Transmission Electron Microscopy Studies On Dynamic Mechanics Of Metallic Glasses

As a new member of amorphous-materials family,metallic glass(MG)has a relatively simple structure,which is very suitable for the study of the essential physical problems of amorphous materials,such as amorphous short-range order or medium-range order and the specific glass transition process.MG has its own excellent physical properties and chemical properties,such as high elasticity,high hardness,wear resistance,corrosion,superplasticity,and sometimes better catalytic or biocompatibility performance than that of crystal,which make it a huge application potential in many fields.However,the study of MGs is still in the initial stage,and many concepts and physical mechanisms remains unclear.Microstructure is used to make the analysis of physical nature and varies properties more easily and intuitive.Transmission electron microscopy(TEM)has its own unique advantages to analyze the microstructure of the material.In this thesis,some kinetic studies on the oxidation resistance,surface diffusion and crystal nucleation of MGs were studied using TEM.1.Aluminiferous MGs has better antioxidative performance than that without aluminum.It is found that a protective layer of alumina was formed on the surface of MG when the electron beam irradiated on it.The protective layer can protect the surface from adhering and implanting oxygen molecules and declines the Mott potential of the system.It also proved that the atomic diffusion of the amorphous surface is easier and faster than that of crystalline countparts.2.It is found that the MG would crystallize at the surface layer when it is annealed below glass transition temperature(Tg)for a long time.The structure is similar to superlattice and the segregation of the original homogeneous component distribution appears.Comparing samples annealed at different temperatures,it is proved that the higher temperature will make the crystal uneven,resulting in polycrystalline or heterogeneous crystals.3.Pd80Si20 MGs nanoparticles which were uniformly distributed on the flat amorphous SiNx supporting film were prepared using a pulsed laser deposition(PLD)method.The critical nucleus size of the MGs was calculated after the MGs were imaged using an aberration corrected TEM.The result is in good agreement with the in-situ observing experiment when the sample was irradiated by the electron beam.It is also agree with the result of molecular dynamics simulation.Two methods of estimating sample’s thickness in FIB-TEM sample fabrication were also proposed to obtain a TEM image of good quality.