Microstructure Control And Strengthening-toughening Mechanisms Of High-strength Co-free Heterostructured FeCrNiAl-based High-entropy Alloys

High-entropy alloys（HEAs）have shown broad application prospects in extreme environments due to their unique microstructure and excellent comprehensive performance.However,issues such as high raw material costs and complex preparation processes limit their engineering applications.Therefore,this study aimed to achieve"low-cost,short process,high-strength and high-toughness"by selecting FeCrNiAl-based HEAs without expensive elements such as Co,Ti,and V as the research object,and using the melting casting process to prepare bulk alloys.Based on thermodynamic calculations and mixed enthalpy differences,the elements were divided into two distinct groups,Ni-Al group with BCC structure and Fe-Cr-Ni group with FCC structure.Multiple kinds of HEAs with novel heterogeneous structure were prepared by optimizing and regulating the alloy composition and solidification microstructure.The main achievements of this study include:With the increasing of（NiAl）ratio,the content of disordered FCC（A1）phase in as-cast FeCrNi（NiAl）_x（0.2≤x≤2）HEAs gradually decreases,while the content of disordered BCC（A2）phase and ordered BCC（B2）phase gradually increases.Meanwhile,the tensile yield strength and ultimate tensile strength significantly increase,whereas the fracture elongation gradually decreases.When x=1.1,1.4,and 1.7,the FeCrNi（NiAl）_x HEAs exhibits good comprehensive mechanical performance,and their tensile properties are comparable to the as-cast Co-contained FeCrNiCo-based HEAs,while the raw material costs are lowered by more than 50%.A heterostructured FeCrNi（NiAl）_1.25 with"core-shell"structure was directly prepared by arc-melting method.The outer shell regions are Fe-Ni enriched A1-structured soft phase,the inner shell regions are Ni-Al enriched B2-structured hard phase,while the core-like regions are composed of A2-structured matrix and B2-structured precipitates.The unique"core-shell"microstructure enables the as-cast FeCrNi（NiAl）_1.25 HEA to exhibit better tensile properties（ultimate tensile strength～1340.2 MPa,elongation～18.0%）than most as-cast alloys.The stacking of geometrically necessary dislocations（GNDs）accumulating at the soft/hard interface not only significantly improves the yield strength of the alloy,but also maintains a high strain hardening rate and then delays the occurrence of necking.Heterogeneous deformation-induced strengthening（HDI）is the main strengthening-toughening mechanism of FeCrNi（NiAl）_1.25HEA.The as-cast Fe_2.0CrNi（NiAl）_x（x=1.2,1.6,1.9）HEA with A1+A2+B2 multiphase structure was prepared by arc-melting method.The main region of Fe_2.0CrNi（NiAl）_1.6 alloy is composed of nanoscale B2-structured shells-like hard region and completely wrapped A1-structured fibers-like soft region.The special fibrous heterostructure enables the alloy to exhibit excellent tensile properties（yield strength of～670.2 MPa,ultimate tensile strength of～1195.1 MPa,tensile elongation of～21.1%）at room temperature.Moreover,fibrous heterostructured Fe_2.0CrNi（NiAl）_1.6 alloy also exhibits excellent strength-ductility matching（yield strength～851.2 MPa,ultimate tensile strength～1412.6 MPa,elongation～18.3%）at an extremely low temperature of 77 K.Experimental results confirm that the combination of a high strength and good ductility can be attributed to the hetero-deformation induced hardening mechanism generated from the soft-fiber and hard-shell interface.A large amount of Mn（～26.3 at.%）was added to the Fe_2.0CrNi（NiAl）_1.6 HEA and Fe:Ni ratio was sustainably increased to prepare as-cast Fe_2+xNi_1.8-xCrAl_0.8Mn_2.0（x=0,0.4,0.7）HEAs.The Fe_2.7Ni_1.1CrAl_0.8Mn_2.0 alloy is composed of a disordered A2-structured Fe-Cr enriched matrix and dual-scale B2-structured Ni-Al enriched precipitates.Its room-temperature yield strength and ultimate tensile strength are 973.1 MPa and 1274.4 MPa,respectively.As-cast Fe_2.4Ni_1.4CrAl_0.8Mn_2.0 HEA exhibits a precipitate-strengthened lamellar heterogeneous structure.The precipitate strengthening induced by high-density ultrafine grain precipitation particles and the hetero-deformation induced hardening originated from lamellar structure makes it exhibit a tensile strength of～1143.7 MPa,yield strength of～712.1 MPa,and elongation of～17.4%.These heterostructured as-cast FeCrNiAl-based HEAs,featuring"low cost,short process,and high strength and toughness",provide experimental basis and reference value for the design of high-performance metallic materials.