamic compression, as more brass {110} texture and Goss {110} texture are generated.

Precipitation strengthening is believed as an important strengthening method that optimizes the strength of alloys. Herein, the possibility of precipitation strengthening is demonstrated by introducing the intermetallic (Cr, Si)-rich σ phase in the novel FeCrNiSix MEAs system. It is found that simultaneous enhancement in strength and work hardening can be attained by adding Si element in the FeCrNi MEA. Specifically, the FeCrNiSi0.2 alloy realizes a good balance of strength and ductility due to the precipitation of ultrafine- or even nanoscale-sized σ phase. Moreover, quantitative analyses are conducted to assess the underlying strengthening mechanisms. [ABSTRACT FROM AUTHOR]