The Thermal Shear-Transformation-Zone Theory: Homogeneous Flow and Superplasticity in Bulk Metallic Glasses (original) (raw)
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Physical Review E, 2004
We extend our earlier shear-transformation-zone (STZ) theory of amorphous plasticity to include the effects of thermally assisted molecular rearrangements. This version of our theory is a substantial revision and generalization of conventional theories of flow in noncrystalline solids. As in our earlier work, it predicts a dynamic transition between jammed and flowing states at a yield stress. Below that yield stress, it now describes thermally assisted creep. We show that this theory accounts for the experimentally observed strain-rate dependence of the viscosity of metallic glasses, and that it also captures many of the details of the transient stress-strain behavior of those materials during loading. In particular, it explains the apparent onset of superplasticity at sufficiently high stress as a transition between creep at low stresses and plastic flow near the yield stress. We also argue that there are internal inconsistencies in the conventional theories of these deformation processes, and suggest ways in which further experimentation as well as theoretical analysis may lead to better understanding of a broad range of nonequilibrium phenomena.
Homogeneous deformation of bulk metallic glasses in the super-cooled liquid state
Materials Science and Engineering: A, 2004
Different Zr-Ti-Al-Ni-Cu bulk metallic glasses have been processed. They have been characterized by neutron and X-ray diffraction in the glassy state and after DSC heating up to crystallization. The homogeneous visco-plasticity of the glasses has been studied in the under-cooled liquid state by compression tests at constant strain-rate and by differential mechanical tests with strain-rate jumps at different temperatures. The flow stress appears to be strongly thermally activated. Another characteristic feature of the deformation of the fully amorphous state is that the flow stress is independent of the strain-rate path followed to reach a given level of strain. The stress-strain curves and their dependence on temperature and strain-rate are then discussed in terms of Newtonian and non-Newtonian viscosity. It is shown that a reduced viscosity can be scaled on a master curve, independently of the strain-rate and temperature. A second analysis of the results is made, in terms of free volume. The temperature and stress variation of the plastic flow are compared with Spaepen model. A constitutive equation of the deformation is proposed. Finally, the effect of crystallization on mechanical behaviour of metallic glasses is treated trough a model involving backstress due to a composite behaviour.
Stress-Temperature Scaling for Steady-State Flow in Metallic Glasses
Physical Review Letters, 2010
Through computer simulation of steady-state flow in a Zr 50 Cu 40 Al 10 metallic glass using a set of realistic potentials we find a simple scaling relationship between temperature and stress as they affect viscosity. The scaling relationship provides new insight into the microscopic mechanism of shear flow in the glassy state, in terms of the elastic energy of the applied stress modifying the local energy landscape. The results suggest that the plastic flow and mechanical failure in metallic glasses are consequences of stress-induced glass transition.
Pure shear deformation and its induced mechanical responses in metallic glasses
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2019
Shear is a basic deformation mode governing yielding, plasticity and fracture in metallic solids. For amorphous metals, due to various constraints, little work is available in addressing directly shear deformation and shear-induced mechanical property changes which are vital to the mechanistic understanding of this new class of disordered materials. Here, by using a finite deformation theory, we examine the pure shear deformation in a bulk metallic glass in a large range of shear strains. With the continuum approach, we show systematically for the first time the detailed shear deformation behaviours, shear-induced normal stress and strain relations, softening in the elastic constants, volume dilatation and free energy change induced by the shear deformation. These results point to two major consequences from the shear deformation, one is the mechanical degradations and the other material degradation which is responsible for the changes in the mechanical properties of the disordered ...
Signature of local stress states in the deformation behavior of metallic glasses
NPG Asia Materials, 2020
The design of ductile heterogeneous metallic glasses (MGs) with enhanced deformability by purposely controlling the shear-band dynamics via modulation of the atomic-scale structures and local stress states remains a significant challenge. Here, we correlate the changes in the local atomic structure when cooling to cryogenic temperature with the observed improved shear stability. The enhanced atomic-level structural and elastic heterogeneities related to the nonaffine thermal contraction of the short-range order (SRO) and medium-range order (MRO) change the characteristics of the activation process of the shear transformation zones (STZs). The experimental observations corroborated by Eshelby inclusion analysis and molecular dynamics simulations disclose the correlation between the structural fluctuations and the change in the stress field around the STZ. The variations in the inclination axes of the STZs alter their percolation mechanism, affect the shear-band dynamics and kinetics,...
Universal mechanism of thermomechanical deformation in metallic glasses
Physical Review B, 2015
We investigated the atomistic structure of metallic glasses subjected to thermomechanical creep deformation using high energy x-ray diffraction and molecular dynamics simulation. The experiments were performed in-situ, at high temperatures as a time dependent deformation in the elastic regime, and ex-situ on samples quenched under stress. We show that all the anisotropic structure functions of the samples undergone thermo-mechanical creep can be scaled into a single curve, regardless of the magnitude of anelastic strain, stress level and the sign of the stress, demonstrating universal behavior and pointing to unique atomistic unit of anelastic deformation. The structural changes due to creep are strongly localized within the second nearest neighbors, involving only a small group of atoms.
International Journal of Plasticity, 2022
Due to the structurally disordered arrangement of atoms and deviation from thermodynamic equilibrium, the physical and mechanical properties of metallic glasses can vary with time, temperature and magnitude of strain or stress. The current work provides a theoretical framework based on the hierarchically correlated atomic theory, which allows a quantitative description of the non-elastic deformation in metallic glasses. The defect concentration is adopted as an order parameter, which can evolve with temperature and non-elastic strain owing to correlated atomic movements. Through our hierarchically correlated atomic theory, we derive the characteristic times for local shear events in metallic glasses that entail activation, growth and/or annihilation of flow defects, which however are not accounted for in the previous mean field theories. Finally, we demonstrate that the current theoretical framework can be validated by the stress relaxation and creep experiments on typical La-based metallic glasses, which in turn provides quantitative insights into the non-elastic deformation mechanisms in metallic glasses.
Scientific Reports
Despite significant research efforts, the deformation and failure mechanisms of metallic glasses remain not well understood. In the absence of periodic structure, these materials typically deform in highly localized, thin shear bands at ambient and low temperatures. This process usually leads to an abrupt fracture, hindering their wider use in structural applications. The dynamics and temperature effects on the formation and operation of those shear bands have been the focus of long-standing debate. Here, we use a new experimental approach based on localized boiling of liquid nitrogen by the heat generated in the shear bands to monitor the tensile plastic deformation of a bulk metallic glass submerged in a cryogenic bath. With the "nitrogen bubbles heat sensor", we could capture the heat dissipation along the primary shear banding plane and follow the dynamics of the shear band operation. The observation of nitrogen boiling on the surface of the deforming metallic glass gives direct evidence of temperature increase in the shear bands, even at cryogenic temperatures. An acceleration in bubble nucleation towards the end of the apparent plastic deformation suggests a change from steady-state to runaway shear and premonitions the fracture, allowing us to resolve the sequence of deformation and failure events.
International Journal of Plasticity, 2019
Structural heterogeneities in monolithic metallic glasses, i.e. free volume variations or stress/strain fields, are known to be pivotal for their plasticity but the mechanisms how they affect irreversible deformation remain poorly understood. We show that flash-annealing is a capable tool for modifying the free volume content and its distribution, which creates non-affine stress fields in a monolithic CuZr-based bulk metallic glass. Rather than the overall free volume, µm-scale regions of relaxed and rejuvenated glass govern plastic deformation. Complementary molecular dynamics simulations underpin the importance of the interfaces between relaxed and rejuvenated volumes for shear band proliferation. Not only can we reach unmatched states of metastability in metallic glasses with the present approach but also better understand the mechanisms underlying plasticity in monolithic but structurally heterogeneous metallic glasses.
Anelasticity and viscoplasticity in metallic glasses
Materials Science and Engineering, 1983
Low level constant-load tensile creep measurements and bend stress relaxation measurements were made on the metallic glasses Fe4oNi4oB2o and Fe32Ni36Cr14P12B 6 at various temperatures. The anelastic (recoverable) and viscoplastic (non-recoverable) components of flow were separated using pre-annealing treatments of various lengths of time, and multiple stress reversals for bend stress relaxation. The influence of the structural relaxation on the anelasticity was studied by continuous measurements of apparent viscosity. It was found that, while viscoplastic deformation drastically decreases with annealing, the anelastic behavior is only weakly affected. The viscosity versus time curves for pre-annealed samples are shifted on the time axis with respect to those for as-quenched samples. The relaxation rates calculated from the final stages of stress relaxation are higher than those estimated for pre-annealing periods. All these findings strongly imply that the spectrum of activation energies for anelastic relaxation is very broad and little affected by thermal treatments.