Glass Forming Ability, Crystallization Mechanisms, Deformation-Induced Microstructure And Mechanical Property Evolutions Of Cu-Based Bulk Amorphous Alloys

Amorphous alloys have become one of the most active fields in the research of advanced materials due to their unique mechanical, physical and chemical properties. However, the criterion of glass forming ability (GFA) of alloys is a critical issue that has not been unsolved yet, and exceptional cases always exist for the known criterions. Traditional quenched amorphous alloys possess very small sizes in at least one dimension, and the obtained strain during deformation at room temperature (RT) is quite limited. The discovery of bulk amorphous alloys (BMA) makes it possible to achieve severe plastic deformation at RT, and provides an opportunity to investigate the microstructural change with changing strains and its influence on various properties of amorphous alloys.In this dissertation, based on thermodynamics and kinetics of glass-forming process, the essence of GFA of alloys is revisited. By using X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and high resolution transmission electron microscopy (HRTEM), the crystallization mechanism during the isothermal and isochronal annealing of the Cu₆₀Zr₂₀Ti₂₀ BMA, the evolution of microstructure and free volume during severe plastic deformation of Cu₆₀Zr₂₀Ti₂₀ and Cu_47.5Zr_47.5Al₅ BMAs are systematically investigated. The microhardness of as-rolled specimens with different amounts of deformation is measured. The present work reveals the evolutions of microstructure, free volume and mechanical property of amorphous alloys with deformation temperature, strain rate and strain during plastic deformation. It provides a new approach to designing the compositions of BMGs and developing amorphous matrix composites.On the basis of the preconditions of the thermodynamics and kinetics of glass forming, a new criterion evaluating the GFA is proposed, i.e. T_k /T_l ( T_k - Kauzmann temperature, T_l - liquidus temperature of an alloy), and D (fragile parameter of alloy liquid). It is found that T_k /T_l can reflect the GFA well for either the alloys with strong liquid behavior or the alloys with fragile liquid behavior. The larger the T_k /T_l , the better the GFA, and the change in D does not change the role of T_k /T_l . Meanwhile, when the GFAs of these two different alloys are compared, and if T_k /T_l is kept to be the same, the larger the D , the higher the GFA.The crystallization of Cu₆₀Zr₂₀Ti₂₀ BMA proceeds through two reactions. The first is a primary crystallization with precipitation of nano-sized Cu₅₁Zr₁₄, and the second is a eutectic one with simultaneous formation of Cu₅₁Zr₁₄ and Cu₂ZrTi. Time-dependent nucleation process exists in both crystallization reactions of Cu₆₀Zr₂₀Ti₂₀ BMA. The nucleation activation energy of primay Cu₅₁Zr₁₄ phase during isothermally annealing the Cu₆₀Zr₂₀Ti₂₀ BMA in the supercooled liquid region is not only dependent on the heating rate before isothermal annealing, but also correlated with the definition of incubation time. A higher heating rate leads to a smaller value of nucleation activation energy. However, the effective activation energy for crystallization is not related to the heating rate or the definition of incubation time.The strain as high as 97% has been achieved during rolling the Cu₆₀Zr₂₀Ti₂₀ BMA at both RT and cryogenic temperature (CT, about 150 K), and the as-rolled specimens with large strains remain ductile. Corresponding to each deformation condition, there is a critical strain, below which only shear bands form in the as-rolled specimens, and above which phase transformation occurs. When the rolling temperature or strain rate is increased, the critical strain for occurrence of phase transformation decreases. Meanwhile, the type of phase transformation varies from phase separation to phase separation plus nanocrystallization, and the deviation of the average chemical composition of phase-separated amorphous phases from that of the original alloy is enhanced. When the Cu₆₀Zr₂₀Ti₂₀ BMA is rolled at CT at the strain rateε& = 1.0×10^-4 s^-1, the specimen remains in a monolithic amorphous state up to the highest strain of 97%. Atε& = 5.0×10-4 s-1, phase separation occurs when the strainεexceeds 93%. With further increasingε& to 5.0×10^-3 s^-1, the critical strain for the occurrence of phase separation drops to 89%. At the higherε& of 5.0×10^-1 s^-1, phase separation plus nanocrystallization occur atε> 67%. At a givenε& of 5.0×10^-3 s^-1, and with increasing the rolling temperature to RT, phase separation plus nanocrystallization also occur atε> 87%. The thermal stability of as-rolled specimens does not obviously change as compared with the as-cast specimen when only shear bands form in the as-rolled and no phase transformation occurs. As phase separation occurs, the thermal stability and crystallization activation energy decrease. If phase separation and nanocrystallization simultaneously take place, the thermal stability and crystallization activation energy will be further reduced.The saturation of the free-volume content in the monolithic metallic glass without phase transformation at high strains are observed in the Cu₆₀Zr₂₀Ti₂₀ BMA during CT-rolling at low strain rates. With increasingε& , the increase rate of the free-volume content in the initial stage of deformation, the critical εabove which the free-volume content in the monolithic metallic glass begins to saturate and the saturated amount of the free-volume content increases. Although the criticalεfor the saturation of the free-volume content in the monolithic metallic glass rises with the strain rate, the criticalεfor phase transformation tends to decrease. The competition of these two critical strains determines whether the saturation of the free-volume content can be obtained when the specimen is still in the monolithic amorphous phase. It is revealed that formation of nano-voids in shear bands through the coalescence of free volume leads to the saturation of free volume. The occurrence of phase separation does not considerably change the free-volume content, while partial nanocrystallization decreases it.Neither phase separation nor crystallization is induced when the Cu_47.5Zr_47.5Al₅ BMA is rolled at RT and CT withε& = 5.0×10^-2 and 5.0×10^-3 s^-1, and only compositional and structural heterogeneity in atomic level exists in the amorphous matrix near the shear bands. The free-volume content continuously increases with increasing strain during the whole rolling process. Since the material in the shear bands is adiabatically heated up to the temperature region of homogeneous flow during plastic deformation, enhancing the annihilation of free volume, results in the fact that more free volume is stored in the CT-rolled specimen as compared with the RT-rolled specimen with the same strain, and the difference in the free-volume content between the CT-rolled and RT-rolled specimens with the same strain decreases with the strain rate. The free-volume evolution with temperature and strain rate during the inhomogeneous deformation is totally different from that predicted by the free volume theory.During rolling the Cu₆₀Zr₂₀Ti₂₀ BMA, microhardness dramatically increases when phase separation or phase separation plus nanocrystallization occurs. Since Cu-rich separated amorphous phases from the matrix possess higher strengths than the original amorphous phase, phase separation is the main reason for strengthening. As the deformation-induced nanocrystallites contain lots of crystal defects, their resistance to yielding is deteriorated. In the deformed amorphous alloy, phase separation may be a more effective way to strengthen amorphous alloys than crystallization.