Investigation On Atomic Level Structural Evolution And Mechanical Behavior Of ZrCuNiAl Metallic Glasses

In the present dissertation, the structural evolution of Zr₅₃Cu_18.7Ni₁₂Al_16.3（Zr53）,Zr_51.9Cu_23.3Ni_10.5Al_14.3（Zr51.9） and Zr50.7Cu28Ni9Al12.3（Zr50.7） metallic glasses （MGs）during melt cooling, heating and quasistatic compression have been carried out by usingin situ synchrotron high energy X-ray diffraction （HEXRD）. The structural relaxation ofZr53, Zr51.9and Zr50.7MGs from room temperature to well bellow the glass transitiontemperature are studied via the shear stress relaxation as well.In situ structural investigation of Zr53, Zr51.9and Zr50.7MGs during melt coolingare performed by combining the HEXRD and melt aerodynamic levitation. The criticalcooling rates Rcof Zr53, Zr51.9and Zr50.7MGs in the aerodynamic levitationcondition are Rc,Zr53>108K/s,39K/s<Rc,Zr51.9<90K/s and Rc,Zr50.7<39K/s, respectively.All of the maximum diffraction peak positions corresponding to Zr53, Zr51.9andZr50.7MGs increased during melt cooling due to the increasing atomic densitycorrelation. The influence of real cooling rate on the atomic level volumetric thermalexpansion is trivial. However, the atomic level volumetric thermal expansion coefficientdecreases as the glass forming ability （GFA） of Zr53, Zr51.9and Zr50.7MGs increases.Structural evolution of Zr53, Zr51.9and Zr50.7bulk metallic glasses （BMGs） inheating process is investigated by using in situ HEXRD. The first and the secondmaximum diffraction peak position q₁and q₂, as well as the first and the second nearestinteratomic distance r₁and r₂on the reduced pair distribution function G（r） areobserved increasing linearly in heating. However, the increments of q₁, q₂, r₁and r₂arenot in the same pace. Particularly, the asynchronous increment of r₁and r₂reveals thatatomic level shear strain could be induced by the thermal expansion and increase alongwith the elevating temperature. This atomic level shear strain could reach the criticalyielding strain at the glass transition temperature.Structural evolution of Zr50.7BMG in heating process is further investigated byusing high resolution XRD. All of the first, second and third maximum diffraction peakposition as well as the peak height and peak width reflect obvious structural relaxationphenomenon. The temperature dependence of the maximum diffraction peak position aswell as the peak height and peak width can be divided into three regions: the lineardecreasing region below the structural relaxation temperature, the nonlinear evolutionregion between the structural relaxation temperature and glass transition temperature, and the linear change region in the supercooled liquid region. The structural relaxationtemperature and glass transition temperature derived based on the temperaturedependence of the maximum diffraction peak position as well as the peak height andpeak width are all different between each other. The variation of the high resolutionreduced pair distribution function G（r） of Zr50.7BMG reveals that the atomic levelshear strain induced by thermal expansion results from the cooperative atomicmovement in the first nearest neighbor shell, while it is controlled by the free volumegeneration and structural relaxation in the second nearest neighbor shell simultaneously.The structural evolution of Zr53BMG in the quasistatic compression isinvestigated by HEXRD. It is found in the plastic deformation stage the apparent atomiclevel strain saturates in the loading direction, while it keeps increasing in the transverseplane perpendicular to the loading direction. The high plasticity of Zr53BMG can beattributed to the abundant homogeneous flows in the amorphous matrix of deformedZr53BMG, except the shear band initiation and expansion in plastic deformation. It isbelieved that this kind of homogenous flow could on one hand facilitate the shear bandinitiation, and on the other hand relax the stress concentration between the shear bandand amorphous matrix which resultantly minimizes the driving force of the catastrophicshear band extension and enhances the plasticity consequently.To check the shear banding behaviors during the plastic deformation of Zr53BMG,transmission electron microscopy is adopted. The evidence presented in the plasticallydeformed Zr53BMG indicates that, except the homogeneous flow accompanied withthe shear banding in Zr53BMG, its high plasticity also benefit from the abundant microshear flows; secondary shear band interactions; as well as the primary shear bandinteractions with secondary shear bands and nanocrystals.The isothermal and the isochronous structural relaxation of Zr53, Zr51.9andZr50.7ribbon metallic glasses （RMGs） as well as Zr50.7BMG are investigated via theshear stress relaxation method from room temperature to well bellow the glass transitiontemperature. Along with the increasing structural relaxation temperature, both of thestructural relaxation rate and structural relaxation strength increase at first but decreasein the end at the elevated temperature. Similarly, all of the activation energy spectra ofthe structural relaxation unit for the studied Zr53, Zr51.9and Zr50.7RMGs as well asthe Zr50.7BMG first increase but finally decrease with the increasing activation energy.The correlation between the GFA and the isothermal structural relaxation property is not obvious. However, the isochronous structural relaxation increases as the GFA increases.Comparison among the activation energy spectra of Zr53, Zr51.9and Zr50.7RMGsindicates that at low temperature （or low activation energy） the activation energy spectraincreases as the GFA increases, while at high temperature （or high activation energy）this correlation is not obvious. Comparison of the structural relaxation between Zr50.7RMG and BMG indicates that at low temperature the activation energy spectra increasesas the experimental cooling rate （or free volume density） increases, while at hightemperature this influence is not obvious.