Softening Stage Research Articles

Influence of build direction on the ratchetting-fatigue interaction of heat-treated additively manufactured 316L stainless steel

This study has investigated the cyclic deformation and ratchetting-fatigue interaction of 316L stainless steel fabricated by laser powder bed fusion (LPBF) and subjected to a 900 °C/2 h post-heat treatment, considering both the vertical and horizontal orientations. Strain- and stress-controlled low cycle fatigue (LCF) tests are conducted to explore the cyclic deformation and the effects of the mean stress and stress amplitude on the fatigue life and ratchetting deformation. A detailed analysis of the cyclic deformation mechanism is conducted through X-ray microtomography and transmission electron microscope (TEM) observations. The results show that post-heat treated LPBF 316L steel exhibits cyclic hardening followed by a non-saturated cyclic softening stage. This behavior is attributed to the evolution of dislocation density and dislocation patterns, and the formation of surface cracks. The ratchetting strain and its rate are sensitive to the mean stress and stress amplitude. The fatigue lives of vertically built specimens are slightly higher under strain-controlled loading conditions, but significantly lower under stress-controlled loading conditions than those of horizontally built ones. Furthermore, ratchetting deformation can promote fatigue damage, resulting in a reduction in fatigue life.

Mesoscopic crack pattern fractal dimension-based concrete damage identification

The randomness and irregularity of concrete cracking present a challenge of quantitatively identifying the developing of damage degree of concrete subjected to complex external conditions. In this study, a concrete meso-mechanical model is established to quantitatively analyze crack propagation and damage evolution under uniaxial tension. Fractal dimension (FD) is used to quantify the crack distribution and track the entire process of concrete cracking. A novel calculation method for the damage identification based on crack FD is proposed, so that the quantitative correlation among the three curves of stress–strain, crack fractal dimension-strain, and damage variable-strain for concrete is obtained. During the hardening stage, the micro-cracks initiate at interface transition zone between aggregates and mortar, causing an increase in crack FD. As the load reaches its maximum, some micro-cracks tend to localize and form a crack aggregation zone. At the subsequent softening stage, the micro-cracks in other regions undergo a rebound phenomenon, resulting in a decrease in crack FD. Once the crack aggregation zone is formed, the macro-crack concentrates and continues to propagate, leading to an increase in crack FD again. This FD-based damage degree accumulates with crack development throughout the tensile process and varies with the aggregate gradation, specimen size, and aggregate distribution. This study suggests that the mutation point of the crack FD-strain curve after the peak stress point is the critical point at which concrete enters the local failure stage, and the corresponding damage value approximates 0.8.

Experimental Study on the Shear Characteristics of Deep‐Sea Sediments Based on a Mining Machine Walking Test Platform

The exploitation and utilization of deep‐sea mineral resources is an important measure to ensure the safety of resources. The shear characteristics of deep‐sea sediments are fundamental parameters for the design of a machine for mining polymetallic nodules. To investigate the shear characteristics of deep‐sea sediments, a comprehensive experimental platform is designed independently to drive mining machines. This platform can perform a series of simulated shear tests between the track board and deep‐sea sediments. The shear tests investigate the effects of earth pressure, shear rate, and overlying water on shear characteristics between tracks and sediments. The results show that the shear deformation process of deep‐sea sediments has four obvious stages, namely linear elastic stage, strain hardening stage, strain softening stage, and residual stress stage. The shear strength of deep‐sea sediments increases with the earth pressure. The shear strength of deep‐sea sediments decreases significantly under the influence of the overlying water. When the overlying water is applied for 1 h, the peak strength decreases by about 26% and the residual strength by about 70% compared to the strengths without water. When the overlying water is applied for 1 day, the peak strength decreases by about 36% and the residual strength by about 90%. This study provides a theoretical basis for analyzing the running behavior and structural design of crawler mining machines.

Study of rotational dynamic recrystallization during warm compression of an Mg-Zn-Nd-Zr alloy

The dynamic recrystallized grains obtained by rotational dynamic recrystallization (RDRX) mechanism are finer and have fewer defects, which can effectively improve the performance of magnesium alloys. However, the impact of temperature and strain on RDRX is currently unclear. In this study, an extruded Mg-Zn-Nd-Zr alloy with trace amounts of rare earth elements and fine grain size was used as the raw material, and a warm deformation temperature interval was selected, which suppressed the large-angle migration, reduced the activity of twinning, and successfully ensured RDRX as the main recrystallization mechanism. The effects of temperature and strain on RDRX of magnesium alloys were systematically investigated, and it was found that the compression temperature and strain jointly affected the formation of RDRX. The RDRXed grains emerged during the dynamic softening stage and abundantly developed during the dynamic equilibrium stage. The percentage of the RDRXed grains increased with the increase of temperature for temperatures below 290 °C and decreased with the increase of temperature for temperatures above 290 °C at low strain levels (0.4). The percentage of RDRX monotonically increased with the increase of compression temperature at high strain (0.7). The size of the recrystallized grains increased with the increase of compression temperature. The formation of the RDRXed grains could effectively release the local strain and form a strain free region without distortion, resulting in strain softening.

Microscopic Mechanisms and Acoustic Emission Characteristics of Sandy Mudstone under Different Water Saturations

In order to study the damage mechanism of surrounding rocks under water-rock coupling, an experimental study on the microscopic law of water absorption and mechanics of sandy mudstone was carried out in the sandy mudstone stratum of Wanfu coal mine in Shandong, China. The results indicate that the strength of sandy mudstone exhibits three stages of relative softening, rapid softening, and decelerated softening. The speed of water molecules entering the microscopic pore structure was analyzed based on the percentage of water molecules in the pore structure. The acoustic emission energy release process is divided into energy accumulation stage, stable release stage and intense release stage. With the increase of water content saturation, Δα (Spectral width of the multifractal spectrum) tends to decrease and the distribution of events is gradually uniform. The action of water gradually expands and penetrates the cracks and pores inside the sandy mudstone, which makes Δf (Frequency of occurrence of the signal) change from negative to positive values.

Peridynamic study of fatigue failure of engineered cementitious composites

Fatigue crack propagation is crucial in studying fatigue in engineered cementitious composites (ECC). This paper established a peridynamics (PD)-based fatigue crack propagation model for ECC, incorporating the “cycle broken bonds number” to enhance calculation efficiency. First, the PD fatigue crack propagation model was constructed, and the validity of the model was verified by simulating the fatigue crack propagation path of an inclined edge crack in the plate. Secondly, an improved PMB model considering the strain softening stage was used to model the ECC cement matrix, and the fibers were modeled as one-dimensional cyclic dynamic rods to establish a PD fully discrete model of ECC. At the same time, an interface exponential friction attenuation model was introduced. Then, the effectiveness and validness of the fully discrete model were verified by simulating single fiber pull-out tests and uniaxial tensile tests of ECC specimens. Finally, the fatigue crack propagation forms of ECC beams with mid span Type I cracks under three-point bending fatigue loads were studied, and the effects of fiber volume fraction, stress level, and fiber length on the crack propagation forms and structural deflection of ECC beams under four-point bending fatigue loads were investigated.

Validation study on light scattering changes in kiwifruit during postharvest storage using time-resolved transmittance spectroscopy

Visible and near-infrared spectroscopy has been well studied for characterizing the organic compounds in fruit and vegetables from pre-harvest to late harvest. However, due to the challenge of decoupling of optical properties, the relationship between the collected samples’ spectral data and their properties, especially their mechanical properties (e.g., firmness, hardness, and resilience) is hard to understand. This study developed a time-resolved transmittance spectroscopic method to validate the light scattering changing characteristics in kiwifruit during shelf-life and in cold storage conditions. The experimental results demonstrated that the reduced scattering coefficient ({mu }_{s}^{prime}) of 846 nm inside kiwifruit decreased steadily during postharvest storage and is more evident under shelf-life than in cold storage conditions. Moreover, the correlation between the {mu }_{s}^{prime} and the storage time was confirmed to be much higher than that using the external color indexes measured using a conventional colorimeter. Furthermore, employing time-resolved profiles at this single wavelength, an efficacious mathematical model has been successfully formulated to classify the stages of kiwifruit softening, specifically early, mid-, and late stages. Notably, classification accuracies of 84% and 78% were achieved for the shelf-life and cold storage conditions, respectively.

Study on the Load Transfer Behaviour and Bond-Slip Model of Fully Grouted Rockbolt

Rockbolts are often subjected to loads during service, and the load transfer behaviour of rockbolts is very important. Therefore, in this study, pullout tests were performed on two kinds of rockbolt systems, with and without defects, and the load transfer behaviour and failure modes were analysed. According to the load transfer process between the rockbolt and cement mortar interface, a bond-slip model considering the yield of the rockbolt was proposed, and the nonlinear behaviour in the softening stage was considered. The test results showed that the evolution of the interface load between the rockbolt and cement mortar in the grouted rockbolt systems without defects underwent four stages of gradual failure, namely, elasticity, yield, softening, and complete slip, and its failure mode involved the rockbolt being pulled out accompanied by splitting cracks and tensile cracks on the concrete surface. The proposed bond-slip models considering the nonyielding and yielding of the rockbolt can accurately reflect the actual load transfer behaviour of a fully grouted rockbolt.

Microstructure evolution and mechanical properties of directionally solidified Al0.5CoCr1.3FeNi1.3(Ti, Si, B, C)0.3 multiphase complex concentrated alloy

Microstructure evolution and mechanical properties of Al0.5CoCr1.3FeNi1.3(Ti, Si, B, C)0.3 multiphase complex concentrated alloy (CCA) prepared by directional solidification combined with quenching during solidification (QDS) technique were studied. The directionally solidified (DS) samples were prepared at constant growth rates V ranging from 2.78 × 10−6 to 5.56 × 10−5 m/s, the constant temperature gradient in liquid at the solid-liquid interface of GL = 16.5 × 103 °C/m and then quenched at a cooling rate of about 50 °C/s. The solidification of the studied CCA begins with the growth of columnar FCC(A1) dendrites and is finalised by the formation of a multiphase microstructure in the interdendritic region. The DS samples subjected to isothermal annealing at 900 °C for 25 h show a strong brittle-ductile transition (BDT) behaviour characterised by brittle-ductile transition temperature (BDTT) between 650 and 700 °C at a strain rate of 1 × 10−4 s−1. The tensile deformation curves show only a strain hardening stage at test temperatures up to 650 °C. At temperatures ranging from 750 to 900 °C, the alloy shows anomalous strain hardening behaviour characterised by one strain hardening stage and three well-distinguished strain softening stages.

Revealing hot deformation behavior of Ti2C-Ti composite and associated dynamic recrystallization mechanism of ceramic matrix

Titanium carbide based composites have attracted much attention owing to their light weight, high elastic modulus, and excellent wear resistance. However, the hot working of these composites has been facing great challenges, especially for the coordinated deformation of brittle phase. Here, soft Ti phase was introduced to improve the thermal deformation ability. In particular, the dynamic recrystallization (DRX) mechanism, crack propagation, and dislocation motion behavior of brittle Ti2C matrix were unveiled in this study. Firstly, a Ti2C-Ti composite was hot compressed at 770–1000 °C with strain rates of 0.012–0.0013 s−1. The results showed that the optimal processing temperature was 950 °C, at which the microstructure can be refined by DRX behavior without the occurrence of macro instability. In the later softening stage of hot compression, the recrystallized Ti2C grains were formed through both discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX). The intergranular Ti phase, which acts as a binder, coordinated the deformation of the Ti2C-Ti composite, and the phase boundaries attracted dislocations on the side of the brittle Ti2C phase. This dislocation motion behavior mitigated stress concentration but inhibited the DRX behavior of the Ti2C phase near the binary phase interfaces. The lamellar Ti precipitates within Ti2C grains facilitated the occurrence of CDRX by promoting the formation of subgrains. The subgrain boundaries were formed when the average distance between dislocation on the dislocation wall was lower than 8.68 nm. Compared with the as-sintered composite, the nanohardness of the Ti2C matrix in the composite deformed at 950 °C increased by 17%. The enhanced hardness was mainly attributed to grain boundary and dislocation strengthening effect, as well as the reduction of Ti precipitates.

Machine learning-assisted constitutive modeling of a novel powder metallurgy superalloy

The flow behavior and microstructure evolution during thermomechanical processing (TMP) of powder metallurgy superalloys are very important for their service behavior. In this contribution, a machine learning-assisted physical model was proposed to predict the flow stress, dynamic recrystallization (DRX) kinetic, and grain size evolution of a novel FGH4113A superalloy during the TMP. The work hardening and dynamic softening stages were modeled separately to predict the flow stress. The novel aspect of the proposed model is that the strong coupled effects of deformation temperature, strain rate, true stain, and primary γ' precipitates on the DRX kinetic were quantitatively described by the genetic algorithm-based artificial neural network (GA-ANN). Besides, the grain size modeling considered the pinning effect by primary γ' precipitates and the deformation-induced DRXed grain growth. Comparing the experimental and predicted flow curves, DRX fraction, and average grain size reveals that the proposed model has a preferable prediction accuracy than the conventional model. Then the specific DRX kinetic and grain size evolution characteristics were revealed. The promoting effect of strain rate on the DRX degree was gradually increased with the deformation temperature. Finally, the 3D processing map based on the developed model was established to guide TMP's optimized design and microstructure control. Three domains were divided and discussed: grain refinement, constant, and growth. The optimal processing window is defined as 1000–1050 °C/0.001–0.006 s−1 and 1075–1100 °C/0.008–0.01 s−1.

Experimental and constitutive modelling studies of type 316L stainless steel based on internal stress under low cycle fatigue and creep-fatigue interaction

Low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behaviour of 316L stainless steel at 550 °C were investigated using experimental and modelling approaches. Results demonstrate that friction and back stress contribute to the initial hardening stage, whereas the former is primarily responsible for the subsequent softening stage. Besides, friction stress also accounts for the non-masing feature and varied softening rate. Increasing holding time induces a rise followed by a decline in friction stress, while the back stress decreases steadily to a saturated value. Moreover, a short tensile holding duration leads to planar structures and higher dislocation density. However, a longer tensile holding time promotes dislocation annihilation and subgrains growth, and elongates dislocation cells. Finally, an improved viscoplastic constitutive model was proposed based on the internal stress analysis. The kinematic hardening rule was improved by considering the cycle-dependent plastic modulus and stress relaxation, and the non-masing and variable softening rate were captured by modifying the isotropic hardening rule. In addition, holding time-related factors were also coupled into the model. The proposed model is robust for describing the cyclic features under both LCF and CFI.

A universal constitutive model considering strain range dependence effect and transient behaviour for both cyclic softening and hardening steels

High-temperature low-cycle fatigue (HTLCF) behaviour of 2.25CrMoV and 316L were investigated by employing internal stress. Results reveal that 2.25CrMoV and 316L show cyclic softening and hardening–softening behaviour, respectively. Moreover, they exhibit strain range dependence effect (SRDE) and transient Bauschinger effect (TBE). The SRDE of 316L was observed throughout the entire fatigue procedure, whereas that of 2.25CrMoV was only observed in the initial softening stage. It is noteworthy that the SRDE is controlled by both back and effective stresses in the case of 316L, and it is primarily dominated by the back stress for 2.25CrMoV. Based on the internal stress analysis, a universal constitutive model was developed. Compared with existing models, a cycle-dependent kinematic hardening factor φ was coupled into kinematic hardening variables to improve the description of the hysteresis loops. Besides, a unified modelling method for SRDE was developed based on a kinematic-isotropic combined hardening rule extended with a strain memory surface. Furthermore, a novel two-dimensional proportionality factor δ related to the plastic modulus of cyclic and monotonic responses was incorporated into the short- and medium-range kinematic hardening variables to account for TBE. Finally, the validity of the proposed model was discussed from the hysteresis loop, peak stress evolution, and peak internal stress evolution. The good agreement between experimental data and simulated results suggests that the proposed model is robust in predicting cyclic softening and hardening–softening behaviours.

Dynamic mechanical properties and constitutive model of photosensitive resin specimens at different temperatures

In order to reveal the dynamic mechanical properties of resin-molded parts prepared from photosensitive resin composition at different temperatures, four typical service temperatures (26 °C, 50 °C, 70 °C and 90 °C) were selected, and the mechanical properties of photosensitive resin specimens under quasi-static and high strain rate (1200 s−1, 1500 s−1 and 1800 s−1) loading were tested by universal material testing machine and split Hopkinson pressure bar (SHPB) experimental device. The stress–strain data of the material were obtained. Results show that the stress of photosensitive resin specimens decreases with the increase of temperature under quasi-static and high strain rate loading conditions, reflecting a certain temperature softening effect. Two typical stages of strain softening and strain hardening exist in the quasi-static compression process of the specimens at room temperature, while the specimens only exhibit strain hardening at 50 °C, 70 °C and 90 °C. Under dynamic loading, the elastic modulus, peak stress and peak strain of the photosensitive resin increase with the increase of the strain rate, reflecting an obvious effect of strain rate strengthening. The nonlinear thermo-viscoelastic constitutive model can better describe the mechanical behavior of the material under high strain rates and service temperatures, and the experimental values are in good agreement with the fitted values of the model. The results can provide theoretical model and method support for the design and development of resin-based materials and the optimization of their mechanical properties.

Experimental and constitutive modeling of the anisotropic mechanical properties of shale subjected to thermal treatment

The physical and mechanical properties of shale exhibit significant anisotropy, impacting the design and construction of high geothermal tunnels and deep resource mining. Laboratory studies on the physical and anisotropic mechanical properties of shale after various thermal treatments showed that rising temperatures caused an exponential decline in shale’s bulk and density, and the uneven expansion of the mineral particles within the rock caused a decrease in its uniaxial compression strength (UCS) and elastic modulus while increasing their anisotropy. The UCS is U-shaped with a change in bedding dip angle, whereas the elastic modulus is U-shaped or shoulder-shaped. Under the assumption that the rock microelement obeyed the Weibull distribution, the microstructure tensor improved Hoek–Brown strength criterion was introduced as the damage basis. Then a thermal–mechanical coupling damage constitutive model considering bedding dip angle and temperature was established. The model was found to be valid via the test results, especially in capturing the compaction and yield softening stages of shale. These findings have reference value for both underground structural engineering and deep resource mining.

Compressive performance of underwater concrete columns strengthened by nondispersive mortar and stainless steel tubes

A novel type of underwater pier reinforcement technology was proposed. A non-dispersive mortar (NDM) and stainless steel tube (SS) were adopted to strengthen an underwater concrete column. The strengthened column takes advantage of the good adhesion of NDM and the corrosion resistance of SS. The NDM pouring and curing processes were both conducted in an underwater environment, which simulated the environment of underwater bridge pier reinforcement. An axial compression test was carried out on 16 strengthened columns and 3 unconfined concrete columns to explore the reinforcement effect of SS thickness and NDM type, including the 2–6 mm thickness of SS, the epoxy-based NDM, and the cement-based NDM. The test results indicated that all the specimens showed extrusion expansion failure. The increase in the thickness of SS resulted in a significant improvement in the load capacity of the specimens, and the stress-strain curves showed the strengthening stage and softening stage according to the confinement factor of the SS thickness. After adding cement-based NDM and epoxy-based NDM, the strength of the specimens increased by 6.2–21.4 % and 8.8–31.5 %, respectively. In the case of the same thickness of SS, cement-based NDM showed a better reinforcement effect. The stress-strain curves for the strengthened columns can be divided into three stages. Calculation models were proposed to predict the full stress-strain curves, including the calculation method of the capacity. The calculated results are in good agreement with the experimental data.

Experimental study of mechanical properties of laser additively manufactured 316L stainless steels

Laser additive manufacturing (LAM), as an advanced additive manufacturing technology, has high efficiency and excellent precision in producing large size 3D printing elements. It has proven potential for application in buildings and civil infrastructure. However, in order to apply LAM technology to the production of components in civil engineering, it is necessary to understand comprehensively the mechanical properties of LAM products. In a previous study, the mechanical properties in the layer plane of LAM 316L stainless steels have been studied, but the mechanical properties in the building elevation plane are still unknown. To this end, an experimental study into the mechanical properties and microstructure of LAM 316L stainless steels has been undertaken and is reported in this paper. A total of 18 tensile coupons with two nominal thicknesses were extracted in various orientation angles relative to the print layer from the LAM prims for the uniaxial tensile tests. The influences of the orientation angle and thickness on the mechanical properties of LAM 316L stainless steels were investigated through the tensile tests. The test results show high anisotropy of the LAM 316L stainless steels in the building elevation plane, for which the anisotropy indexes of all mechanical properties exceed 10%, and the anisotropy indexes of the elastic modulus, Poisson's ratio and the proof stresses even exceed 25%. In addition, a very small Poisson's ratio value and an extreme short softening stage of stress–strain curves after peak values were identified for the coupons with the orientation angle θ = 45°. Microstructural characterisation of the LAM 316L stainless steels was also examined by means of optical microscopy and scanning electron microscope (SEM). The microstructure analyses observe an obvious layered structure perpendicular to the building direction and some columnar crystals perpendicular to the layer plane crossing the boundary, which might explain the reasons causing the high anisotropy of the macroscopic mechanical properties of LAM steels.

Temperature gradient based annealing methodology for tungsten recrystallization kinetics assessment

In future thermonuclear fusion reactors, like ITER and DEMO, plasma-facing components with tungsten armor material will have to sustain high thermal fluxes (10 MW/m² in steady state and 20 MW/m² in quasi-steady-state). For such extreme conditions, tungsten may reach temperatures higher than 1000 °C and, consequently can be prone to recrystallize, which can limit the lifetime of the divertor targets under cycling thermal loadings. Characterization of recrystallization kinetics involves the use of different methods and devices (furnace, laser heating) depending on the annealing temperature regime to be analyzed. For these methods, the heating and the subsequent characterization of around 10 samples (5 × 4 × 4 mm) per annealing temperature is needed. In this article, a method is proposed to quantify the recrystallization kinetics between 1300 °C and 1600 °C using a limited number of samples and heating conditions. The basic idea is to induce a steady-state temperature gradient in tungsten rods by heating one side of the rod using a laser heating system. Tungsten rods (50 × 4 × 5 mm) were annealed with this method. The resulting microstructures are characterized using optical microscopy and hardness measurements. The different stages of tungsten softening are clearly evidenced. These first results can be considered as a proof of concept for the use of such a methodology to assess tungsten recrystallization at high temperature.

Damage softening model and behavior of reinforced soil interface based on the Weibull distribution

A damage-softening model for a reinforced soil interface is proposed based on the trilinear model of a reinforced soil pullout interface, aiming at the softening phenomenon of the geogrid reinforced soil interface during the pullout process and considering the damage softening of the reinforced soil interface during the pullout process. The damage variable factor D is introduced, and it is assumed that D is a function of the interface displacement. The two-parameter Weibull distribution function is used to express the damage evolution law of the reinforced soil interface. Based on the basic control equation of the reinforced soil interface, the calculation model of the stress state of the geogrid at different stages under the pullout load is deduced. To verify the accuracy and applicability of the model, the prediction results of the model are compared with the test results and the ideal elastic–plastic model, hyperbolic model, elastic–exponential softening model, and the damage softening model based on a lognormal distribution function proposed in this paper. The distribution of interfacial shear stress is studied, and the influence of parameters at each stage is analyzed. The results show that in the elastic stage, with the increase of shear stiffness, the maximum shear stress at the interface increases, and the curvature of the curve also increases, while the elastic modulus is the opposite. In the softening stage, with the increase of softening length, the peak value of the curve moves to the free end. In the residual stage, the shear stress increases from the drawing end to the free end and tends to the residual stress. The research results are in accordance with the actual situation and can be applied to reinforced soil engineering.

Bidirectional transformation enabled improvement in strength and ductility of metastable Fe50Mn30Co10Cr10 complex concentrated alloy under dynamic deformation

High strain rate compression experiments performed on a non-equiatomic metastable fcc + hcp Fe50Mn30Co10Cr10 high entropy alloy using split Hopkinson pressure bar setup shows improved flow stress and compression ductility compared to quasistatic deformed samples. The deformation response was characterised by the occurrence of hardening and softening stages compared to sustained strain hardening for quasistatic deformation. Detailed EBSD, BSE imaging analysis coupled with TEM shows significant bi-directional transformation (B-TRIP) and increased fcc γ phase stability in high strain rate regime while only forward (fcc to hcp) transformation dominates with increasing fraction of hcp ε phase in the quasistatic regime of deformation. Bidirectional transformation aided by adiabatic heating and heterogeneous deformation in the high strain rate regime leads to optimal stress and strain partitioning between the two phases and delays the initiation of damage at the interface. The presence of concomitant strain rate hardening and improvement in ductility in the dynamic deformation regime opens up avenues for microstructural tunability to achieve simultaneous improvement in strength and ductility using the metastability paradigm in complex concentrated alloys.