Study On The Relationship Between Structural Asymmetry And Liquid Fragility Of Cu-Zr Alloy Melt

Generally,it is believed that metallic glasses are frozen liquids,whose structure and properties closely associate with their liquid state.Therefore,the study of the melt structure above glass transition temperature Tg can benefit us in understanding the structural characteristics of metallic glasses.In this thesis,the relationship between the asymmetry of metal melt structure and fragility is studied by using molecular dynamics,based on the embedded atom method(EAM)and Finnis-Sinclair many-body potential(F-S potential).Taking the Cu-Zr system as the research object,it provides a new idea for revealing the relationship between structural heterogeneity and dynamic slowing-down.The main contents and results of this thesis are as follows:The dynamics and short program changes of several Cu-Zr systems are studied by molecular dynamics simulation.With the decrease of temperature,the single dynamics relaxation separates into a relaxation and ? relaxation.At this time,there is a platform for both the mean squared displacement and the self-intermediate scattering functions,and the length of the platform increases with the decrease of temperature.The structural relaxation time ??,glass transition temperature Tg and fragility coefficient m were obtained by calculating the autocorrelation scattering function of the system.Due to the thermal expansion effect,the average bond length of atoms in the first shell would increases with the increasing of temperature.However,it has been found that the first peak of the pair correlation functions for metallic melts skews towards left(shorter distance side)with increasing temperature.This abnormal "negative expansion"phenomenon can be attributed to that the formation of atomic clusters with low coordination number is more easily at high temperature and the increase of bond length asymmetry due to the anharmonic nature of the interatomic interaction potential.In this thesis,the skew normal distribution(SND)function is used to fit the part above the full width at half maximum of the first peak of the pair correlation functions,so as to extract the skewness parameter y.Similarly,the width ratio between the two sides at full width at half maximum of the first PDFs peak is used to reflect the magnitude of the asymmetry of the peak.The asymmetry of the first PDFs peak of Cu-Zr system with different components from high temperature melt to TA was systematically studied.Comparing the slope of skewness y with the fragility coefficient m,it is found that fragility liquids seem have larger changing rate of structural asymmetry at relative high temperature,and the same results are obtained in the simulation system using EAM potential and F-S potential.The variation of skewness y of the first peak of three part pair distribution function changing with temperature is analyzed.It is found that Cu-Cu bond and Cu-Zr bond have greater influence on skewness,while Zr-Zr bond has little effect.The atoms in the undercooled liquid can be classified into three groups in terms of the probability distribution of atoms'displacement,named‘fast atoms'(FAs),‘intermediate atoms'(IAs),‘slow atoms'(SAs).The variation of the skewness y of the first PDFs peak of three groups atoms changing with temperature was analyzed.It is found that the SAs were the main factor causing the variation of skewness,which might be due to the "cage effect" aggravated by the change of the bond length asymmetry in the cluster.The change of shape of the first PDFs peak actually reflects the asymmetric change of the cluster structure,and there is a close relationship between them.The bond Length heterogeneity of clusters in amorphous alloys can be described by Bond Length Deviation(BLD).The low bond length deviation indicates that the cluster has higher symmetry or the bond length distribution in the cluster is less deviating from the average configuration of the system.For this reason,we calculated the bond length deviation and Voronoi polyhedron content variation of several Cu-Zr systems during cooling.It is found that the bond length deviations corresponding to the system with a larger skewness change rate k(?)decrease faster with temperature.The complete icosahedron(0 0 12 0)shows the smallest bond length deviations,while the other polyhedral clusters have larger bond length deviations.In the cooling process,Voronoi polyhedra with high five-fold symmetry,such as(0 0 12 0),rise rapidly,while Voronoi polyhedra with low five-fold symmetry,such as(0 4 4 3),(0 3 6 3),and so on,almost keep the same proportion in the system or slightly decrease This causes the symmetry of the system to rise rapidly and y to decrease rapidly.The variation of skewness of Cu-Zr system with temperature under different pressures has been studied.It is shown that the pressure slows down the dynamics process,and the fragility coefficient m and k(?)of the system under high pressure are higher.Previous studies have shown that fragile liquids may have a steeper repulsive potential.By calculating the steepness ? of the repulsive part of the pair correlation functions,we investigate the relation between the k(y)and ?.The result shows that,the steeper repulsive potential which changes faster with temperature may have a larger k(y).The corresponding relationship between structural asymmetry and fragility provides a new idea that the fragility of liquid can be predicted by detecting the asymmetry of PDFs of high temperature melt.