Molecular dynamics insights into nanovoid behavior in metals: from sparsely-arranged nanovoids to densely-arranged nanopores

• Author / Creator

  Cui, Yi

• Atomistic-level study of void behavior in metallic materials is a difficult task for continuum-based methods. In contrast, MD method serves as an ideal tool for real-time computer simulation of all kinds of atomistic phenomena. More and more researchers become aware of this and a few have pioneered in the area of nanovoid simulation. Many problems were nicely addressed, yet not every stone has been turned. Particularly, we provided new understanding to the “shear impossibility” debate in light of our MD investigation. In this work, molecular dynamics simulation is applied to uncover mechanisms regarding the nucleation, growth and coalescence of nanovoids. Molecular dynamics results are examined by using the “relative displacement” of atoms. In doing so, the homogenous elastic deformation has been excluded. The “relatively farthest-travelled” (RFT) atoms characterize the onset of interfacial debonding and void growth due to dislocation formation. Our results indicate that the initiation of interfacial debonding is due to the high surface stress in an initially dislocation-free matrix. Through this approach, we also justified the feasibility of void growth induced by shear loops/curves. At a smaller scale, the formation and emission of shear loops/curves contributes to the local mass transport. At a larger scale, a new mechanism of void growth via frustum-like dislocation structure is revealed. A phenomenological description of void growth via frustum-like dislocation structure is also proposed. As for the shape effect, the simulation results reveal that the initial void geometry has substantial impact on the stress response during void growth, especially for a specimen with a relatively large initial porosity. During void coalescence, the void shape combination is found more influential than the intervoid ligament distance (ILD) on the strength and damage development. The critical stress to trigger the dislocation emission is found in line with the Lubarda model. The dislocation density calculated from simulation is qualitatively consistent with the experimental measurement. For densely-arranged pores, the diamond-array-pore sample exhibits a superior stress response at the same initial porosity. The onset of plasticity is investigated for differently-structured nanoporous samples, which could shed light on the novel designs of nanoporous structure with enhanced structural integrity. Main contributions of this work can be summarized as follows. First, we show that the shape and the arrangement of nanovoids have a great impact on the mechanical performance of nanoporous metals. Secondly, the “relative displacement” is employed to visualize atom movement during interfacial debonding and dislocation formation. Thirdly, the “shear impossibility” debate is preliminarily settled. Fourthly, the Lubarda model for critical stress to trigger dislocation emission is extended to the case of nanoporous geometry.

• Subjects / Keywords

  • Pore Arrangement
  • Void Coalescence
  • Molecular dynamics simulations
  • Void Growth Mechanism
  • Debonding

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R36688Z6K

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.