Investigations Into Phase Behaviors Of Amorphous Alloys

Since it was reported in 1960,amorphous alloys have successfully attracted the attention of a large number of researchers by virtue of their special disordered microstructure and excellent mechanical,soft magnetic properties etc.However,it is gradually found to be plagued by several constraints in scientific research and largescale technological applications,such as the mechanical brittleness as a structural material,and the difficulty of synergy between high saturation magnetic induction strength and low coercivity in soft magnetic applications.Whether these scientific and technical aspects can be reasonably solved depends to a large extent on the understanding of the fundamental scientific issues in amorphous alloys,especially regarding the phase behavior in amorphous alloys,since in actual metallic materials the phase is often the direct carrier of the properties.The understanding of phase behavior in amorphous alloys includes,but is not limited to,thermodynamic aspects such as the existence of anomalous phase separation in amorphous alloys;kinetic aspects such as what is the intermediate state of the amorphous–amorphous phase transition and how the transition occurs;and whether reasonable means of modulating the phase behavior(e.g.,thermal modulation,metastable modulation)can be easily used to achieve the desired properties.This dissertation focuses on a series of investigations into the phase behaviors of amorphous alloys,including anomalous phase separation in amorphous alloys,intermediate states of the amorphous–amorphous phase transition,and methods of metastable phase regulation and its applications.The existence of anomalous phase separation in amorphous alloys is a longstanding unresolved puzzle that has been in controversy since it was first hinted to be possible in 1969,due to microstructural artifacts and the lack of direct large-scale(beyond nanoscale)experimental evidence.In this study,a brand-new ultra-large scale anomalous glass phase separation phenomenon is confirmed in Cr–Fe–Co–Ni–Zr high entropy amorphous alloy with negative mixing heat by combining the interference-free optical microscope,scanning electron microscope,atomic force microscope and double spherical aberration-corrected atomic resolution analysis electron microscope.The separated second phase has an ultra-large scale that has never been reported before,up to tens of microns,which confirmed the existence of anomalous phase separation.Conventional mechanisms and predictions cannot explain this anomalous phase separation phenomenon,a new mechanism like the immiscibility of melt before monotectic reaction is introduced to explain this anomalous phase separation phenomenon.This mechanism not only helps to understand the anomalous phase separation in amorphous alloys,but also helps to understand phase separation phenomena in other fields,such as liquid–liquid phase separation in cells.Since the first report on amorphous–amorphous phase transition in amorphous alloys in 2007,the intermediate process and transformation mechanism remains unclear yet,due to the lack of a suitable system and the convenience of achieving amorphous–amorphous phase transition under in-situ TEM observation conditions.In this work,amorphous elemental Ta is successfully prepared by ion beam sputtering deposition method.Combined with TEM in-situ heating technology,the amorphous–amorphous phase transition process of amorphous elemental Ta phase–crystal Ta phase–another amorphous elemental Ta phase at lower temperature,and the amorphous–amorphous phase transition process of amorphous elemental Ta phase–twinned Ta phase–another amorphous elemental Ta phase at higher temperature are found.The intermediate process of the former amorphous phase transition is the ordinary crystalline phase,which is formed by the nucleation–growth mechanism and collapses resembling Lindemann process during the subsequent heating process to transform into another amorphous elemental Ta phase.In the intermediate process of the latter type of amorphous phase transformation,a twin phase appears,which is formed by Ta atoms attaching to the larger crystal nucleus from the mirror direction for twin growth.During the subsequent heating process,the twin phase transforms into a conventional crystalline phase,which then collapses to form another amorphous phase.These findings have never been reported in the past.This work provides rich information for the study of amorphous–amorphous phase transition,especially the intermediate state of transition,which helps to better understand the amorphous–amorphous phase transition and its intermediate state.Understanding phase behavior helps regulate and apply different phases.Thermal regulation is a quite common means of phase regulation,such as the analysis of the intermediate process of amorphous–amorphous phase transition mentioned above,which relies on the method of thermal regulation of phase behavior.In this work,another typical method of metastable phase regulation,i.e.,ion implantation method and its application in other systems are explored.Taking Li metal battery as an example,it is viewed as the ’holy grail’ of Li batteries due to its highest theoretical specific capacity.However,its practical application is hindered by issues such as uncontrolled dendrite growth and low Coulombic efficiency.Solid-solution-based metal alloy phase electrode is expected to be a good solution to the above issues.However,due to the limitation of equilibrium microstructure,metal–metal solid solutions are usually either interstitial solid solutions or substitutional solid solutions.Whether there is a composite solid solution phase containing both interstitial and substitutional atoms have not been reported,let alone its properties and potential applications.In this work,as a proof-ofconcept investigation,a brand-new Cu–0.01 wt.% Ag composite solid solution phase with enhanced free electron density and higher surface reaction energy given by higher surface stiffness is successfully prepared by non-equilibrium ion implantation method.When used as Li metal battery current collectors,the composite solid solution phase is found to significantly reduce the nucleation overpotential,enhance Coulombic efficiency,ameliorate uncontrolled Li dendrite deposition,and thus significantly improve the cycle life of the battery.The noble metal content used to achieve the significant performance improvement here is the lowest of the results reported so far compared to other reported methods(Figures 4-9).This result provides new insights into the regulation and application of new metastable alloy phases.