| Relaxation behaviors are inherently feature of glassy materials as a result of the nonequilibrium nature and determines the basic characteristics and applications of glassy materials.Therefore,the investigation of glassy materials also focuses on the study of their relaxation behaviors.In recent years,metallic glasses(MGs)played an important role in clarifying many scientific issues and solving difficult engineering applications problems,due to its simple atomic structure and unique mechanical/physical properties.Similar to other glassy materials,MGs also exhibit complex relaxation processes over a wide range of temperature and timescales.In previous works,it has been shown that the relaxation behavior in MGs is closely related to the structure and has an important influence on it properties.However,the relationship between them has not been successfully established.Therefore,it is important to study the relaxation dynamics of MGs in order to clarify its relaxation characteristics and physical mechanism,and reveal its relationship between the structure and properties.Flash scanning calorimetry(FSC)is an important technological breakthrough based on the conventional DSC,with super-high heating and cooling rates,super-wide rate range,and super-high time resolution.It provides a novel method to investigate the relaxation dynamic behaviors of glassy materials.The purpose of this dissertation is to study the relaxation dynamic behaviors of MGs by FSC.The following three parts are included in this dissertation:1.Al-based MGs have very poor glass forming ability(GFA)and are generally prone to crystallization with conventional heating rates.Hence,its glass transition and supercooled liquid properties have not been measured and characterized in detail.In this work,the glass transition process and the supercooled liquid region with a certain temperature width of Albased MGs are clearly captured by the FSC with super-high heating rates,which provide an opportunity to explore the properties of the supercooled liquid of Al-based MG.Subsequently,we systematically investigated the dynamic behaviors of glass transition of20 different Al-based MGs with different chemical compositions.The results indicate that the Al-based MGs have super-high liquid fragility,and some of them are even close to the theoretical upper limit of liquid fragility.At the same time,we also reveal a nonlinear correlation between fragility and the GFA for MGs.Our findings suggest that the superhigh liquid fragility may be the most important controlling factor for the GFA of Al-based MGs,which provides new ideas for the design and development of novel bulk Al-based MGs.2.The FSC can clearly capture the shadow glass transition of the unannealed superquenched MGs at a high heating rate,which provides an opportunity to investigate dynamic behaviors of shadow glass transition in super-quenched MGs.In this work,we use the FSC systematically to study the dynamic behaviors of the shadow glass transition in 24 different MGs with different ? relaxation behavours under a wide range of heating rates.We present evidence that the shadow glass transition correlates with the ? relaxation in MGs:(i)The shadow glass transition and the ? relaxation follow the same temperature-time dependence,and they both merge into the glass transition(or ? relaxation)at higher temperatures;(ii)The shadow glass transition is more obvious in MGs with pronounced ? relaxation,and vice versa,and their relative strengths are proportional to each other.These results fully confirm that the shadow glass transition is a thermodynamic signature of ? relaxation in superquenched MGs.Our findings not only overcome the shortcomings of conventional calorimetric methods that cannot detect the ? relaxation behaviors of glass,but also establish a correlation between glass dynamics and thermodynamics.3.Relaxation dynamic behaviors and properties of Nickel-Phosphorus(Ni-P)MG and nano-glass(NG)have been systematically studied and compared.First of all,compared with general bulk MGs,the supercooled liquid of Ni-P MG exhibits more “fragile” liquid behavior,and this high liquid fragility may lead to its poor GFA.Secondly,the difference of the relaxation dynamics and properties between MG and NG is found: Compared with the same composition MG obtained by melt spined,the NG has a larger enthalpy relaxation area,higher stability,and more obvious ? relaxation peak,but similar anti-crystallization ability.Finally,the relationship between structural relaxation and microstructure is confirmed by the annealing experiment of NG.The present results not only have great significance for the study of broadening the relaxation of the glass state,but also are important guide value for the design of MGs with better glass properties.This dissertation is not only helpful for a deeper understanding of the relaxation dynamic behaviors of MGs from the perspective of calorimetric,but also it has deeply inspire meaning for us to think about how to guide the development of bulk MGs and modulate their properties from the perspective of relaxation.At the same time,it also provides us with new ideas for studying relaxation dynamics in glassy materials. |
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