Mechanical Behaviour and Modelling of Typical CuZr-based Metallic Glasses

Author:Zhang Chun

Supervisor:yao yao


Degree Year:2018





Compared with the traditional crystalline alloy,owing to the atomic structure of short-range order and long-range disorder,bulk metallic glasses(BMGs)exhibit unique properties such as high elastic limit,mechanical strength,hardness,fracture toughness and fatigue resistance,good abrasion resistance and corrosion resistance,and excellent soft magnetic properties.At room temperature,the plastic deformation of BMGs tends to be highly localized in the shear bands,and the deformation can be often characterized by brittle fracture.However,it is still not clear how to change in atomic structure caused by the shear bands.Although the previous studies have proved that plastic deformation of BMGs is related to the density of weak-bonded region,there is no systematic research about how to affect the plastic deformation.At high temperatures,BMGs exhibit good plastic processing properties.However,the systematic study related to mechanical behavior is relative lack so far.CuZr-based BMGs have good glass-forming ability,thermal stability and low-cost,which has aroused the concern of scientists and engineers.In this thesis,the typical CuZr-based BMGs was selected as the research objects.The mechanical behavior and modeling of BMGs are studied from multi-scale analysis,such as atomic scale,atomic cluster scale and large-scale atomic migration.The main contents and conclusions of this thesis are as follows:(1)Starting with the microstructural inhomogeneity of BMGs,the concentration of weak bonded region can be characterized by structural relaxation exothermic enthalpy,which can used to analysis the physical mechanism behind the macro mechanical behavior.Different sizes of Zr50Cu40+x0+x Al10-x(x=0 and 2)BMGs were studied under quasi-static compression,and it is found that samples with smaller aspect ratio of height to diameter exhibit greater macroscopic plastic deformation.For the Zr50Cu40Al10 BMGs,the samples with different diameters contain the similar concentration of weak bonded region,so the size effect belongs to the external size effect.In the case of the same aspect ratio,the thicker specimens have better plastic deformability.While for Zr50Cu42Al8 BMGs the concentration of weak bonded region of 2 mm in diameter is about 2 times than that of 3 mm.It belongs to the intrinsic size effect,and shows that the thinner specimens have better plastic deformability.In the process of plastic deformation,the stress drop in the serrated flow and the concentration of weak bonded region are positively associated.In addition,the average distance of main shear bands is closely related to the concentration of weak bonded region and the location of shear band termination.Also,there is a larger average area of dimples on fracture surface of the specimen with lower concentration of weak bonded region.(2)The size effect of Zr65Cu18Ni7Al10 BMG with the diameter of 2,3 and 5 mm and the aspect ratio of 1 was studied by compression at a high strain rate(250 s-1).The specimen with smaller diameter has higher dynamic compressive strength,which is attributed to its high concentration of weak bonded region.It is predicted that the volume of the shear transition zone of the specimens with diameters of 2,3 and 5 mm are 1.32,0.21 and-0.88 nm3,respectively,in which the negative shear transition zone can be considered as a transition from toughness to brittleness.The theoretical analysis shows that the volume of the shear transition zone decreases with the decrease of the experimental temperature.From the view of the fracture morphology,both the average distance between tearing edges and its own width increase with the decrease of concentration of weak bonded region.(3)The compression deformation behavior of Zr65Cu18Ni7Al10 BMG in the supercooled liquid region is studied.The constitutive equation is obtained.In the process,there is a transformation from Newtonian flow to non-Newtonian flow.The relationship between the normalized viscosity and strain rate is given by using the extended exponential equation.It is pointed out that the parameter?tc represents the characteristic stress transitioning from Newtonian flow to the non-Newtonian flow.The relationship between strain rate sensitivity exponent and strain rate was determined,and the transitional strain rates were obtained at different temperatures.At the same time,it is found that the strain rate of transformation follows the Arrhenius equation,and its activation energy is 5.19 eV,which is in good coincidence with that ofαrelaxation(4.59 eV),frequency factor(5.28 eV)and Newtonian viscosity(4.73 eV).(4)The creep behavior of Zr65Cu18Ni7Al10 BMG is investigated near the glass transition temperature.Similar to the compressive experiment at high temperature,there exists a transition from Newtonian flow to non-Newtonian flow.The activation energy of the transitional strain rate from Newtonian flow to non-Newtonian flow is 3.48 eV,which are very close to that of frequency factor(3.09 eV)and Newton viscosity(3.24 eV).(5)Tensile behavior of Zr41.2Ti13.8Cu12.5Ni10Be22.5(Vit1)BMG is studied by using in situ synchrotron X-ray scattering technique.The atomic scale or atomic shell strain is calculated from the X-ray scattering intensity curve,the structural factor and the reduced radial distribution function.The relationship between the average stress and the atomic scale strain is obtained.(6)The synchrotron X-ray is used to scatter along the fracture surface showing different shear band densities.It is found that the first peak value of X-ray scattering intensity curve,structure factor and reduced pair distribution function in the relatively larger shear band density are obvious higher than that obtained at no obvious shear band.In order to enhance the credibility of the experimental results,the Zr39.6Ti33.9Nb7.6Cu6.4Be12.5 metallic glass composites is studied using the same method,and the results are also the case.The relative relation between the shear band density and the scattering information(atomic strcuture)is qualitatively established,which provides the theoretical basis and practical evidence for the nondestructive testing of BMGs.