Investigations Of The Relaxation Dynamics In Metallic Glassy State

The knowledge of the mobility of atoms(or molecules)in condensed matters is fundamentally important for understanding the condensed state of matters and then improving the properties of materials.Glassy materials are structurally disordered and heterogeneous and,are metastable and nonequilibrium.The relaxation dynamics is thus of crucial importance for understanding supercooled liquid,glass transition and the stability and deformation of glassy materials,and remains a long-standing issue for glass materials and physics.However,relaxations in glassy state involve wide time scales,and within different time and length scales different dynamic modes show correlation but aslo have their own unique features,making them complicated and difficult to study.In this thesis,we have implemented a complementary approach by combining thermodynamics,dynamics and X-ray photon correlation spectroscopy to systematically investigate the relaxation dynamics in metallic glasses(MGs)over the widest possible time and temperature ranges and at different length scales.We found in glassy state a new relaxation decoupling,and constructed the link between different dynamic modes as well as that between macroscopic and microscopic rlaxations.We revealed the origin of the atomic scale intermittent relaxation.Based on the new knowledge of the relaxation dynamics in MGs,we proposed and eventually succeeded in synthesizing ultrastable MGs on room temperature cold substrate.The main results of the thesis are summarized as follows.YNiAl MG systems with hitherto the most prominent?relaxation peak and higher characteristic temperature were developed,providing a new model system for the study of?relaxation and relevant issues.We found that the content of the main element Y and the species of transition metal elements has significant effect on?relaxation,the manifestation of?relaxation correlates closely to the chemical affinity of the system and the fluctuations of mixing enthalpy between the components.We found that boson peak associated with the anomalous atomic vibration follows exactly the structural relaxation and shows memory effect.The boson peak strength decreases as the glass relaxes towards lower energy state,while it increases as the glass shows memory effect relaxing inversely towards higher energy state,and then decreases as the glass relaxes towards lower energy state,going thus all the way with structural relaxation.In addation,the boson peak strength shows a linear dependence on the energy state of the glass.These experimental facts reveal the direct link between structural relaxation and boson peak dynamics.By using a mandrel winding method,we are enabled to observe the long-time slow flow in MGs and found the signature of two-step flow.Then we performed further stress relaxation experiments and we confirmed therein the existence of two-step relaxation.At high temperatures,stress decays with time in a single step,while the relaxation divides into two steps as temperature decreases,indicating the existence of a new relaxation decoupling and distinct dynamic process in glassy state.The faster and the slower relaxation processes exhibit distinct dynamic featurs.The faster relaxation mode is related to the atomic scale internal stress dominated ballisticlike motion,while the slower one is associated with structural rearrangements at larger scales with heterogeneous dynamics.By using X-ray photon correlation spectroscopy,we studied systematically the atomic scale relaxation in MGs with different stability and at different stress states.We found that relaxations in microscopic and macroscopic scales differ a lot but also show close correlation.The microscopic relaxation exhibits intermittency,which becomes strongly suppressed with the increasing glass stability or with the existence of in-situ stress.The intermittent relaxation is always accompanied by the drop of the initial nonergodic plateau of the intensity correlation function,and it drops faster as the intermittency is sharper.These experimental facts unveil the dynamical and structural origin of the perplexing intermittent relaxation on atomic level,that is,intensive activation of additional secondary relaxation process associated with the heterogeneous structure.By controlling the rate of physical vapor deposition,we created ultrastable MGs without heating the substrate,which exhibit~60 K enhanced T_g and stronger resistance against crystallization,more homogeneous structure with less order at longer distances,and more stability and different phase formations during crystallization.Our results circumvent the limitation of the previous works that ultrastable glasses can only be obtained on high temperature substrates,thus are not only of technological significance,but also highlight the key role of surface dynamics in ultrastable glass formation suggestting that it could be much faster-than-expected.Our results deepen our understanding of these important dynamic issues in glasses,such as boson peak,?relaxation,structural relaxation,memory effect and surface dynamics,and provide evidences for distinguishing those numerous theoretical models.There exist far richer-than-expected dynamic modes in glassy state,on which the in-depth investigations will provide new clues for understanding the nature of glass and optimizing the synthesis and properties of materials.