Research On Structural Evolution And Diffusion Mechanism Of CoCrFeMnNi/Al Diffusion Couple

High entropy alloy is a new type of multi-principal alloy,due to its unique microstructure and“four effects”,high entropy alloy has excellent thermal stability and corrosion resistance.In addition to this,it can be designed for high strength or high toughness materials through controlling the composition,such as CoCrFeMnNi high entropy alloy exhibits excellent fracture toughness.Aluminum has the advantages of being lightweight and economical,moreover,it is easy to form intermetallic compounds by diffusion reactions with the constituent elements of high entropy alloys at high temperatures.Metal/intermetallic compound laminated composites with better overall mechanical properties can be produced using high entropy alloys and aluminum as a matrix,and it has potential applications in the field of protective equipment and impact resistant materials because it effectively stops crack propagation with multi-interface structure.What's more,high entropy alloys can be used as coatings to enhance the mechanical properties and corrosion resistance of the base material,and as mould materials for aluminum castings to improve the service life of the mould.The main methods of preparing laminated composites are rolling lamination and hot pressing lamination,and the process parameters for the preparation of intermetallic compound laminated composites with Al and CoCrFeMnNi as a matrix are not yet perfected.Besides,scholars have focused on the overall properties of the high entropy alloy coating materials,but there is a lack of systematic research into the diffusion reactions that occur at the interface.In view of this,this thesis further investigates the evolution of the microstructure,the phase composition of the formed intermetallic compounds and their growth thermo-kinetics during the solid-liquid and solid-solid reactions of CoCrFeMnNi/Al,using scanning electron microscopy,energy dispersive spectroscopy,X-ray diffractometry,focused ion beam and transmission electron microscopy.The main conclusions drawn from this thesis are as follows:(1)In the process of solid-solid diffusion,the diffuse layer grows mainly in a“planar growth pattern”.For the diffusion couples with CoCrFeMnNi(top)/Al(bottom)structure,a density gradient and a tension gradient in the melt region of the diffusion front occur due to the effect of gravity,causing part of the diffusion front to change from a flat interface to a wavy shape.At 600°C,three continuous diffusion layers are formed at the interface.After the temperature rises to 620°C,two-phase layers rich in Cr,Mn and Fe,Co and Ni respectively appear at the interface near the diffusion front on the Al side,and as the temperature increases to 680°C,each phase in the two-phase layer grows as a bulk precipitation phase,while eutectic organization and long strips of precipitation phases rich in Fe,Co and Ni phases appear in the Al region.(2)At the initial interface of diffusion,interdiffusion of diffusion elements occurs.In solid-solid diffusion,the Al element diffuses into the CoCrFeMnNi matrix to form the Al₄(Ni,Mn,Co)₁₅ phase resulting in a high-entropy alloy depleted of Ni and the formation of an enriched phase of Co Cr Fe Ni;In solid-liquid diffusion,it was found that the high entropy matrix transformed from an FCC structure to a BCC structure at the diffusion front with increasing Al content due to the diffusion of Al elements.Because of the large subcooling in the initial phase,a fine crystal region containing a series of complex intermetallic with a gradient of Al content is formed during the diffusion of the constituent elements of CoCrFeMnNi towards Al.(3)In the process of solid-solid diffusion,the intermetallic layers at the interface are formed in the order of Al(Co,Cr,Fe,Mn,Ni),Al₈(Co,Cr,Fe,Mn,Ni)₅,Al₁₃(Co,Cr,Fe,Mn,Ni)₄ phase,Al₉(Co,Cr,Fe,Mn,Ni)₂ and two-phase layer consisting of Al₁₁(Cr,Mn)₄and Al₁₃(Fe,Co,Ni)₄from the high entropy alloy matrix to the Al direction;In the process of solid-liquid diffusion,Al(Co,Cr,Fe,Mn,Ni),Al₁₃(Co,Cr,Fe,Mn,Ni)₄,Al₄(Co,Cr,Fe,Mn,Ni)and two-phase layer consisting of Al₉(Co,Fe,Ni)₂ and Al₇(Cr,Mn)are formed sequentially at the interface.The Cr-rich phase of the two-phase layer in the solid-liquid diffusion changes from Al7(Cr,Mn)to Al4(Cr,Mn)as the temperature increases,and the precipitated phase formed in the internal region of Al is the same composition-Al9(Co,Fe,Ni)2 as the Co-rich phase of the two-phase layer.(4)During the diffusion heat treatment process from 600°C to 680°C,the diffusion layer thickness increased with either the diffusion time or the diffusion temperature,while when the temperature was increased to 700°C,after a holding time of more than 30 min,the diffusion two-phase layer thickness decreased because the Al liquid dissolved part of the two-phase layer,and the dissolution of the two-phase layer Co-rich phase promoted the growth of the Co-rich precipitated phase in the Al region.By investigating the growth kinetics of the intermetallic layers,during solid-solid diffusion,the growth mechanism of Al₁₃(Co,Cr,Fe,Mn,Ni)₄ is bulk diffusion,and Al₉(Co,Cr,Fe,Mn,Ni)₂ is controlled by grain boundary diffusion,and the two-phase layer consisting of Al₁₁(Cr,Mn)₄ and Al₁₃(Fe,Co,Ni)₄ is controlled by interfacial reaction and bulk diffusion mechanism;In the solid-liquid diffusion,Al₁₃(Co,Cr,Fe,Mn,Ni)₄ is controlled by grain boundary diffusion mechanism at 660°C,and as the temperature rises it changes to a growth mechanism controlled by both grain boundary diffusion and bulk diffusion,the Al4(Co,Cr,Fe,Mn,Ni)layer is a mixed growth kinetic mechanism of grain boundary diffusion and bulk diffusion,and the two-phase layer consisting of Al9(Co,Fe,Ni)2 and Al7(Cr,Mn)is mainly controlled by interfacial reactions.(5)Calculated by the Arrhenius relation,the diffusion activation energy of Al₁₃(Co,Cr,Fe,Mn,Ni)₄ phase and Al₉(Co,Cr,Fe,Mn,Ni)₂ phase formed by solid-solid diffusion is 137.98 KJ/mol and 66.01 KJ/mol respectively,and the reaction activation energy of the two phases is 251.76 KJ/mol;The activation energy of Al13(Co,Cr,Fe,Mn,Ni)4 phase and Al4(Co,Cr,Fe,Mn,Ni)2 phase formed by solid-liquid diffusion is 177.36 KJ/mol and177.36 KJ/mol respectively.The diffusion activation energy of Al₁₃(Co,Cr,Fe,Mn,Ni)₄and Al₄(Co,Cr,Fe,Mn,Ni)phases is 177.36 KJ/mol and 200.50 KJ/mol,respectively.