Amorphous alloys (also known as metallic glasses) show broad application prospects in space, national defense, energy and other fields because of their excellent properties. However, amorphous alloys are prone to form nano scale deformation localized shear bands, and the macro brittleness induced by the rapid expansion of shear bands seriously limits their wide application in engineering. Therefore, the shear band problem of amorphous alloys has become an important topic of common concern in the fields of mechanics, physics and materials.
Inherently, the emergence of shear bands in amorphous alloys is a nonlinear process far away from the space-time multi-scale coupling under thermodynamic equilibrium. In space, the inherent structural heterogeneity will cause strong deformation and gradient effect of dynamic behavior. In terms of time, it covers many rate processes, such as atomic vibration, coordinated rearrangement of atomic clusters, plastic flow and so on. These events have their own characteristic time and space scales. Their correlation and coupling control the emergence of shear bands, so that the deformation is highly concentrated in the banded region with a width or thickness of tens of nanometers, and rapidly expands in the mode of near sound velocity. Unlike the crystalline alloys with periodic ordered arrangement of atoms, the deformation of amorphous alloys with long-range topological disordered stacking of atoms implies three highly coupled entangled atomic scale motions:shear, volume expansion and rotation. The strong entanglement of these three local atomic motions is the key bottleneck for the emergence of fine physical images of amorphous alloy shear bands.
Recently, the Dai lanhong research team of the Institute of mechanics of the Chinese Academy of Sciences has made new progress in the research of this problem. Based on the theoretical framework of continuum mechanics, researchers first proposed a two term gradient model (tTG model), which considers both affine and non affine deformation information at the same time, which can completely describe the local deformation field of disordered solid media, breaking through the limitations of the widely used simple affine or non affine models.
Researchers further completed the decoupling of the three highly entangled local motions of shear, swelling and rotation, and defined a new quantitative descriptor of local shear, swelling and rotation events on the atomic scale. In order to characterize the motion of these three kinds of atomic clusters, the concepts and quantitative characterization methods of shear dominated zone (SDZ), dilatation dominated zone (DDZ) and rotation dominated zone (Rdz) are proposed, which overcomes the deficiency that the currently popular shear transformation zone (STZ) cannot characterize the rotation motion of atomic clusters and quantitatively describe the dilatation motion.
On this basis, researchers used large-scale molecular dynamics simulation to accurately characterize the whole process of amorphous alloy from uniform deformation to the emergence of localized shear bands. By tracking the space-time sequence of the motion evolution of SDZ, DDZ and Rdz atomic clusters, it is found that the shear, swelling and rotating cluster motion events in the initial macro uniform deformation stage show a coordinated and consistent behavior similar to the”military action”, which is specifically manifested in the random and synchronous activation of SDZ, DDZ and Rdz in the spatially discrete”liquid like” soft zone.
Based on the statistical extreme value theory analysis, researchers found that at this stage, the spatial distribution of local motion events of body inflation showed more obvious non Gaussian long tailing characteristics than that of shear and rotation events, indicating that the localized flow of body inflation (DDZ) played a leading role. Atomic clusters complete the local softening process through volume swelling movement (DDZ). With the intensification of deformation, this volume swelling local softening further activates the rotation movement of its adjacent hard region, and then gradually breaks the synchronous activation among SDZ, DDZ and Rdz, and turns into the uneven spacing distribution of SDZ, DDZ and Rdz. The enhanced Rdz motion further intensifies the local motion of SDZ and DDZ, and then induces the softening of hard region clusters. When the softening degree reaches a critical point, the hard zone barrier is broken, and the activated SDZ, DDZ and Rdz penetrate each other to form a shear band.
Based on the percolation theory, the researchers further quantitatively analyzed the whole process of the evolution of SDZ, DDZ and Rdz cluster motion events from the random discrete activation in the initial stage of uniform deformation to the emergence of deformation localized shear bands. It was found that the emergence of shear bands belongs to directed percolation and presents a critical power-law scaling behavior.
The two term gradient (tTG) model and three new concepts of atomic cluster motion units (SDZ, DDZ and Rdz) proposed in this work provide basic tools for the quantitative description of the deformation of disordered solid media. The atomic scale fine image and critical behavior of the emergence process of shear bands revealed in this work provide new clues for the further understanding of amorphous alloy shear bands.
The research results have recently been“Hidden spatiotemporal sequence in transition to shear band in amorphous solids”Published onPhysical Review Research 4, 23220 (2022)The first author is Yang Zengyu, a doctoral student. This research work has been supported by the National Natural Science Foundation of China’s major projects”plastic flow and strengthening and toughening mechanism of disordered alloys”, the basic science center project”multi-scale problems of Nonlinear Mechanics”, and the Chinese Academy of Sciences’ class B strategic leading science and technology special project”complex medium system front edge and cross mechanics”.
Fig. 1 rotation (vortex), shear and swelling events in amorphous alloy shear bands
Fig. 2 the correlation between shear swelling events and rotation events is”broken”, and the spatial distribution changes from synchronous activation to alternating interval distribution
Fig. 3 before the emergence of shear bands, the atomic rotating cluster motion (Rdz) is significantly enhanced (white bubbles in the figure represent Rdz, that is, the vortex structure of atomic motion)
Fig. 4 Schematic diagram of atomic scale evolution process of amorphous alloy shear band emergence