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 investigate 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 enriches the traditional tensile deformation theories and provides a new understanding for the physical origins of cavitation in the crack tip of MGs.