The Glass Formation,thermal Stability And Crystallization Behavior For Zr-Ti-Cu-Ni-(Hf,Be) High-entropy Bulk Metallic Glasses

Metallic glasses(MGs)are thermodynamically metastable,and thus enhancing their thermal stability and slowing down their crystallization rate are of great importance in promoting their potential applications.As a new kind of amorphous material,high-entropy bulk metallic glasses(HE-BMGs)not only have the characteristics of long-range disordered atomic packing similar to traditional MGs,but also contain equal-molar ratio of constituents same as crystalline high-entropy alloys(HEAs).Therefore,it is necessary to study the influences of high configuration entropy on the glass forming ability(GFA),thermal stability and crystallization kinetics of HE-BMGs.Up to date,however,such reports are very limited.In this thesis,the GFA,glass transition,crystallization behavior and dynamic evolution of Young's modulus with temperature of the typical Zr-Ti-Cu-Ni-(Hf,Be)HE-BMGs were systematically studied,as compared with the traditional BMG benchmark,Vitreloy 1.The main results and conclusions are as follows:1)Equal-molar ratio Zr-Ti-Cu-Ni-(Hf,Be)HE-BMGs possess higher glass transition temperature(Tg),higher crystallization temperature(Tx),larger crystallization activation energy(Ex)and sluggish crystallization process,but with poorer GFA,as compared with Vitrloy 1.It was found that the HE-BMGs have a larger viscosity in the supercooled liquid region and exhibit a stronger liquid behavior than Vitreloy 1,which results in the sluggish kinetics and high thermal stability of the HE-BMGs.However,the viscosity of the superheated liquid of HE-BMGs is less than that of Vitreloy 1 due to the high configuration entropy,leading to faster atomic diffusion upon undercooling and thus the decrease of GFA.In other words,upon reheating,the high configuration entropy could slow down atomic diffusion in the glassy solids but reduce the viscosity of superheated liquid during undercooling,thereby giving rise to the high thermal stability upon reheating and dramatically reducing the GFA upon cooling.2)The ZrTiCuNiBe and ZrTiCuNiHf HE-BMGs showed larger Young's modulus softening resistance and a much smaller specific heat jump ?Cp(only 1/3 of that of the traditional BMGs)during glass transition than that of traditional Vitreloy 1.In-situ synchrotron X-ray diffraction,low-temperature heat capacity(Boson peak)measurements and ab initio molecular dynamics(MD)simulations indicated that the HE-BMGs possessed a denser,more homogenous,and more ordered atomic structure.The atoms in these HE-BMGs have a smaller degree of translational freedom and a much slower diffusion kinetics,resulting in less structural changes during glass transition and thus inducing the smaller specific heat jump(ACp)and less softening of Young's modulus.3)It was found that the HE-BMGs did not crystallize after the first exothermic peak on their DSC(differential scanning calorimeter)curves.In-situ synchrotron X-ray diffraction and transmission electron microanalysis results revealed that the first exothermic peak associated with an obvious structure ordering process,which underwent the changes of atomic structure over both short-and medium-range length scale.The results of three-dimensional atomic probe tomography analysis confirmed that the structural ordering process is mainly realized by the local diffusion of the small constituent Be.The crystallization process of the HE-BMGs was mediated by such structural ordering process due to the dense packing atomic structure and small degree of freedom of atomic motion during glass transition.4)The in-situ impulse excitation technique was used to monitor the dynamic changes of Young's modulus with temperature for a variety of BMGs with different chemical systems,including the HE-BMGs.It was found that the GFA and plasticity were respectively related to the softening rate right above the glass transition temperature(Tg)and the degree of sub-Tg relaxation.Above Tg,the higher the modulus softening rate is,the lower the GFA of MGs shows;Below Tg,the more obvious degree of the local structure relaxation becomes,the better the plasticity of MGs is.The Young's modulus changes with temperature of ZrTiCuNiHfBe HE-BMG also shows the similar trend,indicating that the above observations are universal.In summary,this thesis not only reveals the characteristics of high thermal stability and the underlying mechanisms of HE-BMGs,but also illuminates the effects of intrinsic structure of the HE-BMGs on the glass formation and crystallization,offering great scientific significance and application value to recognize and understand the relation between intrinsic structure and macroscopic properties of MGs in general,as well as providing new paradigm for enhancing thermal stability of MGs.