The influence of engineering strain rates on the atomic structure, crack propagation and cavitation formation in Al based metallic glass

To enhance the toughness of metallic glasses (MGs), considerable scientific efforts have been made to investigate the physical mechanism of fracture behavior of MGs in past few years. However, many unresolved puzzles remain about the fracture mechanisms of MGs at present. In this paper, Molecular Dynamics (MD) simulations have been used to inves-tigate the tensile fracture behavior of Al based MGs under nine different Engineering strain rates. Our results show that the higher the Engineering strain rate is, the longer the period of plastic deformation, the shorter the crack length. We have observed that Al amorphous with notch gradually crystallizes during tensile fracture, however, there are differences in the degree of crystallization and atomic arrangement as well as micro-structure in different regions with different Engineering strain rates. In the process of tensile fracture of Al based MGs, the crack propagation mainly along the hexagonal-close-packed (HCP) grain boundaries between face-centered-cubic (FCC) crystal blocks. As the Engineering strain increases, numerous HCP atoms located at grain boundaries gradually migrate into the interior of FCC crystal blocks, the velocity of crack propagation will also increase and finally induce the brittle fracture of Al based MGs. Meanwhile, we effectively demonstrated the existence of HCP and FCC atoms in Al based MGs during tensile deformation by analyzing the bond-orientational order parameters ql. The analysis of thermodynamics (entropy) showed that there is a positive correlation between the Engineering strain rate and the strain of maximum entropy when the Engineering strain rate exceeds 0.002 ps = 1. The formation of cavitation ahead of crack could be attributed to the migrate of atoms from the higher entropy cavity region to the lower entropy crystallization region. This study en-riches the traditional tensile deformation theories and provides a new understanding for the physical origins of cavitation in the crack tip of MGs.(c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

This study investigates the tensile fracture behavior of Al based metallic glasses (MGs) under different engineering strain rates using molecular dynamics (MD) simulations. The results show that higher engineering strain rates lead to longer periods of plastic deformation and shorter crack lengths. It is observed that the Al amorphous with notch gradually crystallizes during tensile fracture, and there are differences in crystallization, atomic arrangement, and microstructure in different regions with different engineering strain rates. The crack propagation mainly occurs along hexagonal-close-packed (HCP) grain boundaries between face-centered-cubic (FCC) crystal blocks, and the velocity of crack propagation increases with increasing engineering strain, leading to brittle fracture. The existence of HCP and FCC atoms in Al based MGs during tensile deformation is effectively demonstrated. The analysis also reveals a positive correlation between the engineering strain rate and the strain of maximum entropy. The formation of cavitation ahead of crack can be attributed to the migration of atoms from higher entropy cavity regions to lower entropy crystallization regions. This study enriches the traditional tensile deformation theories and offers new insights into the physical origins of cavitation in the crack tip of MGs.