Investigation On Crystallization And Bonding Zr-base Metallic Glasses Via Molecular Dynamic Simulation

Bulk metallic glass(BMG)is a new type of metal material developed in the1960s.Compared with the traditional metal crystal material,it has a unique atomic stacking structure,which makes it have the advantages of physics,biology,chemistry,etc.,such as:high strength,high elastic limit and better soft magnetic,electrochemical performance.However,at room temperature,the disadvantage that amorphous alloys are difficult to process and form has caused the application of amorphous alloys in the engineering field to be greatly restricted.Therefore,looking for a novel processing technology route plays a vital role in the future development of amorphous alloys.The amorphous alloy welding technology developed in recent years will become an effective way to solve the difficult processing and forming of amorphous alloys.However,due to the cooling rate and other factors during the welding process,the amorphous alloy crystallizes at the joints.As a result,the amorphous alloy at the joint loses its inherent excellent properties.Therefore,aiming at the problems of crystallization and element diffusion at the interface between bulk Zr-based amorphous alloy and crystalline copper during welding,molecular dynamics simulation methods are used to study the atomic accumulation behavior and element diffusion mechanism at the joint.First,using the characteristics of rapid heating and rapid cooling of pulsed laser welding technology,the butt welding and brazing process test of Zr-based amorphous(Zr₄₄Ti₁₁Ni₁₀Cu₁₀Be₂₅)and crystalline copper is performed.By optimizing the parameters,a good welded joint between Zr-based amorphous alloy and crystalline copper is obtained.The results show that there are distinct degrees of crystalline phases in the heat-affected zone and weld zone of the welded joint.The crystalline phase of the heat-affected zone is mainly petal-shaped,while the crystalline phase of the weld zone is mainly small"rice grains"shape.Through XRD diffraction,it is found that the crystalline phases in the heat-affected zone and weld zone are mainly Zr₂Cu and Zr₂Ni.And in the EDS line scan and the electron probe surface scan,it was found that an obvious diffusion layer appeared at the joint.Since two crystalline phases of Zr₂Ni and Zr₂Cu are formed at the joint,the microhardness of the joint is greater than that of the amorphous base material.Then,since Zr atoms are mainly used in Zr-based amorphous alloys,molecular dynamics are first used to analyze the crystallization phenomenon and mechanism of the molten Zr atomic system during the solidification process.Various analysis methods such as potential per atom vs temperature curve(E-T curve),radial distribution function(g(r)),H-A bond index method,common neighbor analysis,etc.are mainly used to analyze the process of atoms from disorderly arrangement to orderly arrangement.Furthermore,the interatomic bonding and the structural transformation of the Zr atom during the solidification process were studied.Studies have shown that when the liquid metal Zr atom system solidifies rapidly at cooling rates of 1×10¹²K/s and 5×10¹¹K/s,the atomic potential energy of the system undergoes two sudden changes,indicating that the metastable structure is produced during the solidification process.Through the analysis of the H-A bond index,it is found that in the metastable state,the structural features of the atom are mainly composed of two bond types,1661 and 1441,while the structure features of the atom after crystallization when cooled to room temperature are composed of two bond types,1421 and 1422.Other bond types have no obvious infl uence on the solidification process of Zr atoms.Judging by common nearest neighbor analysis,it is found that the metastable structure is mainly composed of BCC structure,and when it reaches room temperature,it is composed of HCP structure.And through the study of different initial temperatures,it is found that the higher the initial temperature,the more conducive to the formation of a perfect HCP structure at room temperature.The atomic structure transformation of Cu₄₆Zr₅₄ amorphous alloy was analyzed on the basis of Zr atom solidification crystallization,research shows that when the cooling of1×10¹¹K/s solidification to room temperature,have ordered and disordered arrangement of the two systems Cu₄₆Zr₅₄s a kind of atomic structure.Finally,combined with the molecular dynamics simulation method,the diffusion mechanism of Zr-Cu amorphous alloy and crystalline copper was studied,and the movement behavior of atoms during the diffusion of amorphous alloy and crystal was revealed from the atomic level.The three-dimensional atomic morphology,the atomic concentration in the diffusion direction,and the diffusion coefficient are mainly used to analyze the atomic diffusion behavior.First,the effect of time on diffusion is studied,and the results show that the diffusion coefficient of the two materials in the initial stage gradually increases due to the influence of surface energy.When the diffusion coefficient reaches its maximum value,the interface between atoms gradually becomes blurred,the free surface gradually disappears,and the diffusion coefficient gradually decreases.And from the movement of atoms,it can be seen that in the diffusion process of amorphous and copper crystals,copper atoms diffuse into amorphous alloys.The diffusion coefficient of copper atoms in copper crystals is the largest,followed by copper atoms in amorphous alloys.The diffusion coefficient of zirconium atoms is the smallest,which is exactly in line with the conclusions obtained during the experiment.Furthermore,the effect of diffusion temperature on the diffusion of amorphous alloys and copper crystals has been studied.The study shows that the thickness of the diffusion layer gradually increases with the increa se of the diffusion temperature.