Structural Heterogeneity And Its Correlations With Properties In ZrCu-based Metallic Glasses

Distinct from their crystalline counterparts,metallic glasses(MGs)have longranged disordered atomic structures and thus possess unique properties,which render them great potential in applications for various industries and daily lives.However,compared with the well-established crystallographic theories for crystalline alloys,that for MGs needs to be further developed and improved.As such,characterization of the amorphous structure and establish its relation with macroscopic properties have long been the key scientific issue in the fields of materials science and condensed matter physics.Studies reveal that there are ubiquitous short-to-medium range orders and considerable structural heterogeneity in MGs,which are inherited from the supercooled liquids of MGs and sensitive to the composition,preparation and processing histories.Thus,these complexities of amorphous atomic structures make it challenging to experimentally characterize the local structural features and to establish reliable correlation between the structural heterogeneity and properties.In this thesis,we selected Zr-Cu-based MGs as the prototype to search for the parameters which can reflect the structural heterogeneity via molecular dynamics simulations.The relationship between structural heterogeneity and local dynamical parameters was explored.The critical length scale of structural heterogeneity that determines different macroscopic properties(including glass forming ability,mechanical properties and relaxation behavior)was determined,in an attempt to establish the relationship between structural heterogeneity and macroscopic properties of MGs.The main research contents and conclusions are as follows:(1)The static structural parameters and vibration parameters associated with the local structural heterogeneity,and their correlation with local structural rearrangements under external stimuli were investigated.The critical length scale of structural heterogeneity was determined by analyzing the correlation length of the vibrational parameters and local structural rearrangements.In a typical binary Cu64Zr36 MG,it was found that the atomic vibrational parameters(i.e.,flexibility volume,mean-square displacement,participation ratio to the low-frequency vibrational modes and boson peak intensity)show much better correlation with the propensity for the local structural rearrangement evaluated by the non-affine displacement and local activation energy,as compared with the static structural parameters(i.e.,local five-fold symmetry,atomic volume,atomic packing efficiency and local icosahedra density).In particular,the flexibility volume exhibits the strongest correlation with the propensity for the local structural rearrangement.By comparing the correlation length of vibrational parameters and local structural rearrangements,the critical length scale of structural heterogeneity was determined to be 7?8 ?(around the distance from the central atom to the third coordination shell),i.e.,a middle-range length scale.This finding provides important implications for characterizing the structural heterogeneity.(2)Effects of alloy composition on mechanical and static-structure heterogeneities and the relationship between mechanical heterogeneity and glassforming ability were studied by modeling the nanoindentation behavior with molecular dynamics simulations in Zr-Cu-(Al)MGs.It was found the critical size of mechanical and static structural heterogeneities in Zr-Cu-(Al)MGs were 4 nm and 2 nm,respectively.Voronoi analyses indicate that the short-range ordered structure cannot reflect the mechanical heterogeneity.Besides,we quantitatively measured the degree of mechanical heterogeneities for Zr66.7Cu33.3,Zr50Cu50,Zr56Cu36Al8 and Zr46Cu46Al8 MGs,and found that it is inversely proportional to the glass-forming ability.These results provide a new way of evaluating the structure heterogeneity and glass forming ability in MGs.(3)Effects of cooling rate and composition on structural heterogeneity and mechanical properties of Zr-Cu-(Al)MGs were studied,and the relationship between structural heterogeneity and macroscopic plasticity was established.It was revealed that faster cooling rate leads to more pronounced structural heterogeneity and larger plasticity.For different alloys,the plasticity decreases with the sequence of Zr2Cu,Zr60Cu40,Zr50Cu50,Zr66Cu26Al8,Zr56Cu36Al8 and Zr46Cu46Al8,whilst short-to-medium range orders significantly changed with the alloy compositions.Analysis with the dependence of boson peak intensity on cooling rates and compositions uncovers that the alloy's structural heterogeneity is beneficial to the enhancement of its boson peak intensity.There exists an universal scaling relationship between the boson peak intensity and Poisson's ratio,i.e.,IBP ?(M*v)3/2,where M is the average atomic mass.In other words,the higher the boson peak intensity,the larger the plasticity in MGs.This finding reveals that the macroscopic plasticity is closely related to the atomic structure of medium-range order and beyond in the MGs studied.(4)Dependence of the relaxation behavior on the structural heterogeneity of the Zr50Cu50 MGs alloyed with Al and O have been studied by modeling the dynamic mechanical analysis and local activation-relaxation process.The correlations between structural heterogeneity,local activation energy and relaxation mechanism were established.The results show that addition of 8 at.%Al enhances the structural heterogeneity of the Zr50Cu50 MG,and the average size of activated region decreases from 20 to 14 atoms while the activation energy decreases from 1.30 to 1.20 eV,which facilitates the occurrence of fast ? relaxation with low activation energy within a small length scale.Addition of 1-3 at.%O also enhances the structural heterogeneity of the Zr50Cu50 MG,and the average size of activated region decreases from 20 to 13-15 atoms.Although the local activation energy increases from 1.30 to 1.47 eV,structural analyses indicate that minor addition of O atoms enhances the structural heterogeneity and thereby increases the proportion of unstable local structures,which is favorable for the fast ? relaxation.However,excessive addition of 5 at.%O leads to substantial change in the atomic structures,appreciably enhancing the local structural connectivity.As a result,the fast ?relaxation is suppressed by the transition of atomic relaxation mode from singleatom jump to atomic cooperative motion.These results open up an avenue for tuning the structural heterogeneity and relaxation behavior of MGs.In summary,effects of cooling rate and alloy composition on the structural heterogeneity of ZrCu-based MGs were systematically studied in this thesis.The critical length scale of structural heterogeneity that determines macroscopic properties was determined.Moreover,the correlations between structural heterogeneity and properties including glass-forming ability,plasticity and relaxation behavior were established.This study is not only helpful for understanding the structural characteristics and establishing the structure-property relationships in MGs,but also provides a new avenue for designing MGs with desirable properties via tuning the structural heterogeneity.