573 / 2019-03-19 22:29:42

Unexpected strength behavior of Metallic Glass in Extreme Compressions

Strength; Metallic glass; Extreme compression; ramp compression

全体主题 > High Pressure Physics and Materials Science

We examine the deformation and strength of metallic glass in extreme compressions by employing non equilibrium molecular dynamics (NEMD) in this study. Unlike the expected Mohr-Coulomb law, an unexpected simple form of strength-dependence on pressure is observed. Two kinds of extreme compressions are simulated. One is ramp compression of long sample associated with ramp stress wave propagation. The other is hydrodynamic compression of a smaller sample and the compressed sample’s strength is obtained by a following shear. Long sample and small sample have the same chemical composition and thermodynamic history. We find that the increase in strength in extreme compression is a result of shear modulus increase. Despite the fact that a weak Mohr-Coulomb law manifests in low pressure regime (<5GPa), the scaled strength, v/G (v is von-Mises stress and G is pressure and temperature dependent shear modulus) remain nearly invariant with pressure for all deformations with the same strain rates. Using the strength in shearing with no pressure as a benchmark, v/G in shear deformations with pressure <20 GPa are less 2% higher than the benchmark while v/G in ramp compressions are less 2% lower than the benchmark. We assume that the cooperative shear model (CSM) still manifests in extreme compression. Further considerations are made that the scaling law on PES between the energy barrier and the separation of two energy basins associated with shear transformation zones (STZ) holds true in the course of compression and that the energy barrier increases with decreasing of atomic separations due to increasing pressure. This improved CSM leads to invariant v/G in extreme compressions. This reasoning suggests that extreme compressions does not substantially change STZ features such as the density of states and the geometrical relations between basins on PES. We further develop a fractal analysis method based on 3D atomic strain distribution. The fractal dimensions at atomic scale and at STZ length scale in the course of deformations implies that pressure tunes metallic glass’s structure at atomic scale while pressure has little effect on STZ features on PES. Structure analysis based on Voronoi method suggests metallic glass becomes more and more “order” at atomic scale in ramp compressions even though it yields and plastically flow. This finding contrasts pure shear simulation results. In short conclusion, an unexpected invariant v/G is observed in extreme compressions with same strain rate. This unexpected behavior comes from the pressure effects on PES.