Atomic Structure Of Ductile Metallic Glass

Metallic glasses have excellent properties such as high strength,high hardness,excellent corrosion resistance,biological fusion and other excellent properties.However,poor ductility together with brittle fracture,characteristic greatly limit the application of metallic glasses in engineering.In recent years,researchers have developed three methods,such as the preparation of crystalline-amorphous composite materials by introducing or endogenous second phase,preparing porous metal glass by increasing free volume,and designing dual amorphous phase metallic glass similar to homogeneous polycrystalline structure.A number of materials with certain ductility have been developed and mechanisms for making metal glass malleable have been explored.However,current research rarely reveals the mechanism of ductility from the perspective of atomic structure and cannot meet the needs of research and production.Therefore,in this work,the three types representative and high ductility metallic glasses Zr₅₀Pd₄₀Al₁₀,Cu₄₉Hf₄₂Al₉ and Ti₄₉Nb₄₂Al₉ have been selected in this paper as the objects to study their atomic structures and rebuild the relationship between atomic structures and macroscopic properties,and the corresponding ductility mechanism is discussed.At present,the research on the microstructure of metallic glasses is mainly carried out by X-ray,neutron diffraction combined with Radial Distribution Function?RDF?,however,due to the smaller scattering angle and limited volume of the samples,it is difficult to perform accurate RDF analysis.As for electron diffraction,with large scattering cross section and high resolution,allowed us to focus on study of smaller samples,which facilitates more accurate RDF analysis and reveals the atomic structure of metallic glasses.Therefore,this paper adopts electron diffraction,combined with RDF analysis,and reverse Monte Carlo?RMC?,density functional theory?DFT?simulation optimization to study the atomic structure of metallic glass,to explore the microstructure of metallic glass,and the relationship between macroscopic properties and the ductility mechanism is revealed accordingly.The main research contents and conclusions are as follows:The first chapter demonstrates the research background and significance,reviews the structures and properties of metallic glasses,the research methods of microstructure,the development and application of electron source RDF analysis,and the application of simulation calculation in the research of metallic glass materials.Research content and innovation of this paper have also been pointed out in the end.The following chapter II mainly introduces electron diffraction,radial distribution function RDF analysis,model establishment and RMC,DFT optimization and other research methods and basic theories.In the third chapter,the microdomain atomic structure the Zr₅₀Pd₄₀Al₁₀0 was studied by the electron diffraction.It was found that Zr₅₀Pd₄₀Al₁₀0 metallic glass was a mixed domain of crystal and amorphous structure.The crystal domain of Pd₁₁Zr₉ had a uniform size and a diameter of around 5±2 nm,which was discretely distributed in the amorphous structure of Pd₂₅Zr₅₅Al₂₀,and the volume ratio of the amorphous/crystalline phase fraction was about 1:1.The simulated results showed that the first adjacent distances of Pd-Zr and Zr-Zr atoms to PRDF were 3.00?and 2.92?,respectively,which contributed at most to the peak of the first adjacent distance of material RDF.The competitive growth mechanism of nanocrystals and amorphous phase was highly related to the difference of atomic concentration,radius of Zr,Pd and Al,the strong interaction between Zr-Pd atoms and the differences of their melting points.The Zr-Pd nanocrystals were uniformly distributed in the matrix,and both the nanocrystals and the amorphous phase formed a three-dimensional?3D?interpenetrating interconnected network with smooth interface,which was highly beneficial to the formation of shear bands and made the amorphous composites have superior plasticity.The micro-atomic structure of Cu₄₉Hf₄₂Al₉ metallic glass was studied in the following chapter IV.The RDF analysis of the electron diffraction showed that the metallic glass was completely amorphous after the plastic deformation test.The matrix of the compressed sample was identical in structure to the uncompressed alloy,indicating that the plastic deformation of the MG was confined to the shear band.The free volume was regarded as the second phase in the matrix with shear modulus G=0,and there was a large difference in shear modulus between the matrixes with each other,which can cause stress concentration and promote the proliferation of the shear band,leading to the ductility improvement ultimately.The simulation results showed that the first proximity distances of Cu-Cu,Hf-Cu,and Hf-Hf atoms to Partial Radical Distribution Functions?PRDFs?were 2.56?,2.78?and 3.23?,respectively.Copper atoms were mainly filled between hafnium?Hf?atoms,while Hf-Cu and Hf-Hf were the main contributions of the atomic proximity.The Hf-Hf atom pair had a wide distribution,and when externally applied stress,a stable connection can be formed over a large distance range,leading to the higher ductility fo the metallic glasses.The fifth chapter studied the micro-atomic structure of Ti₄₉Nb₄₂Al₉ metallic glass.Experiments showed that Ti₄₉Nb₄₂Al₉ metallic glass was a completely amorphous structure.According to the micro-region RDF results obtained by electron diffraction,two distinct micro-domain amorphous structures were found:type A,type B,two bi-amorphous phases,of which type A was more easily found in experiments.The PRDFs showed an increase in the first and second proximity distances of Ti-Nb,Nb-Nb,Nb-Al will lead to the shortening of the first and second proximity distances of Ti-Ti.Two ring structures of Ti-Ti-Nb-Ti and Ti-Ti-Nb-Al can be found in the simulated results.Type A has more Ti-Ti-Nb-Ti ring structure,while Ti-Ti-Nb-Al ring structure dominated in Type B.The double amorphous phase causes the atomic proximity distance and chemical composition of the local region of the material to be different.The two amorphous phase interfaces have a high critical shear stress,which prevents crack propagation to a certain extent and is beneficial to the shear band.Proliferation improves extensibility.To conclude,electron diffraction is an effective and significant approach for studying the structure and properties of metallic glasses.In this thesis,the atomic distributions of metallic glasses Zr₅₀Pd₄₀Al₁₀,Cu₄₉Hf₄₂Al₉,Ti₄₉Nb₄₂Al₉ have been illustrated using the RDF analysis.The RMF and DFT simulations were utilized to optimize the atomic model to obtain PRDFs,and the atomic structures of the micro-domains such as the distance between atoms,atomic connections and interactions were also discussed in detail.Finally,from the perspective of atomic structure,three mechanisms of crystal-amorphous composite,free volume and double amorphous phase were proposed to improve the ductility of metallic glass.