Modification Of Microstructure In Zr-based Metallic Glasses

When a metallic liquid is cooled sufficiently fast,its crystallization can be avoided,i.e.,a supercooled liquid transforms into a solid via the glass transition.Such a solid is termed as amorphous alloy,which is also known as metallic glass(MG).It shows long-range disordering and short-range ordering in the microstructure without any crystal-like defects,e.g.,grain boundaries and dislocations.The degree of structural ordering proceeded during cooling is often described in terms of free volume,and occasionally characterized by molecular dynamics simulations.The mechanical performance of MGs is strongly affected by the variations of the local structure,i.e.,the amount and the distribution of free volume.In this case,it is vital to develop methods that are capable of modifying the local structure in order to improve the macroscopic deformation behavior of MGs.In this work,we tried various processing methods to tailor the microstructure of MGs and applied macroscopic measurements for alloys after different treatments.The characterization of internal structures and the evolution of atomic structures were achieved by means of high-resolution transmission electron microscope and high energy X-ray synchrotron radiation in order to establish the relationship between the microstructure and macroscopic properties.The results of this thesis aim to provide more theoretical evidences for deeply understanding the nature of MGs.The research contents are shown as follows:The liquid nitrogen is induced as a refrigerant for the cryogenic treatment.Zr₅₅Cu₃₀Ni₅Al₁₀MGs are subjected for immersing with different durations in order to achieve an overall modification of materials from outside to inside.The influence on the microstructure of MGs under different cryogenic conditions is systematically studied with the aid of X-ray diffraction technology and high-resolution transmission electron microscopy(HRTEM).The alteration of mechanical properties after distinct immersion times is analyzed via room temperature compression tests.The structural evolution at low temperatures is studied by means of high-energy X-ray synchrotron radiation technology so as to establish the relationship between the microstructure and macroscopic properties of MGs.Zr₆₁Cu₂₅Al₁₂Ti₂ MG is selected for the flash-annealing treatment under various conditions in order to obtain samples with different energy states.The influence from pure heating on the internal structure of MGs is analyzed by thermodynamic measurements.The mechanical differences of MGs flash-annealed at distinct temperatures are also displayed and discussed.The alteration of microstructures on nano-scale is characterized by a spherical aberration corrected coefficient(Cs)-corrected HRTEM combining the autocorrelation function(ACF)analysis.The variation of microstructures on atomic scale including the short-range ordering(SRO)and the medium-range ordering(MRO)induced by flash-annealing is studied by high-energy X-ray synchrotron radiation.Since MGs are also known as liquid metals,their structural characteristics are similar to those of the liquid.The structural information of two amorphous systems,i.e.,Zr₆₁Cu₂₅Al₁₂Ti₂and Zr₄₆Cu₄₆Al₈,during overheating and undercooling is obtained by conducting in-situ electromagnetic levitation(EML)experiments.The relationship between overheating and undercooling is discussed.In addition,the inherent structure and property of metallic melts are initially understood.By means of in-situ diffraction results,the icosahedral-like cluster,which is the basic unit in MGs,is found to be existed in its metallic melt as well.The content of this chapter plays a significant role for understanding the liquid structure,also the origin of the structure of MGs.With the help of the new equipment and the new technology:EML,Zr₆₁Cu₂₅Al₁₂Ti₂ MGs with different degrees of the undercooling are obtained.By means of structural and thermodynamic characterizations,the intrinsic relationship between the microstructure and degrees of the undercooling is revealed.The influence of the deep undercooling on the mechanical property of MGs is characterized from a macroscopic perspective.The solidification process of MGs is captured by a high-speed camera.The evolution of atomic structures with different undercooling degrees is analyzed by high-energy X-ray synchrotron radiation.