Research On Design,fabrication And Mechanical Behaviors Of Hierarchically Heterostructural Eutectic High-entropy Alloys

The strength and ductility of materials are well known as the trade off relationship.The as-cast eutectic HEAs（EHEAs）exhibit unique dual-phase lamellar structure formed after solidification,have excellent strength–ductility combination,and feature good casting performance of traditional eutectic alloys,which have attracted extensive research interest.However,the mechanical properties of casting alloys are generally low.How to further improve the strength and ductility of EHEAs and broaden their application scope and field has been the research field of metallurgy and material scientists.However,most of the traditional strengthening methods improve the strength at the expense of plasticity,which still cannot meet the requirements of both high strength and high ductility in engineering applications.In recent years,more and more studies have shown that the architecture of heterogeneous structures in metals and alloys can achieve the synchronous improvement of strength and ductility,while the lamellar structure in EHEAs can also improve strength and ductility to a certain extent.If the two are combined,can high strength–ductility combination of materials be realized?Therefore,in this paper the widely-studied Al Co Cr Fe Ni_2.1 and the recently-reported Al₁₉Co₂₀Fe₂₀Ni₄₁ EHEAs are taken as the research objects.Based on their unique composition and structural characteristics,two types of hierarchically dual-phase heterogeneous lamellar structures are designed and prepared,and the excellent mechanical behaviors induced by this hierarchically heterostructural design are investigated in detail,revealing its unique strengthening mechanism:（1）Through the simple and industrialized cold-rolling and annealing treatment of the as-cast Al Co Cr Fe Ni_2.1 EHEAs,we ingeniously inherited the natural micro-lamellae in eutectics to tailor a dual-phase heterogeneous lamellar structure decorated by ultrafine grains.It was found that upon tensile loading the micro-lamellar-inherited ultrafine duplex heterostructure could induce strong two-hierarchical constraint effect and self-generated microcrack-arresting mechanism,thus achieving superior strength–ductility combination（yield strength of～1245 MPa and uniform elongation of～18%）.Therefore,our current heterogeneous ultrafine-grained structure not only achieves higher strength but also enables better ductility,in comparison to the reported homogeneous ultrafine-grained counterpart（yield strength of～1.1 GPa and tensile ductility of～12%）which elevates the strength by sacrificing ductility seriously.（2）To further improve the strength–ductility combination of ultrafine-grained EHEAs,we propose a sequentially-activated multistage work hardening strategy.This results in high-strength Al₁₉Co₂₀Fe₂₀Ni₄₁ ultrafine-grained EHEAs（1.22±0.02GPa）with a sustained and effective strain-hardening capability,thus inducing excellent uniform elongation of 24.24±1.26%and ultrahigh tensile strength of 1.52±0.02 GPa.Moreover,at such high yield strength,the strength difference between tensile and yield strengths as high as 300 MPa is extremely rare,further indicating the high effectiveness of the sequentially-activated multistage hardening strategy.It has been demonstrated that this multistage hardening strategy in ultrafine-grained EHEAs is derived from the coordinated three-level design:lamellar heterogeneity,grain-size control and intragranular composition in-situ modification.（3）By means of multi-scale structural characterizations and quantitative stress-test analysis,it is found that during tensile loading,the two types of hierarchically-hetero-lamellar structures undergo multiple heterogeneous deformations,which enable remarkable heterogeneous deformation-induced（HDI）hardening.As a consequence,the ultrafine-grained EHEAs not only have high strength but also exhibit excellent ductility.Due to the coordinated three-level design in Al₁₉Co₂₀Fe₂₀Ni₄₁ EHEAs,the sequential multi-type mechanical nano-twins are introduced into the heterostructure and coupled to the HDI hardening.Finally,a sequentially-activated multistage hardening is realized in ultrafine grains.Therefore,compared with Al Co Cr Fe Ni_2.1 heterostructural EHEAs,the multi-type twin-reinforced Al₁₉Co₂₀Fe₂₀Ni₄₁ heterostructural EHEAs have better strength–ductility synergy.