Grain size effect is one of the key factors governing the dynamic mechanical response of metallic materials. Phase transformation iron is selected as the model material, and a series of nanocrystalline polycrystals with identical topology and grain orientation distributions but different grain sizes are constructed to investigate size effects under a fixed grain configuration. Molecular dynamics simulations show that, under high strain rate uniaxial compression, all models undergo elastic deformation, α→ε phase transition, and high-pressure phase plastic deformation. During the elastic stage, grain boundaries act as a soft layer, leading to lower stresses in the fine grain models than in the coarse grain model. After the structural phase transition, grain boundaries hinder the plastic development of the new phase, so that the fine grain models exhibit higher stress than the coarse grain models. At the onset of phase transition, the threshold of phase transition of smaller grains is lower, and the transformed phase in fine grains mainly forms stacking fault structures, whereas twinning structures appear in relatively larger grains. With increasing strain, the disappearance of twinning and the reconstruction of stacking faults are observed in large grains. Under high strain rate tension, shear strain of grain boundary in the large grain models is highly localized, readily forming continuous shear bands that serve as preferred paths of crack propagation. After grain refinement, shear strain of grain boundary gradually evolves into a diffuse mode, and the effective paths of crack propagation are constrained by the network of grain boundaries. The change of grain boundary effects leads to a non-monotonic variation of fracture strength with the grain size.

The optimization design of Al-Cu intermetallic compounds is crucial for the mechanical properties of Al-Cu alloys. Molecular dynamics (MD) simulation can provide microscopic processes of the mechanical behavior of Al-Cu alloys, and the interatomic potential is the key physical basis to ensure the reliability of the MD simulation. This work constructed a depth potential (DP) function for the Al-Cu system based on first-principles calculations, and compared the physical properties predicted by DP (crystal structure, energy-volume curve, pressure-volume curve, and phonon spectrum) with density functional theory (DFT) and embedded atom method (EAM) results. The generalization ability and accuracy of the DP model were verified. Based on the DP potential, MD simulations were conducted on the compression process of five Al-Cu intermetallic compounds (θ-Al₂Cu, θ′-Al₂Cu, Al₃Cu, Al₄Cu₉, and AlCu₄ phases). The characteristics and laws of yielding phenomena in structures such as θ-Al₂Cu, θ′-Al₂Cu and AlCu₄ were presented. The yield stress and shear stress of θ-Al₂Cu, θ′-Al₂Cu and AlCu₄ increase with the increase of strain rate, and the yield strain also increases correspondingly. This phenomenon arises from the enhancement of phonon drag obstruction to atomic slip. Among them, θ-Al₂Cu has the best compressive performance, yielding at a strain rate of 4×10⁹ s⁻¹ when compressed to 17.4%, with a yield strength of 51.15 GPa. Screw dislocations are produced, and the atoms slip along the [ $\overline{1}11$], [111] and [ $11\overline{1}$] directions. θ′-Al₂Cu yields when compressed to 10.0%, and the atoms slip in the plane perpendicular to the compression. Yielding occurs when the AlCu₄ phase is compressed to 13.4% and the atoms slip along the [401] and [ $40\overline{1}$] directions.