Design And Study Of The Plastic Deformation Mechanism Of High Strength And Toughness Heterostructure High Entropy Alloys

Metal structure materials with high strength and high ductility play an irreplaceable role in transportation,defense equipment,aerospace and other important fields.However,the strength and ductility of metal materials are inverted,high strength materials tend to have lower ductility,and high ductility materials tend to have lower strength,which is known as the strength-ductility trade-off.Therefore,it is an international scientific challenge to develop advanced metal materials with high strength and high ductility.High-entropy alloy is a new alloy system that has developed rapidly in recent years.Compared with traditional metal materials,it has excellent comprehensive properties,such as high strength,high plasticity,high hardness,high corrosion resistance,high thermal stability and other functional properties,representing the future development direction of alloy materials.However,similar to the development bottleneck of traditional metal materials,the mechanical properties of high-entropy alloys still have a strength-ductility inversion relationship,which greatly restricts the development and application of high strength and high toughness high-entropy alloys.For example,（1）high-entropy alloys with body-centered cubic structures（mainly refractory high-entropy alloys）have high strength and low plasticity,and（2）face-centered cubic high-entropy alloys have high ductility and low strength.Heterogeneous materials are different from homogeneous materials in strengthening and toughening effect by introducing heterogeneous structures from nanometer to micrometer scales,achieving high strength and toughness mechanical properties,breaking the long-standing strength-ductility trade-off relationship of metal materials.Inspired by the design concept of heterogeneous materials,in order to break through the bottleneck of material strength-ductility trade-off and develop novel high-entropy alloys with high strength and high ductility,this thesis carried out the following two aspects of work.First,the aim is to design a new type of refractory high-entropy alloy with high ductility by using the design concept of heterogeneous materials,chemical composition regulation,and systematically study its plasticity deformation mechanism and strengthening and toughening mechanism to provide a solution for the high strength and high ductility refractory high-entropy alloy system.Second,the high strength gradient heterogeneous face-centered cubic high-entropy alloy was designed by rolling,annealing and shot peening.The microstructure and deformation mechanism were systematically studied to reveal the strengthening and toughening mechanism of heterogeneous materials,which provided a research idea and technical route for the development of high strength and high toughness face-centered cubic high-entropy alloys.The main innovations,research contents and conclusions of this thesis are summarized as follows:1.A novel Hf Nb Ta Ti V refractory high-entropy alloy was designed by selecting ductile elements as the alloy component to maximize the lattice distortion of the alloy and form atom-scale heterogeneity.The alloy exhibits excellent mechanical properties;for example,the yield strength at room temperature is up to 1350 MPa,the compressive plasticity is more than 45%,and the yield strength is increased by 50%when compared with the Hf Nb Ta Ti Zr alloy.At the same time,the alloy exhibits excellent mechanical properties at high temperature and high temperature softening resistance;for example,the alloy retains 53%room temperature strength and 77%compressive plasticity at700°C.These results are better than those of refractory high-entropy alloys and other high-performance metal materials.The lattice distortion of the alloy was revealed qualitatively and quantitatively by TEM characterization at the atomic scale level,revealing that the high strength of the alloy is mainly contributed by lattice distortion.The solution strengthening amount caused by lattice distortion was calculated to be up to 1094 MPa.2.A novel Hf Nb Ti V refractory high-entropy alloy was designed based on a heterogeneous refractory high-entropy alloy design concept via spinodal decomposition.The alloy achieves high yield strength（1100 MPa）and high tensile ductility（28%）,which are among the best reported values for refractory high-entropy alloys.The excellent mechanical properties of the Hf Nb Ti V alloy are attributed to the specialβ（BCC₁）andβ*（BCC₂）dual-phase structures.It was found that contiguous inhomogeneities were created by decomposing the BCC structure intoβ（BCC₁）+β*（BCC₂）via spinodal decomposition,producing modulation of the chemical composition.The heterostructure produces a gradient lattice strain effect,which effectively blocks dislocation motion,causing a traffic jam and cross-slip and facilitating dislocation interactions,multiplication,and accumulation.More importantly,dislocation cross-slip forms wavy dislocations that entangle with dislocation planar slip bands,further improving the strain hardening.The heterogeneous structure enhances the uniform plastic deformation of the material,which effectively solves the scientific problem that it is difficult to obtain tensile ductility of the refractory high-entropy alloy.3.A method to improve the mechanical properties of Co Cr Fe Mn Ni face-centered cubic high-entropy alloys was developed by using a gradient grain and nanotwin heterogeneous design strategy.First,this article is based on the non-equiatomic design concept to design Co_21.5Cr_21.5Fe_21.5Mn_21.5Ni₁₄ high-entropy alloys,guaranteeing the formation of a single-phase FCC structure while simultaneously reducing the stacking fault energy and promoting the formation of faults and deformation twins.Second,a heterogeneous structure with gradient grains and nanotwins was created in the non-equiatomic alloy by using a thermomechanical processing strategy of cold rolling,heat treatment and rotationally accelerated shot peening.The tensile yield strength of Co_21.5Cr_21.5Fe_21.5Mn_21.5Ni₁₄ heterogeneous high-entropy alloys is 750 MPa,which is375%of that of the as-cast alloy.The toughness of the alloy was 2.53×10¹⁰k J/m³,which is 200%of that of the as-cast alloy.Compared with the strengthening and toughening method of a single structure,the advantages of gradient isomerization are as follows:Activation of dislocation,stacking faults,deformation twins and micro-deformation bands and other plastic deformation modes.The interaction of dislocation,stacking faults,twins and microbands contributes to achieving a high strain hardening of the alloy that helps to assure steady plastic deformation and avoid stress concentration and necking and thereby high ductility.Meanwhile,the contribution of grain boundary strengthening,dislocation strengthening,twin strengthening,and hetero-deformation strengthening associated with the heterogeneous microstructure of the gradient heterogeneous alloy is revealed by quantitative calculation.In summary,the aim of this thesis is to use the design concept of heterogeneous materials to explore and resolve the common strength-ductility trade-off of high-entropy alloy materials by designing and regulating the microstructure of high-entropy alloy materials.The design concept,preparation method,microstructure control,mechanical properties,deformation mechanism and strengthening and toughening mechanism of heterogeneous materials were systematically studied.The relationship between the microstructure,mechanical properties and deformation mechanism of the high-entropy alloy was established.The aim is to provide a theoretical basis and experimental guidance for achieving high strength and high ductility mechanical properties of high entropy alloy materials through the design concept of heterogeneous materials.