Amorphous alloys display topological structures with short-range order and long-range disorder. This poses a great challenge to the traditional mechanism of crystalline plasticity based on dislocations, twining, etc. It has been generally accepted that the unit process of amorphous plasticity is the “shear transformation (ST)” of local atomic groups. However, whether this enthalpy-entropy driven, thermally activated event has a clear structural origin or not still remains much controversies. The present thesis aims at this controversy, and explores the spatiotemporal correlation between amorphous structures and resulting plastic flow, by combining synchrotron radiation X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS), dynamic thermomechanical analysis (DMA) with continuum mechanics modeling. The main results we obtained are as follows.
Using the synchrotron radiation SAXS experiments, the structural heterogeneity of the Vitreloy 1 amorphous alloy in the elastic deformation stage is obtained. The results indicate that the scattering intensity curve of the Vitreloy 1 amorphous alloy exhibits the positive deviation of Porod law. According to the Porod's law, it is revealed that the diffuse interface exists between the two phases, associated with the density fluctuations in either of phases. Furthermore, we demonstrate that the shape of scatterer is far from a sphere and their characteristic sizes measured by the radius of gyration are mainly distributed between 0.8 nm and 1.6 nm. The radius of gyration is almost not changed in the elastic deformation stage of the amorphous alloy. Finally, based on the correlation function defined by Debye, we analyze the correlation of electron density fluctuation between two arbitrary scatterers. The result indicates that the nanoscale scatterers in the amorphous alloy are strongly correlated only within a range of about 1 nm, which is consistent with the short-range ordered and long-range disordered structural features of the amorphous alloy.
Using the synchrotron radiation XRD and SAXS experiments, the atomic-scale-to-nanoscale structural evolution of the amorphous alloy is studied in its elastoplastic deformation stage. The basic image of amorphous plastic flow is revealed by the clue of local dilatation. Based on XRD analysis, it is found that there are inherent local dilatation in the plastic yielding and softening stage. Mediated by the local dilatation, local atomic rearrangements can operate at the left-subpeak of the second shell and/or at the fourth shell with a characteristic lengthscale of about 1 nm, pointing to the nanoscale ST operation. The obtained dilatancy signatures of amorphous plasticity are further confirmed by SAXS in terms of the nano-scale structural heterogeneity.
With the dynamic thermal analysis, the relaxation kinetics serves as a bridge to link the structural signature and the plastic deformation of amorphous alloys. The temperature dependence of dynamic modulus at different deformation stages show that the deformation increases the local translational motion at low temperatures and the large-scale diffusion motion at high temperatures, which makes it easier to relaxation. Therefore, both β relaxation and α relaxation occur at lower temperatures. It is found from the frequency dependence of dynamic modulus that, the β relaxation has the same activation energy as that of basic ST events, which can effectively indicate the yielding, softening and steady-state flow behavior of amorphous alloys, implying that they have the same structural origin.
The amorphous plastic constitutive model based on shear transformation and free volume (local dilatation) interaction is extended to the three-dimensional stress state, further taking hydrostatic pressure, free volume diffusion and temperature effects into account. The extended constitutive model is realized by finite element simulation, which can correctly predict stress overshoot and strain softening, and reproduce the typical behavior of amorphous plastic flow. Based on the model, the effects of initial structural heterogeneity, inherent local dialation effect, loading strain rate and ambient temperature on the amorphous plastic flow behavior is also studied. These results confirm that the basic carrier of amorphous plasticity is STs, whereas the free volume dominates the mechanical behavior after plastic yielding.