Formation mechanism of hierarchical twins in the CoCrNi medium entropy alloy

The three-dimensional hierarchical twin network has been proved to be the source of the excellent strength-ductility combination in the CoCrNi medium entropy alloy.Revealing the formation mechanism of hierarchical twins,however,remains a challenge using either the post-mortem or the in-situ microstructural characterization.In this study,the atomistic formation mechanism of hierarchical twins was investigated using molecular dynamics simulations,with special focus on the effects of strain rate and deformation temperature.Compared to the primary twin boundaries kink-driven hierarchical twinning tendency in pure FCC metals,the chemical inhomogeneity in CoCrNi can reduce the necessary kink height to trigger conjugate twins(CTWs),fascinating the formation of twin networks.At room temperature,the plastic deformation is dominated by primary twins(PTWs)and conjugate slips at a relatively lower strain rate(e.g.,5×10^(7)s^(−1)).The hierarchical twins can be activated in cases of deforming at a higher strain rate(e.g.,2×10^(8)s^(−1)).Further increasing the strain rate(e.g.,1×10^(10) s^(−1))leads to the phase-transformation induced plasticity.At cryogenic temperatures,the hierarchical twins are promoted within a large range of strain rates(e.g.,5×10^(7)–1×10^(10) s^(−1)).A higher temperature leads to the synergy of CTWs and primary slips at a lower strain rate,but hierarchical twins at a higher strain rate.On this basis,a qualitative comparison and scalable trends between experiments and simulations were revealed.The present study would not only provide the basic understanding for the twinning behavior found experimentally,but also contribute to the design of medium/high entropy alloys with excellent mechanical performances by tuning microstructures.

Dislocation me diate d dynamic tension-compression asymmetry of a Ni_(2)CoFeV_(0.5)Mo_(0.2) medium entropy alloy

Although tension-compression(T-C)asymmetry in yield strength was rarely documented in coarse-grained face centered cubic(FCC)metals as critical resolved shear stress(CRSS)for dislocation slip differs little between tension and compression,the T-C asymmetry in strength,i.e.,higher strength when loaded in compression than in tension,was reported in some FCC high entropy alloys(HEAs)due to twinning and phase transitions activated at high strain regimes in compression.In this paper,we demonstrate a reversed and atypical tension-compression asymmetry(tensile strength markedly exceeds compressive strength)in a non-equiatomic FCC Ni_(2)CoFeV_(0.5)Mo_(0.2) medium entropy alloy(MEA)under dynamic loading,wherein dislocation slip governs dynamic deformation without twins or phase transitions.The asymme-try can be primarily interpreted as higher CRSS and more hard slip modes(lower average Schmid factor)activated in grains under dynamic tension than compression.Besides,larger strain rate sensitivity in dy-namic tension overwhelmingly contributes to the higher flow stress,thanks to the occurrence of more immobile Lomer-locks,narrower spacing of planar slip bands and higher dislocation density.This finding may provide some insights into designing MEAs/HEAs with desired properties under extreme conditions such as blast,impact and crash.

Chemical inhomogeneity inhibits grain boundary fracture:A comparative study in CrCoNi medium entropy alloy

Grain boundary(GB)fracture is arguably one of the most important reasons for the catastrophic failure of ductile polycrystalline materials.It is of interest to explore the role of chemical distribution on GB defor-mation and fracture,as GB segregation becomes a key strategy for tailoring GB properties.Here we report that the inhomogeneous chemical distribution effectively inhibits GB fracture in a model CoCrNi medium entropy alloy compared to a so-called‘average-atom’sample.Atomic deformation kinematics combined with electronic behavior analysis reveals that the strong charge redistribution ability in chemical disor-dered CrCoNi GBs enhances shear deformation and thus prevents GB crack formation and propagation.Inspects on the GBs with different chemical components and chemical distributions suggest that not only disordered chemical distribution but also sufficient“harmonic elements”with large electronic flexibility contribute to improving the GB fracture resistance.This study provides new insight into the influence mechanism of GB chemistry on fracture behavior,and yields a systematic strategy and criterion,from the atoms and electrons level,forward in the design of high-performance materials with enhanced GB fracture resistance.

Thermal Deformation Behavior and Processing Map of a Novel CrFeNiSi_(0.15)Medium Entropy Alloy

The thermal deformation behavior of a novel CrFeNiSi_(0.15)medium entropy alloy(MEA)was studied via isothermal compression experiments,with the processing parameter range of 900–1200℃and 0.001–1 s^(−1).According to experimental data,the modified constitutive equation had been obtained,which precisely predicted the flow behavior of CrFeNiSi_(0.15)MEA during thermal deformation.At the same time,the processing map was established on the basis of the dynamic material model(DMM)theory.According to the map,the optimal processing parameters were determined at 1130–1200℃/0.06–1 s−1,under which the power dissipation efficiency could reach above 34%.The peak efficiency was above 38%,which occurred at 1200℃/1 s^(−1).In such parameter,complete dynamic recrystallization(DRX)also occurred.The flow instability of CrFeNiSi_(0.15)MEA was estimated to occur at 900–985℃/0.12–1 s^(−1),which was shown as grain boundaries cracking.Furthermore,both the continuous DRX(CDRX)and discontinuous DRX(DDRX)occurred simultaneously during thermal deformation.Meanwhile,some twins were also newly formed during DRX process,most of which were primary twins.The occurrence of twinning was beneficial to promote the development of DRX behavior.

Microbiologically Influenced Corrosion Behavior of Fe_(40)(CoCrMnNi)_(60) and Fe_(60)(CoCrMnNi)_(40) Medium Entropy Alloys in the Presence of Pseudomonas Aeruginosa

In this work,the microbiologically influenced corrosion(MIC)of Fe_(40)(CoCrMnNi)_(60) and Fe_(60)(CoCrMnNi)_(40) medium entropy alloys(MEAs)induced by Pseudomonas aeruginosa(P.aeruginosa)was investigated.Corrosion behaviors during 14 days of immersion in sterile and P.aeruginosa-inoculated culture media are presented.Under sterile conditions,both MEAs exhibited good corrosion resistance against the culture medium solution.In the presence of P.aeruginosa,the pitting corrosion of MEAs was promoted.The results of inductively coupled plasma‒mass spectrometry(ICP‒MS)and potentiodynamic polarization tests showed that the presence of P.aeruginosa promoted the selective dissolution of passive film and accelerated the corrosion of MEAs.The results of X-ray photoelectron spectroscopy(XPS)and Mott-Schottky measurements further demonstrated the degradation effect of P.aeruginosa on the passive film.Compared with Fe_(60)(CoCrMnNi)_(40),Fe_(40)(CoCrMnNi)_(60) manifested better resistance to the MIC caused by P.aeruginosa,which may be attributed to more Cr oxides and fewer Fe oxides of the passive film.

Combinatorial development of antibacterial FeCoCr-Ag medium entropy alloy

Antibacterial activity and mechanical properties of FeCoCr-Ag medium entropy alloys were studied via combinatorial fabrication paired with high-throughput characterizations.It was found that the antibacterial activity and mechanical properties exhibit non-linear dependence on the content of Ag addition.Within the studied alloys,(FeCoCr)_(80)Ag_(20) possesses an optimized combination of different properties for potential applications as antibacterial coating materials.The underlying mechanism is ascribed to the formation of a dual-phase structure that leads to competition between the role of Ag phase and FeCoCr phase at different Ag content.The results not only demonstrate the power and effectiveness of combinatorial methods in multi-parameter optimization but also indicate the potential of high entropy alloys as antibacterial materials.

Microstructure and mechanical properties of CoCrNi-Mo medium entropy alloys:Experiments and first-principle calculations

The effect of Mo additions on the microstructures and mechanical properties of CoCrNi alloys was investigated,meanwhile,ab initio calculations are performed to quantitatively evaluate the lattice distortion and stacking fault energy(SFE).The yield strength,ultimate tensile strength,and elongation of(CoCrNi)_(97)Mo_(3)alloy are 475 MPa,983 MPa and 69%,respectively.The yield strength is increased by~30%and high ductility is maintained,in comparison with CoCrNi alloy.Besides the nano-twins and dislocations,the higher density of stacking faults is induced during the tensile deformation for(CoCrNi)_(97)Mo_(3)alloy.Ab initio calculation results indicate the mean square atomic displacement(MSAD)and SFE value of(CoCrNi)_(97)Mo_(3)alloy is 42.6 pm^(2)and-40.4 mJ/m^(2)at 0 K,respectively.The relationship between mechanical properties and MSAD,SFE for various multiple principal element alloys is discussed.

Origin of strong solid solution strengthening in the CrCoNi-W medium entropy alloy

Solid solution strengthening is one of the most conventional strategies for optimizing alloys strength,while the corresponding mechanisms can be more complicated than we traditionally thought specifically as heterogeneity of microstructure is involved.In this work,by comparing the change of chemical distribution,dislocation behaviors and mechanical properties after doping equivalent amount of tungsten(W)atoms in CrCoNi alloy and pure Ni,respectively,it is found that the alloying element W in CrCoNi alloy resulted in much stronger strengthening effect due to the significant increase of heterogeneity in chemical distribution after doping trace amount of W.The large atomic scale concentration fluctuation of all elements in CrCoNi-3W causes dislocation motion via strong nanoscale segment detrapping and severe dislocation pile up which is not the case in Ni-3W.The results revealed the high sensitivity of elements distribution in multi-principle element alloys to composition and the significant consequent influence in tuning the mechanical properties,giving insight for complex alloy design.

Refined microstructure and enhanced mechanical properties of AlCrFe_(2)Ni_(2) medium entropy alloy produced via laser remelting

A Co-free as-cast AlCrAlCrFe_(2)Ni_(2)medium entropy alloy(MEA)with multi-phases was remelted by fiber laser in this study.The effect of laser remelting on the microstructure,phase distribution and mechanical properties was investigated by characterizing the as-cast and the remelted AlCrAlCrFe_(2)Ni_(2)alloy.The laser remelting process resulted in a significant decrease of grain size from about 780μm to 58.89μm(longitudinal section)and 15.87μm(transverse section)and an increase of hardness from 4.72±0.293 GPa to 6.40±0.147 GPa(longitudinal section)and 7.55±0.360 GPa(transverse section).It was also found that the long side plate-like microstructure composed of FCC phase,ordered B2 phase and disordered BCC phase in the as-cast alloy was transformed into nano-size weave-like microstructure consisting of alternating ordered B2 and disordered BCC phases.The mechanical properties were evaluated by the derived stressstrain relationship obtained from nano-indentation tests data.The results showed that the yield stress increased from 661.9 MPa to 1347.6 MPa(longitudinal section)and 1647.2 MPa(transverse section)after remelting.The individual contribution of four potential strengthening mechanisms to the yield strength of the remelted alloy was quantitatively evaluated,including grain boundary strengthening,dislocation strengthening,solid solution strengthening and precipitation strengthening.The calculation results indicated that dislocation and precipitation are dominant strengthening mechanisms in the laser remelted MEA.

Dynamic deformation behavior of a FeCrNi medium entropy alloy

Deformation behavior of a FeCrNi medium entropy alloy(MEA)prepared by powder metallurgy(P/M)method was investigated over a wide range of strain rates.The FeCrNi MEA exhibits high strain-hardening ability,which can be attributed to the multiple deformation mechanisms,including dislocation slip,deformation induced stacking fault and mechanical twinning.The shear localization behavior of the FeCrNi MEA was also analyzed by dynamically loading hat-shaped specimens,and the distinct adiabatic shear band cannot be observed until the shear strain reaches~14.5.The microstructures within and outside the shear band exhibit different characteristics:the grains near the shear band are severely elongated and significantly refined by dislocation slip and twinning;inside the shear band,the initial coarse grains completely disappear,and transform into recrystallized ultrafine equiaxed grains by the classical rotational dynamic recrystallization mechanism.Moreover,microvoids preferentially nucleate in the central areas of the shear band where the temperature is very high and the shear stress is highly concentrated.These microvoids will coalesce into microcracks with the increase of strain,which eventually leads to the fracture of the shear band.

Microstructures and properties of CuZrAl and CuZrAlTi medium entropy alloys prepared by mechanical alloying and spark plasma sintering

Equiatomic CuZrAl and CuZrAlTi medium entropy alloys were designed and synthesized by mechanical alloying and spark plasma sintering technique.The alloying behavior,phase evolutions,microstructures and properties of samples were investigated by X-ray diffraction,differential scanning calorimetry,field emission scanning electron microscopy,microscopy/Vickers hardness testing and electrochemical polarization measurement.The results indicate that the final products of as-milled alloys consist of amorphous phases.Ti addition improves the glass forming ability of as-milled alloys.The as-sintered CuZrAl alloy contains face-centered cubic(fcc)solid solution,Al_(1.05)Cu_(0.95) Zr and AlZr_2 phases at different sintering temperatures.With Ti addition,the as-sintered sample is only composed of intermetallics at 690°C,while fcc1,fcc2 and CuTi3phases are formed at 1100°C.CuZrAlTi-1100°C alloy exhibits the highest hardness value of 1173HV0.2owing to the high sintering density,solid solution strengthening and homogeneous precipitation of nano-size crystalline phase.CuZrAlTi-690°C alloy presents a similar corrosion resistance with304 Lstainless steel in seawater solution and further possesses the lower corrosion rate.

Probing deformation mechanisms of gradient nanostructured CrCoNi medium entropy alloy

The gradient nanostructured medium entropy alloys(MEAs) exhibit a good yielding strength and great plasticity. Here, the mechanical properties, microstructure, and strain gradient in the gradient nanostructured MEA CrCoNi are studied by atomic simulations. The strong gradient stress and strain always occur in the deformed gradient nanograined MEA CrCoNi. The origin of improving strength is attributed to the formation of the 9 R phase, deformation twinning, as well as the fcc to hcp phase transformation, which prevent strain localization. A microstructure-based predictive model reveals that the lattice distortion dependent solid-solution strengthening and grain-boundary strengthening dominate the yield strength,and the dislocation strengthening governs the strain hardening. The present result provides a fundamental understanding of the gradient nanograined structure and plastic deformation in the gradient nanograined MEA, which gives insights for the design of MEAs with higher strengths.

A quasi-in-situ EBSD study of the thermal stability and grain growth mechanisms of CoCrNi medium entropy alloy with gradient-nanograined structure

The thermal stability and mechanical properties of a gradient-nanograined structure(GNS)CoCrNi medium entropy alloy(MEA)processed by ultrasonic surface rolling were studied by using isothermal/isochronal annealing tests combined with quasi-in-situ electron backscatter diffraction(EBSD)characterization and Vickers micro-hardness(HV)measurements.A layer by layer high-throughput investigation method was used to quantitatively study the grain growth kinetics and grain boundary evolution with different initial grain sizes,which could effectively save specimen and time costs.The grain nucleation and growth,as well as shrink and disappearance process throughΣ3 coincidence site lattice boundary migration with slightly lattice rotation during annealing were directly revealed.The layer by layer grain growth kinetics and calculated activation energy indicate that the thermal stability of nanograined top surface layer is relatively higher than that of nano-twined subsurface layer for the gradient CoCrNi MEA processed by ultrasonic surface rolling.Further analysis show that the grain boundary relaxation and dynamic recrystallization of the topmost nano-grains led to the decrease of grain boundary energy,thus improving their thermal stability.The present work provided theoretical basis for the application of CoCrNi MEA at high temperatures.Moreover,the high-throughput method on the investigation of grain stability by using gradient structure can be easily extended to other materials and it is of great significance for understanding the microstructural evolution of gradient materials.

Precipitation and heterogeneous strengthened CoCrNi-based medium entropy alloy with excellent strength-ductility combination from room to cryogenic temperatures

The improved understandings of the mechanical properties as well as deformation mechanisms at cryogenic temperatures are the prerequisite for realizing the application of any new engineering materials to cryogenic industries.Here,a(CoCrNi)_(94)Al_(3)Ti_(3) medium entropy alloy(MEA)with nanoscale L12 coherent precipitates and heterogeneous grain structures was prepared by codoping Al and Ti elements with subsequent cold rolling and heat treatment processes.The mechanical properties were evaluated at the temperature range of 293–113 K.The ultimate strength of the MEA increases almost linearly from 1326 to 1695 MPa as the temperature decreases from 293 to 113 K,while the total elongation remains approximately constant of~35%.The underlying deformation and strengthening mechanisms were investigated using various characterization techniques.Due to the effect of co-doped Al/Ti on channel width of the matrix and the increasing critical twinning stress induced by heterogeneous ultrafine grain size,the formation of deformation twins is inhibited at all temperatures.Consequently,only a slight increase of the deformation twins and stacking faults in the deformed specimens with a decreasing temperature,which leads to the relative temperature-independence of the ductility.The dislocation cutting mechanism of L1_(2) coherent precipitates and the heterodeformation induced(HDI)hardening both significantly contribute to the strain hardening so that an excellent combination of strength and ductility is obtained.Additionally,the evolution of lattice friction stress with deformation temperature is determined by quantitative analysis,indicating an approximately linear relationship between the lattice friction and temperature.The present work provides new insights into the strategy of achieving outstanding strength-ductility synergy of the MEA under the wide temperature range by coupling heterogeneous ultrafine-grained structure and coherent precipitation strategy.

Twinning induced remarkable strain hardening in a novel Fe_(50)Mn_(20)Cr_(20)Ni_(10) medium entropy alloy

The microstructures and tension properties of Fe_(50)Mn_(20)Cr_(20)Ni_(10) medium entropy alloy(MEA)were investigated,which was produced by vacuum induction melting and subsequently was homogenized at 1200 C for 6 h.Microstructure characterization shows the single-phase solid solution with face-centered cubic structure by means of transmission electron microscopy and scanning electron microscopy combined with energy disperse spectroscopy.Our Fe-MEA has an ultimate tensile strength of 550±10 MPa and a high strain hardening exponent,n,of 0.41 as well as a higher ductility(60%)than those of CrMnFeCoNi alloy.The single-phase solid solution deforms plastically via dislocations and twins.Twin boundaries associated with deformation twinning impede dislocation motion,enhancing the strain hardening capacity.This article focuses on the insights into the concept of Fe-MEAs and provides a potential direction for the future development of high entropy alloys and MEAs.

Achieving high strength and ductility in Fe_(50)Mn_(25)Ni_(10)Cr_(15) medium entropy alloy via Al alloying

The microstructure and tensile properties of(Fe_(50)Mn_(25)Ni_(10)Cr_(15))_(100-x)Al_(x)(x=0-8 at.%)medium-entropy alloys(MEAs)were investigated.It was found that the crystalline structure changes from face-centered cubic(FCC)single phase to FCC+body-centered cubic(BCC)dual-phase with the increase of Al content.Therefore,the addition of Al elements with large atomic size could induce solid solution strengthening and dual-phase heterogeneous structure strengthening.Correspondingly,the present MEAs exhibit excellent combinations of yield strength,ultimate tensile strength(UTS)and ductility both at 298 and 77 K.Among the MEAs,the(Fe_(50)Mn_(25)Ni_(10)Cr_(15))_(95)Al_(5) alloy has a remarkable combination of cryogenic UTS(1077 MPa)and ductility(~85%),and has lower raw material costs than the reported high-entropy alloys(HEAs)and MEAs.The correlation among microstructure and mechanical properties and the corresponding strengthening mechanism were clarified.

In situ neutron diffraction unravels deformation mechanisms of a strong and ductile Fe Cr Ni medium entropy alloy

We investigated the mechanical and microstructural responses of a high-strength equal-molar medium entropy FeCrNi alloy at 293 and 15 K by in situ neutron diffraction testing.At 293 K,the alloy had a very high yield strength of 651±12 MPa,with a total elongation of 48%±5%.At 15 K,the yield strength increased to 1092±22 MPa,but the total elongation dropped to 18%±1%.Via analyzing the neutron diffraction data,we determined the lattice strain evolution,single-crystal elastic constants,stacking fault probability,and estimated stacking fault energy of the alloy at both temperatures,which are the critical parameters to feed into and compare against our first-principles calculations and dislocation-based slip system modeling.The density functional theory calculations show that the alloy tends to form shortrange order at room temperatures.However,atom probe tomography and atomic-resolution transmission electron microscopy did not clearly identify the short-range order.Additionally,at 293 K,experimental measured single-crystal elastic constants did not agree with those determined by first-principles calculations with short-range order but agreed well with the values from the calculation with the disordered configuration at 2000 K.This suggests that the alloy is at a metastable state resulted from the fabrication methods.In view of the high yield strength of the alloy,we calculated the strengthening contribution to the yield strength from grain boundaries,dislocations,and lattice distortion.The lattice distortion contribution was based on the Varenne-Luque-Curtine strengthening theory for multi-component alloys,which was found to be 316 MPa at 293 K and increased to 629 MPa at 15 K,making a significant contribution to the high yield strength.Regarding plastic deformation,dislocation movement and multiplication were found to be the dominant hardening mechanism at both temperatures,whereas twinning and phase transformation were not prevalent.This is mainly due to the high stacking fault energy of the alloy as estimated to be 63 mJ m^(-2) at 293 K and 47 mJ m^(-2) at 15 K.This work highlights the significance of lattice distortion and dislocations played in this alloy,providing insights into the design of new multicomponent alloys with superb mechanical performance for cryogenic applications.

High-throughput simulation combined machine learning search for optimum elemental composition in medium entropy alloy

In medium/high entropy alloys, their mechanical properties are strongly dependent on the chemicalelemental composition. Thus, searching for optimum elemental composition remains a critical issue to maximize the mechanical performance. However, this issue solved by traditional optimization process via "trial and error" or experiences of domain experts is extremely difficult. Here we propose an approach based on high-throughput simulation combined machine learning to obtain medium entropy alloys with high strength and low cost. This method not only obtains a large amount of data quickly and accurately,but also helps us to determine the relationship between the composition and mechanical properties.The results reveal a vital importance of high-throughput simulation combined machine learning to find best mechanical properties in a wide range of elemental compositions for development of alloys with expected performance.

Revealing the Local Microstates of Fe–Mn–Al Medium Entropy Alloy:A Comprehensive First-principles Study

Entropy-stabilized multi-component alloys have been considered to be prospective structural materials attributing to their impressive mechanical and functional properties.The local chemical complexions,microstates and configurational transformations are essential to reveal the structure–property relationship,thus,to promote the development of advanced multicomponent alloys.In the present work,effects of local lattice distortion(LLD)and microstates of various configurations on the equilibrium volume(V0),total energy,Fermi energy,magnetic moment(μMag)and electron work function(Φ)and bonding structures of the Fe–Mn–Al medium entropy alloy(MEA)have been investigated comprehensively by first-principles calculations.It is found that theΦandμMag of those MEA are proportional to the V 0,which is dominated by lattice distortion.In terms of bonding charge density,both the strengthened clusters or the so-called short-range order structures and the weakly bonded spots or weak spots are characterized.While the presence of weakly bonded Al atoms implies a large LLD/mismatch,the Fe–Mn bonding pairs result in the formation of strengthened clusters,which dominate the local microstates and the configurational transitions.The variations ofμMag are associated with the enhancement of the nearest neighbor magnetic Fe and Mn atoms,attributing to the LLD caused by Al atoms,the local changes in the electronic structures.This work provides an atomic and electronic insight into the microstate-dominated solid-solution strengthening mechanism of Fe–Mn–Al MEA.

Atomic-level mechanism of spallation microvoid nucleation in medium entropy alloys under shock loading

Spallation, rupture under impulsive tensile loading, is a dynamic failure process involving the collective evolution and accumulation of enormous microdamage in solids. In contrast to traditional alloys, the spallation mechanism in medium entropy alloys, the recently emerged multiprinciple and chemically disordered alloys, is poorly understood. Here we conduct molecular dynamics simulations and first principle calculations to investigate the effects of impact velocities and the local chemical order on spallation microvoid nucleation in a CrCoNi medium entropy alloy under shock wave loading. As the impact velocity increases, the microvoid nucleation site exhibits a transition from the grain boundaries to the grains to release redundant imposed energy. During the intragranular nucleation process, microvoids nucleate in the poor-Cr region with a large local nonaffine deformation, which is attributed to the weak metallic bonds in this position with sparse free electrons. For intergranular nucleation, a Franke-like dislocation source forms through the dislocation reaction, leading to enormous dislocations piling up in a narrow twin stripe, which markedly increases the local stored energy and promotes microvoid nucleation. These results shed light on the mechanism of spallation in chemically complexed medium entropy alloys.