Micromechanical model of nanoparticle compaction and shock waves in metal powders
AE Mayer, INTERNATIONAL JOURNAL OF PLASTICITY, 147, 103102 (2021).
A micromechanical model of metal powder compaction is developed as a generalization of the previous model proposed by Mayer et al. (2020). The model is applicable for an arbitrary axisymmetric deformed state including the uniaxial compression and the uniform tri-axial one. It describes the stage of compression of initially spherical particles, the percolation transition to the system of isolated pores and the stage of the collapse of pores. At the first stage, the metal particles are approximated by spheres with indented sectors with two independent indentation depths: One along the dedicated axis and another one along the perpendicular directions. The model of porous medium is formulated in finite deformations and the dislocation-driven plasticity model (Krasnikov and Mayer, 2015) is used to describe the pore collapse at the last stage. MD simulations of uniform tri-axial and uniaxial compression of iron, copper and magnesium nanoparticles of 6, 12 and 18 nanometers in diameter are performed for a number of temperatures. Activity of dislocations, which form dense structures, is the main driver of the plastic deformation of nanoparticles during compaction, while the presence of twins is revealed for copper and magnesium. MD data is used for identification of the model parameters and verification of the model. Bayesian algorithm is implemented for parameter identification, which reveals itself as an efficient tool and allows us to fit the model with the same set of parameters to MD data for all investigated temperatures, particle diameters and modes of compression. The developed and parameterized micromechanical model is embedded into the continuum model of shock wave propagation in metal powder as a constitutive model of the substance. Both partial and complete compaction by the shock wave is investigated by an example of iron. The presence of elastic precursor is revealed for the case of nanopowders because of high yield strength of nanoparticle material achieved due to the strain hardening during the preliminary relaxation under the action of surface tension. At the same time, the precursor is negligible for submicro - and microparticles. Velocities of both the precursor and the plastic shock wave are considerably lower than the sound speed of the bulk metal. This is because the average elastic modulus of powder during compaction is much smaller than that for a compact material.
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