| For most structural materials,high strength and high toughness are both essential,but unfortunately,there is often a trade-off between them,it is difficult to ensure that the material possesses high strength and hardness while maintaining excellent fracture toughness.The same is true of high-entropy alloys which have been popular in the field of structural materials in recent years,although the high-entropy alloy with face-centered cubic structure(FCC)has high toughness and low temperature toughness,excellent thermal stability and good fatigue resistance,it is difficult to overcome the contradiction between strength and toughness.The high-entropy alloy with body-centered cubic phase(BCC)has high strength but poor plasticity.To tackle this problem,a new eutectic two-phase high-entropy alloy AlCoCrFeNi,had been studied in this paper,with homogenous distribution of BCC and FCC lamellar phases inside a transmission electron microscope.The distribution of different elements in two phases was different,and the semi-coherent interface between the two phases was chemically disordered,on which a series of dislocations formed by lattice mismatch were homogeneously distributed.In-situ tensile test,we observed that these dislocations on the phase boundary provide a large number of dislocation sources for the initial deformation stage of the alloy,and most dislocation movements basically occurred in the FCC phase.However,even dislocation motion in FCC phase was affected by lattice resistance,which was more difficult than that in conventional alloys,cause the abundant RT cross-slip in FCC phase,and the interaction between primary slip arrays and cross-slip systems during deformation increased the rate of dislocation accumulation by forming complex dislocation substructures,thus making the FCC phases exceptional strong,while almost no dislocation movement and plastic deformation were found in the BCC phase at the initial stage of alloy deformation.At the later stage of deformation,tiny cracks began to appear in the FCC phase.Under the significant stress concentration and plastic deformation at the crack tip,the high density dislocations accumulated at the phase boundaries to prevent the crack tip from penetrating and cracked in zigzag mode in the FCC phase.When the stress concentration on the phase boundary was large enough,some dislocations would penetrate or emit from the phase boundary and fracture along the cleavage plane of BCC phase.However,the lattice friction of dislocation movement in BCC phase was larger than that in FCC phase,and the complex dislocation nuclei in ordered B2 structure made it difficult to initiate slip in BCC phase.When the crack continued to propagate into FCC phase,the crack propagation would be changed by the cross-slip of dislocations in FCC phase.This indicates that FCC phase was not only the main carrier of plastic deformation,but also playing a key role in improving the damage tolerance of HEAs..Base on that,the effect of pre-stra:in on the macro-strength of high-entropy alloys was also studied in this paper.The stress-strain curve shown that the cold-rolled AlCoCrFeNi alloy had more dislocation density in FCC phase,and its yield strength was about five times that of the as-cast alloy.Two kinds of high-entropy alloys with different components were prepared by changing the content of Ni in the alloys,AlCoCrFeNi2.1 and AlCoCrFeNi2.4 respectively.It was found that the AlCoCrFeNi2.4 alloy had higher FCC ratio but higher strength,which was inconsistent with the traditional principle.This was mainly due to the hindrance of dislocation motion by the chemically disordered semi-coherent interface,which accumulated a large amount of dislocations in the FCC phase,the strengthening effect even exceeds that of BCC phase,which improved the strength of the alloy. |
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