Investigation of Rejuvenation and Relaxation Behaviors in Fe-based Metallic Glasses


Student thesis: Doctoral Thesis

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Award date7 Aug 2023


Metallic glasses (MGs) possess a disordered structure that defines their properties and are characterized by being metastable and non-equilibrium materials. Understanding the relaxation dynamics of glassy materials is critical to comprehending phenomena such as supercooled liquids, the glass transition, and the deformation of MGs. The rejuvenation phenomenon in MGs is a reversal of the relaxation process. Along with greatly extending their energy range, it also enhances plasticity at ambient temperature. By comprehending the underlying mechanisms of rejuvenation, it is possible to modulate the properties of MGs over a broad range, potentially achieving a high degree of rejuvenation with fundamental and practical implications. In this thesis, we have implemented a complementary approach by combining thermodynamics, dynamics, and in-situ X-ray diffraction to systematically investigate the relaxation and rejuvenation mechanism in Fe-based MGs. The main results of the thesis are summarized as follows.

Extensive cryogenic thermal cycling (CTC), up to 1000 cycles was applied to a Fe88Zr8B4 metallic glass. This MG exhibits exceptional rejuvenation capacity under CTC treatment, with the stored energy reaching 304% of the as-cast sample after 450 treatment cycles. This value is greater than that of most other MGs. It has been confirmed that the outstanding energy storage behavior induced by CTC treatment is not only due to the local atomic motion occurring in the flow units associated with the β relaxation but also from the penetration of flow units in the elastic matrix associated with the α relaxation. The activation of two relaxation modes leads to a decoupling between enthalpy and hardness in MGs. The results of synchrotron diffraction indicate that the structural information of MGs, such as atomic packing density and ordering, is impacted by the mechanisms of rejuvenation rather than being solely dependent on energy states. Our finding offers a holistic comprehension of the rejuvenation mechanism in MGs and provides a novel approach to adjusting the properties of MGs. This approach involves the manipulation of relaxation modes through CTC treatment.

The relaxation behavior of Fe88Zr8B4 MG after rejuvenation was studied using differential scanning calorimetry and synchrotron X-ray scattering. Before the glass transition, a larger exothermic peak is witnessed by the release of the introduced rejuvenated volume. The analysis of the structure factor indicates that the sharper scattering peaks are caused by the annihilation of the increasing free volume after rejuvenation. In addition, the relaxation of these extra volumes reduces the impact of thermal expansion during heating, leading to a smaller volume change when compared to the as-cast MG. The rearrangement of atoms in the second coordination shell has a greater impact on the relaxation enthalpy and free volume change compared to that in the first shell. Finally, the mechanical properties of the rejuvenated MGs were evaluated at different annealing temperatures, revealing that the restoration of both hardness and elastic modulus is achieved through structural relaxation.

The sub-Tg relaxation of multicomponent FeSiBPC metallic glasses (MGs) has been systematically investigated. A two-stage relaxation behavior in a series of Fe-based MGs has been observed, indicating the pinning effect of Curie temperature. The origin of this behavior resides in the local structure departure from short-range order to medium-range order upon annealing. Analysis of diffraction and thermophysical data shows that the structural ordering involved in the β relaxation is confined and, meanwhile, facilitated by the magnetic ordering. It is mainly the medium-range rearrangements of atoms that yield the influence on magnetic ordering. This work provides new insights into the interaction between structural arrangement during relaxation and intrinsic magnetic ordering and helps to understand the inner magnet-influenced β relaxation in MGs.