| Conventional alloy design has been largely based on one or two principal elements alloyed with small additions of other elements,to obtain desired properties.However,this strategy limits alloy design to the corners of phase diagrams,leaving unexplored the vast central regions of multi-component phase space.The high-entropy alloy(HEA)strategy proposes to design alloy with more than four principle elements and extend the component space of the alloy to the central region of the multicomponent phase diagram,which provides a broad space for the development of the alloy.Single phase HEAs are reported to be either high strength or good tensile ductility,but it is still a huge challenge to have both high strength and good ductility in HEAs.Recently,the eutectic high entropy alloy(EHEA)AlCoCrFeNi2.1 has been reported could achieve a good balance of strength and ductility meanwhile possessed the excellent casting properties of the eutectic alloys,resuting in broad attention from the material researchers.In this paper,the microstructure,deformation behavior and performance optimization of AlCoCrFeNi2.1 EHEA were studied.The main content of this paper is as follows:1.The microstructure of the EHEA AlCoCrFeNi2.1 in the as-cast condition has been investigated systematically from macro scale to submicron scale.The eutectic structure in the alloy was composed of FCC and B2 phases instead of L12 and B2 phases.Atomic analysis of the FCC and B2 phases,we found there exist nanoscale L12 and BCC precipitates in the FCC and B2 phases,respectively.The orientation relationship(OR)between FCC and B2 phases in the EHEA has been determined to be K-S OR with<110>FCC?<111>B2 and {111}FCC?{110}B2.This OR has two variants due to the growth twin in the FCC phase.The EHEA displays three kinds of interfaces,namely(112)FCC?(321)B2,(332)FCC?(011)B2,and(552)FCC?(231)B2.The formation mechanism of the preferred interfaces is revealed using atomistic geometrical analysis according to the criteria of low interfacial energy based on the coincidence-site lattice(CSL)theory.2.The plastic deformation mechanism of the dual-phase alloy and the influence of the heterophase interface on the deformation behavior and mechanical properties of the alloy were explored.The plastic deformation of the FCC phase in the EHEA is governed by the planar slip of the 1/2<110>dislocation on the{111} planes,while the primary deformation mode of the B2 phase is associated with the pure screw dislocations,<111>dislocations slip on the {110} lattice planes.Based on the analysis of the geometrical possibility of slip-transfer and atomic interface characteristics of the interface in the alloy,it is found slip-transfer is facilitated across the K-S interface,thereby the co-deformation of the two phases of the eutectic alloy realizing,which further improves the tensile plasticity of the alloy.The K-S interface in the EHEA can strengthen the alloy by preventing dislocation movement,and its strengthening effect also comes from the interface shear modulus strengthening and lattice parameter mismatching strengthening.3.The superposition of heterogeneous structure strengthening and precipitation strengthening was realized and a novel precipitate strengthened heterogeneous structure is tailored by simple industrialized thermo-mechanical treatment to optimize the mechanical properties of the alloy.The precipitate enhanced heterogeneous structure is characterized with nanoscale L12 and BCC precipitates uniformly distributed inside the alternating recrystallized FCC/B2 lamellae,respectively.The precipitation strengthened dual-phase heterogenous structure can achieve an exceptional combination of high strength(yield strength?1008.8 MPa,ultimate strength?1475.6 MPa)and high ductility(elongation-to-fracture?19.2%).The achievement can be rationalized as the heterogenous structure strengthens and toughens the material simultaneously,and the precipitates generate additional strong strengthening effect on the material without sacrifice ductility. |
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