The Development of Interatomic Potentials and Their Applications in Unveiling the Deformation Mechanisms of Mg Alloys

Abstract

Plastic deformation is a key factor affecting the performance of magnesium (Mg) alloys as structural materials. However, our understanding of the intricate relationship between stacking faults, dislocation core structures, twins, precipitates, and solute interactions is still incomplete. This work aims to bridge this knowledge gap through the use of multiscale simulations, including molecular dynamics (MD), atomistic first-principles modeling, and Monte Carlo (MC) simulations. These approaches enable the bridging of time and length scales and provide valuable insights into the complex deformation mechanisms in Mg alloys.

Accurate interatomic potentials that can reproduce the essential characteristics of materials are vital for conducting meaningful and insightful MD simulations. We thus first developed interatomic potentials for Mg and Mg alloys. The training of the interatomic potentials is performed within the framework of the extended modified embedded atomic method (XMEAM), which is developed from the modified embedded atomic method (MEAM) potentials. The particle swarm optimization (PSO) algorithm is used to automatically select the optimal potential parameters. In particular, the developed Mg-Y potential successfully captures the solute-solute pair interactions in Mg, a crucial property that many other potentials struggle to reproduce. The potential also successfully predicts the solute-induced pyramidal II to pyramidal I dislocation transitions and Y-induced {1 1 2 1} twin nucleation. The XMEAM Mg-Al potential also captures most bulk properties of pure Al, such as the surface energies of the (1 0 0), (1 1 0) and (1 1 1) planes and the self-interstitial atom formation (SIA) energy of FCC-Al. Additionally, the Mg-Al potential reproduces the key characteristics of Mg-Al alloys, such as solute-solute pair interactions, solute-dislocation interactions, and solute-twin interactions. The development of these accurate interatomic potentials lays a crucial foundation for studying the deformation mechanisms of Mg alloys.

With the developed XMEAM interatomic potentials as the foundation, we started the investigation from dislocations in pure Mg. With the consideration of energy sustainability, the non-alloying method has gained attention. Consequently, the strengthening of dislocations has emerged as an important approach to improving the strength and toughness of pure Mg. Therefore, a thorough investigation of dislocation dynamics in pure Mg was conducted, which led to the successful unveiling of the deformation mechanisms of dislocation loops in this material. The results of the study show that the ⟨𝐜 + 𝐚⟩ dislocation loop on the prism plane initially expands when an external shear stress is applied and then undergoes cross slip to the pyramidal I plane at the critical resolved shear stress. Subsequent increase in shear stress induces a double cross slip to pyramid II plane. In addition, the prismatic ⟨𝐜+𝐚⟩ dislocation loops with hexagonal shape on the basal plane can also cross slip to pyramidal II plane with lower critical resolved shear stress. These ⟨𝐜 + 𝐚⟩ dislocation loops can be activated and provide a sustained source of mobile ⟨𝐜 + 𝐚⟩ dislocation slip through cross-slip or double cross-slip during subsequent tensile deformation, resulting in large tensile ductility in pure Mg. This dislocation engineering approach, which leverages the behavior of dislocation loops, has the potential to replace the need for alloying elements in Mg alloys, promoting the sustainable development of pure Mg and Mg alloys.

Besides dislocations, deformation twins can also be activated as a secondary deformation mechanism in Mg alloys. By using the developed XMEAM Mg-Y and Mg-Al potentials in the atomistic simulations of a blunted crack under Mode II loading, we show that only 3.0% Y can activate {1 1 2 1}⟨1 1 2 6⟩ twin mode and make it competitive to the regular {1 0 1 2}⟨1 0 1 1⟩ extension twin in Mg. Using the reduced-constraint (RC) slip path and DFT calculation, we show that in the HCP structures, the localized slip occurring in the ⟨1 1 2 6⟩ twinning direction leads to simultaneous shear and ⟨1 1 0 0⟩ directional displacements in the corrugated {1 1 2 1} twinning plane. Such shifts play a crucial role in reversing the asymmetry of the {1 1 0 0} atomic planes to fulfill the {1 1 2 1}⟨1 1 2 6⟩ twin symmetry. However, in pure Mg and Mg-Al alloy, these displacements are not sufficient, resulting in a high energy barrier for twin nucleation via the multilayer slip path. The presence of Y atoms, whether in a random distribution with statistical fluctuations or with short-range ordering (SRO), amplifies slip-induced redistribution to a level that effectively reverses the asymmetry of the {1 1 0 0} atomic levels. This slip-controlled, solution-modulated mechanism of twin nucleation is expected to apply generally to HCP metals and alloys.

Finally, the plastic deformation of Mg alloys is also strongly influenced by the presence of SRO structure and precipitates. Molecular dynamics, supported by the MC algorithm, is well suited for modeling and studying these mechanisms. Although the embedded atomic method (EAM) potential has successfully implemented a parallelizable MC scheme, it is inaccurate for modeling and simulating solute precipitation or segregation in Mg alloys because it does not accurately capture SRO structures. The MEAM or XMEAM potential is limited to single-threaded operations, which is time-consuming and impractical for simulations with large dimensions required for the study of precipitation or segregation phenomena of solutes. This motivated us to develop a parallelizable MC algorithm specifically designed for MEAM/XMEAM potentials. Since EAM and XMEAM have similar basic principles and differ only slightly in their formulation, we can implement the parallelizable MC scheme of EAM to XMEAM. This progress has led to a significant acceleration in the computational speed of MC simulations. We have also simplified the energy calculation of XMEAM to further speed up the calculations. In the MC simulations, we only calculate the energy change caused by the exchanged atom instead of iterating over all atoms. This simplified method has no influence on the final results but it significantly improves the calculation speed. By employing the parallelizable MC algorithm and using high-precision XMEAM potentials, we have achieved successful predictions of solute segregation in Mg alloys. This breakthrough enables more efficient and accurate simulations that provide valuable insights into the behavior of solute precipitation and segregation in Mg alloys.

Future work will focus on studying the interactions between SRO or precipitates, solutes, dislocations, and twins, and their collective impact on the deformation mechanisms of alloys. This knowledge will contribute to the rational design of advanced metallic materials with tailored mechanical properties, opening up new possibilities for applications in various industries, from automotive and aerospace to energy and biomedical fields.

Date of Award	22 Nov 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhaoxuan WU (Supervisor)