Deformation Mechanisms Of Metallic Glasses Research Articles

Evolution of atomic rearrangements in deformation in metallic glasses.

Atomic rearrangements induced by shear stress are fundamental for understanding deformation mechanisms in metallic glasses (MGs). Using molecular dynamic simulation, the atomic rearrangements characterized by nonaffine displacements (NADs) and their spatial distribution and evolution with tensile stress in Cu50Zr50 MG were investigated. It was found that in the elastic regime the atomic rearrangements with the largest NADs are relatively homogeneous in space, but exhibit strong spatial correlation, become localized and inhomogeneous, and form large clusters as strain increases, which may facilitate the so-called shear transformation zones. Furthermore, initially they prefer to take place around Cu atoms which have more nonicosahedral configurations. As strain increases, the preference decays and disappears in the plastic regime. The atomic rearrangements with the smallest NADs are preferentially located around Cu atoms, too, but with more icosahedral or icosahedral-like atomic configurations. The preference is maintained in the whole deformation process. In contrast, the atomic rearrangements with moderate NADs distribute homogeneously, and do not show explicit preference or spatial correlation, acting as matrix during deformation. Among the atomic rearrangements with different NADs, those with largest and smallest NADs are nearest neighbors initially, but separating with increasing strain, while those with largest and moderate NADs always avoid to each other. The correlations in the fluctuations of the NADs confirm the long-range strain correlation and the scale-free characteristic of NADs in both elastic and plastic deformation, which suggests a universality of the scaling in the plastic flow in MGs.

On the deformation mechanisms in metallic glasses

The deformation of metallic glasses (MG) may be considered on the basis of three models: the STZ (shear transformation zone) model; the free-volume model; and dislocation plastic flow. For the example of MG on the iron base, it is shown that dislocation plastic flow occurs at low temperatures; the activation energy is a few tens of eV. At higher temperatures, plastic deformation occurs with an activation energy of a few eV, which is close to the activation energy of self-diffusion. That corresponds to the first two models.

Transiently suppressed relaxations in metallic glass

We show suppressed relaxation behavior and an apparent hardening effect under repeated loading in metallic glasses through cyclically tensile stress relaxation experiments. The effects are attributed to the gradual exhaustion of deformation units via activation energy spectra analysis. We find the suppression is partly alleviated and rapidly saturates with increasing the time interval between the cyclic loadings, reflecting the observed phenomena are associated with the un-recovered activated deformation units. These results may shine light on the long-standing issues such as the fatigue damage, cyclic hardening, and the deformation mechanism of metallic glasses.

Correlation between elastic structural behavior and yield strength of metallic glasses

Tremendous research effort has been put into the study of plastic deformation mechanisms of metallic glasses (MGs) in an attempt to elucidate the origin of their high fracture strength. Little attention has, however, been paid to the elastic deformation of MGs. In this paper, a series of MGs with different yield strengths are studied, with a focus on the fine structural evolution (at the atomic level) during elastic deformation. Our results reveal that an atomic reorientation happens in the first nearest-neighbor shell due to elastic deformation. This reorientation subsequently leads to a drop in the local stress, which further results in a cooperative shift of surrounding atoms to counterbalance this change in local stress level. A concordant region is formed as a result. The relation between this concordant region and the yield strength is thoroughly discussed in terms of its size and the stress level in this region. It is proposed that this concordant region could be the missing part that bridges the macroscopic yield strength and the microscopic atomic structure.

Assessing the shear band velocity in metallic glasses using a coupled thermo-mechanical model

Shear band (SB) velocity is a key parameter in assessing the dynamic behavior and also deformation mechanisms of metallic glasses. Using a coupled thermo-mechanical model, we calculated the speed of SB propagation in Vitreloy 1 (Zr41.25Ti13.75Ni10Cu12.5Be22.5) under compression at a strain rate of 10 s−1. The propagation of the SB exhibits a non-uniform speed: a rather slow stage followed by a rapid propagation when the SB is close to the sample surface or at failure. The average speed is estimated to be about 100 m/s, while the maximum speed could reach 1400 m/s. The impact of the varying SB velocity on measurement relying on surface detection such as thermographic images, strain gauge, and surface offsets is discussed.

Internal stresses induced by plastic shear deformation of Zr–(Cu,Ag)–Al bulk metallic glasses

Effective internal shear stress σi induced by torsional deformation of Zr46(Cu4/5Ag1/5)46Al8 and Zr46Cu46Al8 bulk metallic glasses different by the glass-forming ability of the maternal melts has been determined by measurements of stress relaxation upon stepwise unloading. It has been found that the ratio σi/σ0 (σ0 is the initially applied shear stress) decreases upon increasing the temperature from ≈0.8 at T=450K (T≈0.64×Tg) to ≈0.08 at T=638K (T≈0.91×Tg) independently of σ0 and glass composition. The obtained result is in good agreement with earlier data obtained on ribbon metallic glasses. The origin of deformation-induced internal stresses and their connection with deformation mechanisms of metallic glasses has been briefly discussed.

Quasi phase transition model of shear bands in metallic glasses

A quasi phase transition model of shear bands in metallic glasses (MGs) is presented from the thermodynamic viewpoint. Energy changes during shear banding in a sample–machine system are analyzed following fundamental energy theorems. Three characteristic parameters, i.e. the critical initiation energy ΔGc, the shear band stability index k0, and the critical shear band length lc, are derived to elucidate the initiation and propagation of shear bands. The criteria for good plasticity in MGs with predominant thermodynamic arrest of shear bands are proposed as low ΔGc, large k0, and small lc. The model, combined with experimental results, is used to analyze some controversial phenomena of deformation behavior in MGs, such as the size effect, the effect of testing machine stiffness and the relationship between elastic modulus and plasticity. This study has important implications for a fundamental understanding of shear banding as well as deformation mechanisms in MGs and provides a theoretical basis for improving the ductility of MGs.

Plasticity of Ductile Metallic Glasses: A Self-Organized Critical State

We report a close correlation between the dynamic behavior of serrated flow and the plasticity in metallic glasses (MGs) and show that the plastic deformation of ductile MGs can evolve into a self-organized critical state characterized by the power-law distribution of shear avalanches. A stick-slip model considering the interaction of multiple shear bands is presented to reveal complex scale-free intermittent shear-band motions in ductile MGs and quantitatively reproduce the experimental observations. Our studies have implications for understanding the precise plastic deformation mechanism of MGs.

Mechanical Properties and Deformation Mechanism of Metallic Glasses

Mechanical Properties and Deformation Mechanism of Metallic Glasses