Study On Mechanical Coupling Strengthening And Toughening Mechanism Of Amorphous/Crystalline Nanocomposite Structure

The combination of metallic glass with crystal is an important approach to enhance the strengthening and toughening property of metallic glass.However,it is unclear about the microscopic plastic deformation process and the strengthening and toughening mechanism of amorphous/crystalline nanocomposites.By using massively parallel molecular dynamics simulation method and theoretical analysis,this work focuses on the plastic deformation mechanism of Cu-Zr amorphous/crystalline system under different structures such as nanolamintes and metallic glass matrix nanocomposites,and then the mechanical coupling strengthening and toughening mechanism is studied.Molecular dynamics simulations are performed on Cu₅₀Zr₅₀0 amorphous/Cu crystalline nanolaminate to analyze the mechanical properties and plastic deformation behaviors.Significant size effect in strength is demonstrated.The nanolaminate structures include various modulation ratios and various modulation periods,and both single cystal and poly-crystal.The study shows laminate structure has significant suppression effect on the nucleation of shear transformation zones.The bifurcating,bending and deflecting of immature shear band are observed when its propagation is blocked by the amorphous/crystalline interface.Crystalline layer hinder the propagation of mature shear band effectively.Dislocation interacted with amorphous/crystalline interface causes the formation of new shear transformation zones within the amorphous phase near the interface.The immature shear bands/shear transformation zones and dislocations contribute to the network pattern which provides the global plastic deformation.The size effect relationship between strength and layer thickness is well-explained by the confined layer slip model when layer thickness is from 27.1 nm to 4 nm.When layer thickness decreases to less than⁴ nm,the nanolaminate exhibits global shear deformation because of the destruction of the amorphous/crystalline interface.The strengths of nanolaminates with various modulation ratios are explained by the simple mixture rule,and the plasticity of nanolaminate is higher than both of the pure metallic glass and pure crystal.When introducing grain boundary in or grain size is less than the crystal layer thickness,the existence of grain boundary cause more instability area to emit dislocations in crystal,and the nanocomposites tend to overall shear deformation.Amorphous/crystalline interface play the role of an intermediary,connecting the different layers and transmitting the strain within the nanolaminate structure.Interface acted as the source of the shear transformation zones and as both the source and sink of the shear band and the dislocations.The mechanical properties and microscopic plastic deformation evolution of Cu₅₀Zr₅₀amorphous/B2-CuZr crystalline nanolaminate is investigated by using molecular dynamics simulations.The study shows that the martensitic phase transformation from B2 structure to transitional R phase provides the plasitic to crystalline layer.Shear transformation zones can be induced in where the phase transformation zones interact with the amorphous/crystalline interface.At the same time,under the mechanisms of that the shear transformation zones are suppressed by the laminates structure and immature shear band/shear band is obstructed by amorphous/crystalline interface,the adjustable shear transformation zones distribute within the metallic glass layer evenly.The release of stress by immature shear band/shear band within metallic glass can induce the reverse phase transformation in crystalline layer.Martensite phase transformation of crystal and its reverse phase transformation can coordinate with the deformation of metallic glass.The global plasticity is provided by the phase transformation zones and shear transformation zones cooperatively.The mechanical properties and plastic deformation behaviors of Cu₅₀Zr₅₀ metallic glass matrix composites reinforced by Cu crystalline phases are studied by molecular dynamics simulation.The results point out that the reinforced crystalline phase suppresses the development of immature shear band/shear band.The amorphous/crystalline interface can act as the role to induce shear transformation zones and immature shear band.The interaction between dislocations and interface can also induce the new shear transformation zones in metallic glass near the interface,which is similar to the laminate case.Increasing the crystalline composition and designing the proper distribution pattern of crystalline phase can improve the mechanical strengthening and toughening property.The architecture of structure is very important and proper designing achieves the shear transformation zones/immature shear band stagnation in the local area or making it more evenly distributed within the nanocomposite structures.The zones between crystalline phase restrict the shear transformation zones/immature shear bands,which facilitate the silting-up effect.This paper aims to investigate the interaction rules between shear transformation zones,immature shear band,shear band,dislocation,phase transformation zones.The results are significant for the research about plastic deformation behaviors and strengthening and toughening mechanism.The work provides the theoretical basis and advice for a wider range of experiment research and application.