Compositionally complex coherent precipitation-strengthened high-entropy alloys:a critical review

The coherent precipitation-strengthened high-entropy alloys(CPS-HEAs)as a new type of structural materials are expected to possess many unique mechanical properties,such as the outstanding strength-ductility com-bination at cryogenic and room temperatures.Apart from this,most of their strengths can even be well retained at elevated temperatures.The compositionally complex matrix and nanoprecipitation phases,as well as the coherent interfaces between them,can potentially bring novel merits of these CPS-HEAs,including sluggish dif-fusion,excellent thermal stability,and controlled magnetic properties.Note that the ductile coherent L1_(2)-nanoparticles can improve the strength of alloys without too much reduction of plasticity,while the coherent B2-nanoparticles strengthened HEAs display completely brittle failure upon tensile test at room temperature.An overview of the alloy design,microstructure evolution,oxidation resistance,mechanical and magnetic properties of CPS-HEAs are briefly discussed here.The advantages of multicomponent coherent precipitation-strengthened HEAs as well as the limitations in this field are also summarized.In addition,this review also points out the future research directions and prospects.

Evolution of helium bubbles in FeCoNiCr-based high-entropy alloys containing γ′ nanoprecipitates

A series of high-entropy alloys(HEAs) containing nanoprecipitates of varying sizes is successfully prepared by a non-consuming vacuum arc melting method.In order to study the irradiation evolution of helium bubbles in the FeCoNiCrbased HE As with γ' precipitates,these samples are irradiated by 100-keV helium ions with a fluence of 5 × 10^(20) ions/m^(2) at 293 K and 673 K,respectively.And the samples irradiated at room temperature are annealed at different temperatures to examine the diffusion behavior of helium bubbles.Transmission electron microscope(TEM) is employed to characterize the structural morphology of precipitated nanoparticles and the evolution of helium bubbles.Experimental results reveal that nanosized,spherical,dispersed,coherent,and ordered L1_(2)-type Ni_(3)Ti γ' precipitations are introduced into FeCoNiCr(Ni_(3)Ti)_(0.1) HEAs by means of ageing treatments at temperatures between 1073 K and 1123 K.Under the ageing treatment conditions adopted in this work,γ' nanoparticles are precipitated in FeCoNiCr(Ni_(3)Ti)_(0.1) HE As,with average diameters of 15.80 nm,37.09 nm,and 62.50 nm,respectively.The average sizes of helium bubbles observed in samples after 673-K irradiation are 1.46 nm,1.65 nm,and 1.58 nm,respectively.The improvement in the irradiation resistance of FeCoNiCr(Ni_(3)Ti)_(0.1) HEAs is evidenced by the diminution in bubbles size.Furthermore,the FeCoNiCr(Ni_(3)Ti)_(0.1) HEAs containing γ' precipitates of 15.8 nm exhibits the minimum size and density of helium bubbles,which can be ascribed to the considerable helium trapping effects of heterogeneous coherent phase boundaries.Subsequently,annealing experiments conducted after 293-K irradiation indicate that HEAs containing precipitated phases exhibits smaller apparent activation energy(E_(a)) for helium bubbles,resulting in larger helium bubble size.This study provides guidance for improving the irradiation resistance of L1_(2)-strengthened high-entropy alloy.

Computer simulation of strain-induced morphological transformation of coherent precipitates

The coherent elastic strain-induced morphological transformation of a binarycubic model alloy was simulated with different strain energy parameters. The microscopic diffusionequation was combined with the theory of microscopic elasticity. The results show that when thestrain energy is neglected, the randomly distributed equiaxed particles are obtained with isotropiccharacteristic. It is coarsening that follows the Ostwald ripening mechanism: smaller particlesdwindle and larger particles grow; when the elastic strain is considered, plate precipitates tend toalign along the elastically soft directions <01> with anisotropic characteristic. The particlesgrow in the soft directions and coarsen further; particles dwindle in out of the soft directions.While the coarsening of the particles localized in the same row or column follows the rule: smallerparticles shrink and larger ones grow.

Designing ultrastrong maraging stainless steels with improved uniform plastic strain via controlled precipitation of coherent nanoparticles

The development of ultrastrong maraging stainless steels(MSSs)is always in high demand.However,traditional high-strength MSSs generally exhibit early plastic instability with a low uniform strain since the precipitated nanoparticles are non-coherent with the body-centered-cubic(BCC)lath martensitic matrix.Here,we design a novel ultrahigh strength MSS(Fe-5.30 Cr-13.47 Ni-3.10 Al-1.22 Mo-0.50 W-0.23 Nb-0.03 C-0.005 B,wt.%)using a cluster formula approach.A fabulous microstructure consisting of a uniform distribution of high-density coherent B2-Ni Al nanoprecipitates(3-5 nm)in BCC martensitic matrix was successfully obtained.This alloy has not only an exceedingly high ultimate tensile strength of 2.0 GPa,but also a decent uniform elongation of 4.2%-5.1%,which is almost triple of the value observed in existing MSSs.We present an in-depth discussion on the origins of ultrahigh strength and uniform plastic strain in the new alloy to validate our design strategy and further offer a new pathway to exploit highperformance MSSs.

Formation of coherent BCC/B2 microstructure and mechanical properties of Al-Ti-Zr-Nb-Ta-Cr/Mo light-weight refractory high-entropy alloys

The present work investigated the formation of coherent body-centered-cubic(BCC)/B2 microstructure in Al-Ti-Zr-Nb-Ta-Cr/Mo refractory high-entropy alloy system,in which three alloys with the composition of Al_(2)Ti_(6) Zr_(2)Nb_(3)Ta_(2)(Cr/Mo)_(1)were designed.These as-cast alloy ingots were homogenized at 1573 K for 2 h and then aged at different temperatures for 24 h.It is found that the BCC/B2 coherent microstructure with cuboidal BCC nanoprecipitates into the B2 matrix would be formed in873 K-aged Al_(2)Ti_(6) Zr_(2)Nb_(3)Ta_(2)Cr_(0.5)Mo_(0.5)alloy with half Cr and half Mo,which is ascribed to a moderate lattice misfit(ε=0.94%)between BCC and B2 phases.It is due to the special coherent microstructure that results in a prominent mechanical property with the highest compressive yield strength(σ_(YS)=1314 MPa)and good plasticity among these alloys,while coarse Cr2 Ti and Zr_(5)Al_(3)particles are existed in 873 K-aged alloy with Cr1.After 1073 K aging,all these alloys with Cr_(1),Cr_(0.5)Mo_(0.5)and Mo_(1)consist of BCC,B2 and Zr_(5)Al_(3),in which BCC/B2 coherent microstructure is destabilized.The strengthening mechanism of BCC/B2 coherent microstructure in 873 K-aged alloy with Cr_(0.5)Mo_(0.5)was also discussed.

Thermal stability and deformation mechanisms in Ni-Co-Fe-Cr-Al-Ti-Nb-type nanoparticle-strengthened high-entropy alloys

The precipitate morphologies,coarsening kinetics,elemental partitioning behaviors,grain structures,and tensile properties were explored in detail for L1_(2)-strengthened Ni_(39.9)Co_(20)Fe_(15)Cr_(15)Al_(6)Ti_(4-x)Nb_(x)B_(0.1)(x=0 at.%,2 at.%,and 4 at.%)high-entropy alloys(HEAs).By substituting Ti with Nb,the spheroidal-to-cuboidal precipitate morphological transition,increase in the coarsening kinetics,and phase decomposition upon aging at 800°C occurred.The excessive addition of Nb brings about the grain boundary precipitation of an Nb-rich phase along with the phase decomposition from the L1_(2)to lamellar-structured D019 phase upon the long-term aging duration.By partially substituting Ti with Nb,the chemically complex and thermally stable L12 phase with a composition of(Ni_(58.8)Co_(9.8)Fe_(2.7))(Al_(12.7)Ti_(5.8)Nb_(7.5)Cr_(2.3))ensures the stable phase structure and clean grain boundaries,which guarantees the superb high-temperature mechanical properties(791±7 MPa for yielding and 1013±11 MPa for failure)at 700℃.Stacking faults(SFs)were observed to prevail during the plastic deformation,offering a high work-hardening capability at 700°C.An anomalous rise in the yield strength at 800℃was found,which could be ascribed to the multi-layered super-partial dislocations with a cross-slip configuration within the L1_(2)particles.

Enthalpy induced phase partition toward hierarchical,nanostructured high-entropy alloys

Heterogeneous nanostructured metals are emerging strategies for achieving both high strength and ductility,which are particularly attractive for high entropy alloys(HEAs)to combine the synergistic enhancements from multielement composition,grain boundaries,and heterogeneity effects.However,the construction of heterogeneous nanostructured HEAs remains elusive and can involve delicate processes that are not practically scalable.Herein we report using composition design(i.e.,enthalpy engineering)to create hierarchical,nanostructured HEAs as demonstrated by adding Ni into FeCrCoAlTi_(0.5)HEA.The strong enthalpic interaction between(Ni,Co)and(Al,Ti)pairs in FeCrCoAlTi_(0.5)Nix(x=0.5–1.5)induced phase partitions into B2(ordered phase,hard)matrix and A2(disordered phase,soft)precipitates,resulting in a hierarchical structure of B2 grains and sub-grains of near-coherent A2 nanodomains(~12.5 nm)divided by A2 interdendritic regions.As a result,the FeCrCoAlTi_(0.5)Ni_(0.5)HEA with this unique hierarchical nanostructure exhibits the best combination of strength and plasticity,i.e.,a 2-fold increase in compressive strength(2.60 GPa)and significant enhancement of plastic strain(15.8%)as compared with the original FeCrCoAlTi_(0.5)HEA.Enthalpy analysis and simulation study reveal the phase partition process during cooling induced by an enthalpy-driven order-disorder transition while the order parameters illustrate the strong ordering in(Ni,Co)(Al,Ti)-rich B2 phase and high entropy mixing in less interactive FeCrCo-rich A2 phase.Our work therefore provides a strategy for hierarchical nanostructured HEA formation by composition design considering enthalpy and entropy interplay.