Experimental And Constitutive Investigations On The Mechanical Behavior Of The Gradient Nanostructured High Entropy Alloy

With the rapid development of the high-end equipment manufacturing industry,the importance of metallic materials with high strength and ductility is increasingly prominent.How to effectively strengthen and toughen metals has become an urgent issue to be solved.As a new type of metallic material with multiple principal elements,high entropy alloys（HEAs）can have many mechanical properties that are superior to conventional alloys by widely changing the element types and atomic ratios.Through the proper design of components,multiple strengthening and plastic mechanisms can be simultaneously activated,which is an efficient approach to achieve strong and ductile HEAs.In addition to element adjustment,heterogeneous microstructure adjustment is also an effective way to balance strength and ductility.As one of the heterostructures,gradient nano-grained structure orderly combines the high-strength nanograins and the high-toughness coarse grain,which effectively solves the mismatch of strength and ductility caused by grain refinement.Meanwhile,it can improve the surface hardness and fatigue life of the material,resulting in higher engineering serviceability.Gradient nanostructures have been successfully applied to a variety of conventional alloys.Introducing the gradient nanostructure into HEAs is expected to improve the synergy of strength and ductility further.Such an approach realizes the combination of element adjustment and microstructure adjustment.The mechanical properties of gradient nanostructured（GNS）HEAs better satisfy the requirements of actual service requirements.However,now the experimental investigation of GNS HEA is still lack.Moreover,establishing a constitutive model that can reasonably describe the materials’mechanical behavior is crucial to service safety assessment and property optimization.At present,most of the existing constitutive models for GNS materials can only consider the effect of one or two initial gradient microstructures.Such kind of models is not suitable for the GNS HEA with multiple plastic mechanisms.Furthermore,the theoretical models considering the influence of gradient twins and martensites cannot reflect their evolutions during the deformation process,which to some extent limits the in-depth analysis of the strength,strain hardening and ductility in GNS materials.For solving the issues described above,in this paper,gradient nanostructures were introduced in the non-equiatomic metastable Fe_49.5Mn₃₀Co₁₀Cr₁₀C_0.5（at.%）HEA（iHEA for short）by surface mechanical attrition treatment（SMAT）,resulting in GNS iHEA.Furthermore,macroscopic mechanical tests,microscopic characterizations,and crystal plasticity constitutive modeling were conducted for the GNS iHEA.The main work contains the following three aspects:（1）Uniaxial tensile and nano-indentation tests were carried out for the GNS iHEAs prepared by different-time SMAT.The stress-strain responses and hardness distributions of the different specimens were obtained.Thus,the evolution law of strength and ductility with increasing the SMAT time was summarized.By analyzing the surface strain field and fracture morphology,the fracture mechanisms of the iHEAs before and after SMAT were compared.The intrinsic reason for the loss of ductility after long-time SMAT was revealed by various microscopic characterizations.In addition,the gradient distributions of grain size,dislocation density and martensite volume fraction were measured.Finally,the grain refinement mechanism and microstructural evolutions of the iHEA during the SMAT process were analyzed.（2）Based on the experimental results,a crystal plasticity constitutive model considering multiple plastic deformation mechanisms of iHEA was developed,including dislocation slip,deformation twinning and martensitic transformation in austenite,as well as dislocation slip and cross slip in martensite.The equations describing the nucleation and growth of twin and martensite were established based on the stacking fault mechanism.The slip resistances of dislocations contained the contributions of solid solution strengthening,precipitation strengthening,back stress,etc.Moreover,the grain size effect was considered in the different mechanisms.Thus,a size-dependent multi-mechanism-based crystal plasticity constitutive model was established,which was linked with the finite element simulation software Abaqus by DAMASK.The simulated tensile results of the iHEAs with different grain sizes fitted the experimental results well,verifying the validity of the developed model.Meanwhile,by analyzing the evolutions of microstructures,the intrinsic reasons for the differences in strength and ductility of iHEAs with different grain sizes were further revealed.Finally,the effect of the initial martensite volume fraction on the uniaxial tensile deformation behavior of iHEA was discussed.（3）The transition from the simulated deformation response of a single crystal to the macroscopic scale was achieved by the isostrain homogenization scheme,which effectively reduced the size of the GNS finite element model.According to the microscopic characterizations,different initial gradient microstructures were introduced by different parameters in simulations.Based on the newly developed crystal plasticity constitutive model,the uniaxial tensile deformations of iHEAs with different SMAT times were simulated.The accuracies and rationalities of the constitutive model and simulation method were verified.By analyzing the gradient distributions and evolutions of the field variables and internal variables in simulations,the intrinsic reason for the surface strain localization in the iHEA after long-time SMAT was further revealed.Finally,the contributions of different gradient microstructures to the strength and ductility of the GNS iHEA were quantified.