Study On Ductile-brittle Transition Behavior Of Zr-based Metallic Glasses Under Multi-axial Stress State

Amorphous alloy,also known as metallic glass,is considered to be a high-quality structural material with great potential due to its unique properties.However,the intrinsic brittleness of amorphous alloys greatly limits its wide application as structural materials.Therefore,systematic research on the deformation and fracture behavior of amorphous alloys and exploration of the micro-deformation mechanism of amorphous alloys provide new ideas for the design of ductile amorphous alloys.In this dissertation,the free volume change of amorphous alloys during shear deformation is taken as the starting point and the method of combining experiment and numerical simulation is adopted to study the influence of free volume evolution on deformation and fracture behavior of amorphous alloys under multiaxial stress state.The main research content includes:（1）Under uniaxial compression,the plastic strain increases from 1.4%to 4.3%with the increase of glass forming ability（GFA）of Zr Ti Be Cu metallic glasses（MGs）,revealing the dependence of plastic（ε_p）on GFA（）,namely:ε_p=873-4109+4845².It is proposed that the total free volume of amorphous alloy consists of two parts:intrinsic free volume（V₀）and excess free volume（V_G）.However,since MGs with high GFA have more frozen excess free volume,they have a larger total free volume content.In addition,the serrated flow behavior reveals that MGs with high GFA has high elastic energy,and the release of elastic energy causes the temperature in the shear band to rise beyond the melting point,which leads to local softening in the shear band.Due to the combined action of free volume and adiabatic shear,the viscosity in the shear band of MGs is significantly reduced,and the shear band expands and interacts strongly,significantly improving the plastic deformation capacity of MGs.（2）Under multi-axial loading,Zr-based MG sheets with thickness of 50～700μm exhibits the dependence of deformation behaviors.During the deformation process,the stress state is different due to the thickness difference of MG sheets.As the thickness of MG sheet decreases,the deformation factor（α=?）increases from 0.4 to 1,and the deformation mode changes from brittleness to toughness.The deformation factor is negatively correlated with the thickness of MG sheets（T）:α=1-7.8T.The smaller thickness of amorphous alloy exhibits greater plasticity,and the relationship between the thickness of MG sheets and deflection（D）is as follows:D=909-0.84T.When the thickness of MG sheets is 200～500μm,the deformation mode changes significantly,that is,the two critical deformation factorsα_c1andα_c2 induce the formation of circumferential and radial shear bands,respectively.Specifically,whenα≤α_c2, only a few microcracks appear on the amorphous surface.Whenα_c1≥α≥α_c2,multiple radial shear bands are activated and distributed uniformly on surface of MG sheets.However,whenα≥α_c1,multiple circumferential shear bands and a large number of radial and circumferential shear bands are generated on the surface of MG sheets under high shear stress.The formation and interaction of multiple radial and circumferential shear bands significantly improve the toughness of MGs.（3）In the annealing state,MGs（below T_g,between T_g and T_x,above T_x）exhibit typical ductile-brittle transition.In the multi-axial loading mode,the deflection（D）of MGs is negatively correlated with the annealing temperature（T_A）:D=616-0.74T_A.At the same time,it can be observed on the surface of MGs:（ⅰ）circumferential shear band+radial shear band（below T_g）;（ⅱ）Radial shear band（between T_g and T_x）;（3）Radial crack（above T_x）.Then,the plastic strain（ε_p）of MGs is also negatively correlated with the annealing temperature（T_A）under uniaxial compression mode:ε_p=4.9-0.008T_A,which is consistent with the deformation behavior under multi-axial loading mode.The relationship between the free volume（Δv_f）and the annealing temperature isΔv_f=9-0.02T_A.The higher the annealing temperature is,the smaller the free volume content is,which leads to the typical ductile-brittle transition phenomenon of MGs.（4）Under multi-axial loading,Zr_xTi_65-xBe_27.5Cu_7.5 MGs exhibits the dependence of ductile-brittle transition on GFA in the composition range of x=20～50 at%.When the Zr element composition is 30～45 at%,the plasticity of the amorphous alloy is greater,which is attributed to the formation of a larger content of excess free volume induced by high GFA,revealing the dependence of free volume（Δv_f）on GFA（）:Δ_f=562-2681+3223².Since the atomic sizes of Zr and Ti are similar,the intrinsic free volumes of MGs studied are basically the same.However,MGs with high GFA have a large amount of excess free volume,which is conducive to the formation and expansion of multiple shear bands and significantly improve the plastic deformation ability of MGs.