H. L. Peng

Effect of local structures and atomic packing on glass forming ability in CuxZr100−x metallic glasses

Molecular dynamics simulations are performed for CuZr metallic alloys to study the structural and dynamical features for glass forming ability (GFA). Our analysis shows that in CuZr metallic system, although ⟨0,0,12,0⟩ icosahedral clusters are important, some Zr-centered clusters such as ⟨0,1,10,4⟩ and ⟨0,1,10,5⟩ play a key role in slowing down the dynamics. It is found that these Zr-centered clusters are intrinsically slow and fundamentally determine the stability and slow dynamics. Due to the strong spatial correlation between ⟨0,0,12,0⟩ and Zr-centered clusters, their relative population influences the dense packing and dynamics in metallic glasses, and further the GFA.

Evolution of structural and dynamic heterogeneities and activation energy distribution of deformation units in metallic glass

We present experimental results on the distribution and evolution of energy barriers of deformation units in metallic glass (MG) via an activation-relaxation method. Our results show that the dynamical heterogeneity of metallic glass arises from its structural inhomogeneity, and there exist the close correlations between the deformation units, dynamical and structural heterogeneities, and relaxation behaviors in metallic glasses. The results might provide insights on the heterogeneities, plastic deformation, and relaxations behaviors of metallic glass.

Characterization of mechanical heterogeneity in amorphous solids

The structural geometry and size distribution of the local atomic rearrangements induced by external stress in amorphous solids are investigated by molecular dynamics studies. We find that the size distribution exhibits a generic power-law behavior and their structural geometry shows fractal feature. This indicates that the local atomic rearrangements in amorphous solids are self-organized during deformation. A simple theoretical model based on the interaction of the heterogeneous elastic field sources is proposed which predicts the power-law scaling and characterizes the properties of the local atomic rearrangements in amorphous solids.

Stress-versus temperature-induced structural evolution in metallic glasses

Structure evolution induced by shear deformation was investigated via molecular dynamic simulation on CuZr metallic glass system and compared with that induced by temperature. Voronoi tessellation analysis found that the local structures evolve to a liquid-like state as shear stress increases, similar to the temperature-induced structure evolution. However, shear stress induces little change to the radial distribution functions (RDFs) compared to temperature, indicating that the global glassy state still sustains. This is in contrast to the opinion that shear deformation may be similar to temperature increase and induce a metallic glass system to experience glass-liquid transition. The analysis of anisotropic part of RDFs reveals that shear deformation can induce significant anisotropic structural evolution, while pure increase of temperature cannot. Our findings demonstrate that shear deformation may induce different structural evolution in metallic glasses compared to temperature.

Crystal growth in fluid flow: Nonlinear response effects

We investigate crystal-growth kinetics in the presence of strong shear flow in the liquid, using molecular-dynamics simulations of a binary-alloy model. Close to the equilibrium melting point, shear flow always suppresses the growth of the crystal-liquid interface. For lower temperatures, we find that the growth velocity of the crystal depends nonmonotonically on the shear rate. Slow enough flow enhances the crystal growth, due to an increased particle mobility in the liquid. Stronger flow causes a growth regime that is nearly temperature-independent, in striking contrast to what one expects from the thermodynamic and equilibrium kinetic properties of the system, which both depend strongly on temperature. We rationalize these effects of flow on crystal growth as resulting from the nonlinear response of the fluid to strong shearing forces.

Velocity autocorrelation function in supercooled liquids: Long-time tails and anomalous shear-wave propagation

Molecular dynamic simulations are performed to reveal the long-time behavior of the velocity autocorrelation function (VAF) by utilizing the finite-size effect in a Lennard-Jones binary mixture. Whereas in normal liquids the classical positive t^{-3/2} long-time tail is observed, we find in supercooled liquids a negative tail. It is strongly influenced by the transfer of the transverse current wave across the period boundary. The t^{-5/2} decay of the negative long-time tail is confirmed in the spectrum of VAF. Modeling the long-time transverse current within a generalized Maxwell model, we reproduce the negative long-time tail of the VAF, but with a slower algebraic t^{-2} decay.