Microstructure Mechanical Property And Deformation Mechanism Of Zr-Cu-Al Based Bulk Metallic Glass And Its Composites

Due to their unique long-range disordered structure,metallic glasses exhibit a series of excellent physical and chemical properties.Among them,mechanical properties such as high strength and high hardness are particularly prominent,which makes them have broad application prospects in the engineering application of macro and micro-sized structural devices.However,in the process of exploring the application of bulk metallic glasses,there are still many key questions.Among them,the room temperature brittleness and strain softening of bulk metallic glasses are limiting its engineering application.Therefore,The problem of the plastic source and plasticization mechanism of bulk metallic glasses needs to be solved urgently.In this study,this thesis takes the Zr-Cu-Al metallic glasses as the model object,and mainly studies the influence of the preparation process and composition optimization on the structure of the metallic glasses.To establish the relationship between structure and mechanical properties especially the plasticity of the metallic glasses and its composites in different structural states.In-situ observation shows the deformation process of the two-phase structural bulk metallic glasses composite,which shows great plasticity,The interaction between the secondary phase and the shear band are studied.Interpretation of the initiation and expansion of the shear band are also focused.Analysis of the serrated flow behavior under quasi-static tension and compression,and explore the self-organized critical behavior of bulk metallic glasses composites during deformation are carried out.This thesis aims to explore the plastic origin and plasticization mechanism of bulk metallic glasses composites,reveal the nucleation and expansion rules of amorphous alloy shear bands,and provide theoretical support and experimental basis for the engineering application of bulk metallic glasses.The Zr-Cu-Al amorphous alloy was successfully prepared by using tungsten arc melting and melt extracted method.Through process optimization and composition optimization,the purpose of Zr-Cu-Al bulk metallic glasses composites structure control was achieved;and the optimized Zr₄₈Cu₄₈-_xAl₄Co_xand Zr₄₉Cu₄₅Al₆ were tested and characterized.The results show that the Zr₄₈Cu₄₈Al₄ metallic microwires have no tensile ductility,but have good bending ductility,and multiple shear bands appear on the sample surface.The addition of Co promotes the formation of crystal phases and increases the alloy’s cooling rate It is sensitive and easily leads to the“agglomeration”of the crystal phase.Co element controls the mechanical properties by controlling the structure of the crystal phase.The cooling rate has a severe effect on the structure.With the decrease of the cooling rate,the internal crystallization degree of the alloy and the crystal phase size are improving.Through the controlling of the crystal phase,the mechanical properties of the metallic glass can be effectively adjusted.With the increase of the crystal phase content,the material exhibits brittle,low-strength.With the increase of the crystal phase content,the metallic phase exhibits high elasticity and strength.Only when the second phase is uniformly dispersed in the matrix,the volume fraction is about 50%,and the crystal phase size is about50～100μm,the material will show excellent macroscopic plasticity.Among them,the Zr₄₉Cu₄₅Al₆ alloy has a uniform and dispersed complex phase structural features.The size of the second phase is 10～70μm.The glass transition temperature is 669 K,and the crystallization temperature is 771 K.The second phase is a B2-Cu Zr phase with a simple cubic Pm-3m structure.In the tensile mechanical performance test,it shows a stable engineering strain range of6%～14%.By in-situ observation methods,tensile test was carried out on the Zr₄₉Cu₄₅Al₆ bulk metallic glasses composite exhibiting excellent plasticity at room temperature.The results show that the deformation process of bulk metallic glasses composite can be divided into three parts:elastic stage,hardening stage and softening stage.In the elastic stage,the two phases undergo coordinated elastic deformation,and the elastic limit and the first yield strength are mainly affected by the volume fraction of the second phase.The hardening stage can be divided into elastic-plastic deformation zone and plastic-plastic deformation zone.In this stage,the second phase yields first,and irreversible plastic deformation occurs.The metallic matrix yields after elastic deformation under complex stress conditions and then begins to plastically deform.During the plastic deformation of the metallic matrix and the second phase,the metallic matrix yields maintaining the softening trend,and the second phase of Zr-Cu-Al bulk metallic glasses composite undergoes martensitic transformation under strain,which in turn provides the possibility of work hardening effect.When the work hardening effect of B2 phase is greater than that of the matrix,althought the softening of the matrix initiating,the stress-strain relationship maintains the work hardening trend.But when the work hardening of the B2 phase is less than the softening of the metallic matrix,the stress-strain relationship enters the softening zone.The shear band of the bulk metallic glasses composite originates at the interface of the two phases,and the initial shear band is generated perpendicular to the direction of the external stress.As the deformation in the same position increases,multiple shear bands are formed locally.The martensitic transformation of the secondary phase provides the work hardening effect.When multiple shear bands expand and the interface is"de-bonding",the work hardening effect no longer dominates the deformation process,and the softening effect of the metallic matrix will promote the softening and fracture of this alloy.A comparative analysis of the serrated flow phenomenon in the plastic section of the tensile deformation process and the compression deformation process is carried out.Serrations mainly depends on the two-phase interaction and the accumulation and release of elastic energy,which is specifically manifested in the initiation and interaction of shear bands.Statistical analysis of the serrated flow behavior of bulk metallic glass composites reveals a complex,scale-free deformation process.This process is related to the shear slip phenomenon in the deformation process of bulk metallic glasses.Through statistical analysis of the accumulation of elastic energy,it is found that the process obeys the power law relationship.Through time series analysis,it is found that this is a dynamic self-organization process with convergent trajectory.