Investigation on the Mechanical Properties of Polycrystalline Mg Using Molecular Dynamics Simulation

Abstract

Magnesium (Mg) and its composites have been widely used in different fields, but the mechanical properties and deformation mechanisms of polycrystalline Mg (polyMg) at the atomic scale are poorly understood. In this paper, the effects of grain size, temperature, and strain rate on the tensile properties of polyMg are explored and discussed by the Molecular dynamics (MD) simulation method. The calculated results showed that there exists a critical grain size of 10 nm for the mechanical properties of polyMg. The flow stress decreases with the increase of grain size if the average grain size is larger than 10 nm, which shows the Hall-Petch effect, and the deformation mechanism of large grain-sized polyMg is mainly dominated by the movement of dislocations. When the average grain size is less than 10 nm, it shows the reverse Hall-Petch effect that the flow stress decreases with the decrease of grain size, and the deformation mode of polyMg with small grain-size is the movement and deformation of atoms at the grain boundary. Due to the more active motion of atoms as the system temperature increases, the material can easily reach the plastic stage under tensile loading, and the mechanical properties of polyMg decrease at high temperatures. The strain rate has a hardening effect on the properties of composite. Based on our calculated results, it can provide theoretical guidance for the applications of Mg metal and Mg matrix composites.

The Plastic Deformation Mechanism in Nano-Polycrystalline Al/Mg Layered Composites: A Molecular Dynamics Study

“…Separate polycrystalline Mg and Al with different grain sizes, at a strain rate of 10 6 s −1 , were used to examine the potential influence of concurrent tension on the deformation behaviour and structural evolution of bonded Al and Mg, and the resulting stress-strain curves are presented in Figure 5a,b. It can be seen that, contrary to the study by Liu et al [30], no clear relationship between yield stress and grain size is observed for neither polycrystalline Mg nor Al at relatively low strain rates, while the elastic modulus rises with increasing grain size and the toughness increases with decreasing grain size. Similar to the stress fluctuations in the stress rise section of the Al/Mg layered composites mentioned earlier, the structural evolution of polycrystalline Mg is further analysed, as shown in Figure 6, where the stacking faults within the grains at a strain of 6.5% have been nucleated, the stress first reaches the peak, and then the stress decreases.…”

“…The deformation behaviour of the polycrystalline system was simulated using the open-source Lammps software (LAMMPS 64-bit, version 3Nov2022-MPI; Simulation tool for particle-based materials modeling at the atomic, meso, and continuum scales; Sandia National Laboratories: Los Alamos, NM, USA, 2022) [ 44 ], and the results were analysed and visualised with Ovito [ 45 , 46 ], using the dislocation analysis (DXA) method and the common neighbour analysis (CNA) method. In Liu [ 30 ] and other recent studies, the Hall–Petch effect has been observed in Al and Mg polycrystalline models with a grainsize under 10 nm during tensile loading. The simulation boxes for polycrystalline Al and polycrystalline Mg were constructed with 10 nm length in each direction, containing 2, 4, 8, 16, and 32 grains at random, respectively.…”

“…By understanding how different factors, such as layer thickness, composition, and interface characteristics, influence the plastic deformation, strategies can be developed to enhance the composite’s resistance to deformation, and the microstructure and properties of composites can be tailored to meet specific application requirements [ 24 , 25 ]. Extensive research efforts have been devoted to investigating the plastic deformation behaviour of materials on the nanoscale under various conditions, including grain size [ 26 , 27 , 28 , 29 , 30 , 31 , 32 ], strain rate [ 31 , 32 , 33 ], grain boundaries (GBs), and interfacial interactions [ 5 , 23 , 34 , 35 , 36 , 37 , 38 ]. For instance, Liu [ 30 ] utilised the molecular dynamics (MDs) method to examine the plastic deformation phenomenon of nano-polycrystalline Mg, and observed the inverse Hall–Petch relation at the grain sizes below 10 nm.…”

“…Extensive research efforts have been devoted to investigating the plastic deformation behaviour of materials on the nanoscale under various conditions, including grain size [ 26 , 27 , 28 , 29 , 30 , 31 , 32 ], strain rate [ 31 , 32 , 33 ], grain boundaries (GBs), and interfacial interactions [ 5 , 23 , 34 , 35 , 36 , 37 , 38 ]. For instance, Liu [ 30 ] utilised the molecular dynamics (MDs) method to examine the plastic deformation phenomenon of nano-polycrystalline Mg, and observed the inverse Hall–Petch relation at the grain sizes below 10 nm. Xue et al [ 38 ] investigated the deformation process in nano-polycrystalline Mg through MD simulations and transmission electron microscope (TEM) experiments.…”