Relaxation Dynamic Behaviors In Metallic Glasses

Metallic glasses are non-equilibrium materials with complex relaxation behaviors that would determine the physical and chemical properties.However,the evolvement of relaxation behaviors is still not clear.Therefore,it is important to clarify the mechanisms of relaxation dynamics in metallic glasses.In this dissertation,the phase transition kinetics,relaxation dynamic behaviors and memory effect of metallic glasses were investigated by high-rate and high-precision differential scanning calorimeter.?1?The effect of Cu content on glass transition behavior of(La_0.5Ce_0.5)₆₅Al₁₀Co_25-xCu_x?x=0,5,10,15,20,25?metallic glasses was studied.It is found that the glass transition temperature?T_g?decreases with the increase of the Cu content.The fragility?m?increases first and then decreases with the increasing of Cu content,with a maximum m=63.6 for x=10.A correlation is confirmed between the enthalpy of mixing fragility.These results suggest that metallic glasses with desirable supercooled liquids characters can be designed by alloying elements with suitable mixing enthalpy.?2?Isothermal annealing is a very useful strategy in modulating the properties and structures of metallic glasses,which has been regarded as a single relaxation progress.In this work,the enthalpy relaxations of Au-and Fe-based metallic glasses are studied using a high-precision calorimeter.An intriguing transition from?relaxation to?relaxation was confirmed during isothermal annealing.Energy landscape model is proposed to quantitatively explain how the relaxation modes transform.It is found that a small enthalpy decrease?about 0.8 kJ/mol?in initial state causes an extremely large increase?about 100 kJ/mol?in relaxation barrier,which is attributed to the enhanced cooperative atomic motion.These results open a gate for precisely understanding the role of different relaxation modes in modifying the properties of metallic glasses.?3?Studying the competition between glass formation and crystallization is helpful for understanding the physical origin of glass formation and benefits the development of new bulk metallic glasses.In this paper,we studied in situ the phase transition kinetics of an Au-based metallic glass using a high-rate high-precision differential scanning calorimeter.Two phase diagrams are established to illustrate the transformations between glass,liquid and crystal upon heating and cooling,which are more concise compared to the classical temperature-time-transformation diagram.We further found that metallic glasses located in different positions in the phase diagrams exhibit different resistance to crystallization.The relationship between enthalpy and relaxation activation energy was studied by changing cooling rate.It is interesting that the enthalpy collapses into a single curve versus activation energy,revealing the inherent relationship between enthalpy and activation energy.?4?The enthalpy relaxation kinetics of Au-based metallic glass is studied by single and two-step annealing process using a high-precision calorimeter.We find that the responses of the memory effect are closely related to pre-annealed kinetic behaviors.The quantitative relation between the glass enthalpy and the memory effect of relaxation is studied.Moreover,the hierarchical dynamics in sample suggest that the more entangled states of?relaxation compared to?relaxation.