In-situ Scattering Study Of Multistage Phase Transformations In Zr-Cu-Ni-Al-Ti Bulk Metallic Glasses

Metallic glass,which is also named amorphous alloy,is a kind of alloy with long ranged disordered structure.It is formed by rapidly cooling the molten alloys.In this thesis,we systematically investigated an unusual pathway of amorphous-to-crystalline phase transformation for the Zr-based bulk metallic glass Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅?commercial name is BAM 11?with excellent glass forming ability.The results of differential scanning calorimetry show that there existed two exothermic processes in the supercooled liquid region of BAM 11.Researchers have found that the first anomalous exothermic peak?AEP?would vanish completely while the second one preserved after appropriate isothermal treatment.Someone reported that the abnormal exothermic peak may correspond to the precipitated crystalline phase or chemical phase separation.However,due to the absence of direct evidence,this subject is still controversial.To solve this long-standing issue,we carried out a series of in-situ measurements,including synchrotron X-ray diffraction,in-situ transmission electron microscopy,synchrotron simultaneous small/wide angle high energy X-ray scattering and X-ray photoelectron spectroscopy,to study the evolution of the structure of BAM 11during isothermal annealing in the supercooled liquid region.The results of in-situ scattering revealed an unusual crystalline pathway of BAM 11 upon isothermal annealing,which includes five stages:1)incubation time;2)liquid-to-liquid phase transition;3)chemical phase separation;4)crystallization stage I;5)crystallization stage II.Experimental results suggest that a liquid-to-liquid phase transition would occurr firstly before phase separation in BAM 11sample.At the same time,the previous phase transition may lead to the corresponding changes of the driving force and the energy barrier of the subsequent phase transition process toward crystallization.The analysis of in-situ pair distribution function indicated the connectivity of clusters in medium range order played an important role at the early phase transition.According to the results of in-situ small angle X-ray scattering,diffused interface play an essential role in the subsequent phase transition.The effect of oxygen and other impurities on the anomalous exothermic peak was ruled out according to the results of X-ray photoelectron spectroscopy.Mechanical experiments show that the hardness of alloys with liquid phase transition can be improved without loss of elasticity.To sum up,our study revealed a unique pathway of amorphous-to-crystalline phase transition upon annealing the BAM 11 at the AEP temperature range,which not only would be an important reference for nailing down the controversial issue but also shed light on exploring nanostructured bulk metallic glasses.