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Development of a Temperature-Dependent Plasticity Model for Compaction of Aluminum Nanopowder with a Surface Oxide Layer to Enable Multiscale Thermo-Mechanical Simulation

Shekooh Haghighi, Mohammad Danesh | 2025

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58725 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Khoei, Amir Reza
  7. Abstract:
  8. A thermo-mechanical multi-scale framework is presented in this study for simulating the compaction process of aluminum nanopowders coated with a surface oxide layer. The primary objective is to link atomistic-scale information with continuum-scale models in order to improve the accuracy of predictions of material response under varied mechanical loading and thermal conditions. This approach enables analysis of the role of atomic-scale mechanisms in the overall material response and aims to achieve improvements in both predictive accuracy and computational efficiency relative to conventional methods. In the first step, the mechanical properties of the nanopowders were extracted at 300, 400, 500 and 600 K using atomistic molecular dynamics (MD) simulations employing the ReaxFF reactive force field. For each temperature, hydrostatic and triaxial tests were simulated to obtain stress–strain and stress–relative density curves. From these data, Young’s modulus, bulk modulus and Poisson’s ratio were computed as functions of relative density and temperature. The calculated Young’s modulus and Poisson’s ratio fall within the ranges of 28–108 GPa and 0.03–0.42, respectively. The results indicate that increasing confining (hydrostatic) pressure leads to material stiffening and a reduction in the strain-rate of change. The results indicate that an increase in confining pressure leads to material stiffening and a reduction in the rate of strain change, a phenomenon attributable to closer particle packing and reduced freedom of particle motion. Moreover, increasing temperature facilitates densification: for identical displacements, the stresses developed in the nanopowder decrease at higher temperatures. The relative density versus modulus of elasticity, relative density versus bulk modulus and relative density versus Poisson’s ratio curves also show noticeable temperature-dependent changes, in particular a significant change in slope at elevated temperatures. The computed values were compared with available experimental data, showing good agreement and thereby validating the adopted methodology. To transfer atomistic information to the continuum scale, a temperature-dependent double-surface plasticity model was formulated and its parameters were calibrated using the mechanical properties obtained from the MD simulations. Subsequently, a macroscopic analysis based on nonlinear thermo-mechanical finite-element simulations was carried out to simulate material behavior under compaction at different temperatures. Numerical simulations demonstrated that increasing temperature facilitates densification and, on average, results in a 10% reduction in the stress required to achieve higher densities. In summary, the proposed thermo-mechanical multi scale framework by transferring quantities extracted from molecular dynamics to temperature-dependent continuum models renders the compaction process of alumina coated aluminum nanopowders quantifiable. The findings indicate that increasing confining pressure promotes particle densification, restricts particle mobility, reduces strain rate changes and increases effective stiffness, while increasing temperature facilitates compaction and reduces the force required for a given displacement. Additionally, Young’s modulus, bulk modulus and Poisson’s ratio depend directly on density and temperature, with pronounced changes in the slopes of the relative density–modulus of elasticity, relative density–bulk modulus and relative density–Poisson’s ratio relationships at elevated temperatures. The temperature-dependent double-surface plasticity model, parameterized from atomistic data, successfully quantifies the influence of microscale mechanisms on macroscale response and shows substantial agreement with experimental observations. From an applied perspective, controlled heating of the nanopowder can reduce the required compaction force consumption and thus facilitate attainment of higher densities.
  9. Keywords:
  10. Multiscale Modeling ; Molecular Dynamics ; Temperature-Dependent Double-Surface Plasticity Model ; Nanopowder Compaction ; Aluminum with an Alumina Coating Nanopowders

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