Process And Mechanism For Diffusion Bonding Of AlCoCrFeNi_2.1 High Entropy Alloy

Due to the subversive concept of material design,high entropy alloy breaks the dilemma that traditional metals are difficult to possess both strength and plasticity,and it has broad application prospects in aerospace equipment,polar scientific research,nuclear power and other fields.However,the unclear welding process and joining mechanism of high entropy alloy have been a bottleneck restraining the engineering application of high entropy alloy.In addition,due to the high cost of high entropy alloys,in the practical application of nuclear power and other fields,it is necessary to incorporation with traditional structural materials such as stainless steel,and then give full play to the advantages of dissimilar metals and further promote the engineering application process of high entropy alloys.Due to the high quality and small deformation of the joint,diffusion bonding was widely used in aerospace,nuclear industry and other high-tech fields.Therefore,it owns important theoretical meaning and engineering value to conduct systematic research on the weldability of high entropy alloys and further study the joining mechanism and welding process of diffusion bonding for high entropy alloy/stainless steel,which can clarify the relationship between interfacial microstructure and mechanical properties,further to promote the reliable application of high entropy alloy in homogeneous and composite components in the field of high-end equipment.This thesis takes AlCoCrFeNi_2.1 eutectic high entropy alloy and 304 stainless steel as the research object.The high entropy alloy joint and high entropy alloy/stainless steel dissimilar joint are prepared by diffusion bonding,and electron probe micro-analyzer,scanning electron microscope,universal material testing machine,X-ray diffractometer and other equipments are used to study the microstructure,mechanical properties and bonding mechanism of high entropy alloy and high entropy alloy/stainless steel diffusion bonded joints,in order to systematically study the evolution of microstructure and mechanical properties of the joints of high entropy alloy and high entropy alloy/stainless steel with process parameters,deeply discuss the mechanism of high entropy effect and sluggish diffusion effect of high entropy alloy on diffusion bonding process,and further clarify the joining mechanism for the joints of high entropy alloy and high entropy alloy/stainless steel and obtain the welding process specification of joints with high strength and toughness.The main findings are listed as follows:The weldability of eutectic high entropy alloy is studied,and the sound joint of AlCoCrFeNi_2.1 eutectic high entropy alloy is obtained by diffusion bonding.With the bonding temperature rising,the shear strength and the deformation rate of joints gradually increase,and the fracture mode transforms from brittle fracture to ductile fracture.When the bonding temperature rises to 1050°C,the maximum shear strength is 648 MPa.Moreover,the cleavage plane and the dimples simultaneously appear on the fracture surface due to the uncoordinated deformation between FCC phase and B2 phase at 1000°C,which shows that the fracture mode of the joint is compound type fracture.The new bonding mechanism of void disappearance and closing of eutectic high entropy alloy diffusion bonded joint is clarified.The interfacial diffusion mechanism dominates the disappearance of voids below 1000°C,and the slow thermal motion of atoms at low temperature and the sluggish diffusion effect of high entropy alloy lead to a large number of voids.While the bonding temperature increases to 1050°C,the mechanism of void disappearance changes from interfacial diffusion to viscoplastic deformation.Compared with the traditional metals,the effect of viscoplastic deformation on the voids disappearance is much greater than that of interfacial diffusion due to the sluggish diffusion effect of high entropy alloy.The shrinkage rate of voids increases under the effect of viscoplastic deformation,and then the number of voids decreases sharply and the interface disappears,meanwhile,a continuous solid solution structure forms at the original interface,which can improve the shear strength of the joint.Moreover,the voids still exist at the interface near the B2 phase at 1050°C due to the synergistic effect of inferior plasticity and low diffusion rate of elements in the B2phase.When the bonding temperature is lower than 1000°C,the width of diffusion zone increases with the increase of temperature.While increasing the temperature to 1050°C leads to no significant change in the width of diffusion zone compared to that at 1000°C,because the diffusion of elements is inhibited by the sluggish diffusion effect.With the increase of bonding temperature,the grain size of the joint gradually decreases,and the recrystallization ratio near the interface is doubled.On the basis of the above,the"interlocking"characteristic diffusion bonding between AlCoCrFeNi_2.1 eutectic high entropy alloy and 304 stainless steel are realized.The solid solution zone and the Fe₂Al₅ intermetallic compound zone are formed on both sides of the diffusion zone,respectively.Based on the different diffusion rate of the elements between two base metals and two phases of eutectic high entropy alloy,a solid solution region composed of FCC phase and?-Fe phase is formed on the high entropy alloy side near the interface.While the Fe₂Al₅ intermetallic compound grows in different degrees on the stainless steel side near the interface,and then the interlocking structure with"interlocking"characteristics is formed.Finally,under the coupling effect of solution metallurgy and interlocking structure in the diffusion zone,the compressive shear strength of the joint reaches the shear strength of stainless steel base metal.Moreover,with the increase of bonding temperature and holding time,the shear strength of the joint first increases and then decreases,and the deformation rate of the joint gradually increases.In this study,the maximum shear strength of 355MPa and the deformation rate of 1.6%are obtained under the optimal bonding process of 980°C/60 min/30 MPa,and the fracture mainly occurs around the Fe₂Al₅ intermetallic region on the stainless steel side.