Low Cycle Fatigue Behavior And Surface Nanocrystallization Of Ni₂Fe₁Co₁Mo_0.2V_0.5 High Entropy Alloys

High-entropy alloys are composed of five or more principle elements,whose atomic percentage is between 5%and 35%.Due to their distinct atomic structure,high-entropy alloys exhibit cocktail,high-entropy,lattice distortion and sluggish diffusion effects.These effects offer high-entropy alloys superior properties such as high strength,high hardness,excellent corrosion and radiation resistance.In recent years,the development of the concept of heterogeneous high-entropy alloys has provided new insights for significantly elevating the mechanical properties of high-entropy alloys.Heterogeneous high-entropy alloys with mixtures of soft and hard zones display unparalleled combination of strength and ductility.Herein,the strain-controlled low-cycle fatigue properties and deformation mechanisms of heterogeneous high-entropy alloy Ni₂Fe₁Co₁Mo_0.2V_0.5fabricated by hot forging were systematically investigated.Electron backscatter diffraction（EBSD）,transmission electron microscope（TEM）and scanning electron microscope（SEM）were applied to explore deformation mechanisms during fatigue.Second,a nano gradient Ni₂Fe₁Co₁Mo_0.2V_0.5high-entropy alloy is attained through rotationally accelerated shot peening（RASP）treatment.Grain refinement and deformation mechanisms are characterized by TEM and EBSD analysis.The main experimental conclusions are as follows:（1）Low-cycle fatigue（LCF）properties and deformation mechanism of Ni₂Fe₁Co₁Mo_0.2V_0.5 high-entropy alloyThe Ni₂Fe₁Co₁Mo_0.2V_0.5 alloy exhibits continuous cyclic hardening without softening during the LCF test.After the fatigue test,pronounced pile-ups and plenty of slip bands were induced on the sample surface.The fatigue fracture surface is not flat,and the main crack is prone to propagate in the fine grains（hard zone）.The fatigue fractography consists of brittle and ductile fatigue striations.TEM characterization revealed the dislocation slip mode is single planar slip.In the fatigue process,the single planar slip promotes the formation of persistent slip bands,which drives the crack initiation and thereby reduces the LCF life of the sample.In the meanwhile,the single planar slip is unfavorable for the formation of low energy structures（e.g.dislocation cells）,which well substantiates the phenomenon of continuous cyclic hardening without softening.Stress concentration is more likely to occur in the fine grains,which induces the brittle fatigue striations.While some cracks propagate in large grains,the ductile fatigue striations are the typical fractography.（2）Surface nano-crystallization and deformation mechanisms of Ni₂Fe₁Co₁Mo_0.2V_0.5high-entropy alloyThe nano gradient Ni₂Fe₁Co₁Mo_0.2V_0.5 high-entropy alloy is attained through rotationally accelerated shot peening（RASP）treatment.The grain size from surface to matrix increases from nanometer to micrometer.The average grain size at the surface is estimated to be 37 nm.The deformation mechanism of Ni₂Fe₁Co₁Mo_0.2V_0.5 high-entropy alloy treated with RASP can be divided into two situations:（a）Deformation twinning is the grain refinement mechanism in the top surface（high strain rate）.（b）Planar dislocation slip is the major deformation mechanism in the region with low strain rate that close to the matrix.