Plastic deformation in nanoporous aluminum subjected to high-rate uniaxial compression
MJ Diwu and XM Hu, ACTA PHYSICA SINICA, 64, 170201 (2015).
The mechanical behavior of nanoporous monocrystal aluminum subjected to uniaxial compressive loading at a rate of 2 x 10(9) s(-1) along 110 crystallographic orientation is studied using molecular dynamics simulations. Subjected to such a loading, nanovoids act as the effective sources of dislocation nucleation and emission, four of the twelve 111 < 110 > slip systems may be activated. With the same strain of 3.8%, dislocation nucleation will occur in both the sample of multiple voids and that with a single void. The configuration of multiple voids decreases the required stress for the onset of dislocation nucleation and emission in comparison with the sample with an isolated void of the same size. Because of the emission of trial partials, the accumulation of dislocation density can be changed into a piecewise linear process by the dislocation density propagation rate d rho(d)/d epsilon: in the initial stage of plastic deformation we obtain d rho(d)/d epsilon approximate to 1.07 x 10(18) m(-2), but this changes to d rho(d)/d epsilon approximate to 5.36 x 10(18) m(-2) at higher deformation. The velocity of dislocation is calculated to be subsonic and is a variable value during the plastic deformation. Dislocation loop pairs emit from the same void, glide and approach to each other, leading to the reduction of dislocation velocity. Then one loop of each pair continues to glide to intersect mutually and finally interact with the loops emitted from other voids, causing a strain hardening to reach the peak flow stress of 4.3 GPa. There is a post-yield softening corresponding to the onset of rapid dislocation density proliferation at higher dislocation densities. With the temperature evolution of the sample with multiple voids during plastic deformation, the density of mobile dislocations is calculated to be one magnitude lower than the total dislocation density. There is a decrease of mobile dislocation densities at large strains, showing that the mobile dislocation are diminished by the formation of dislocation forest and junctions. At the onset of their nucleation, the dislocations are all Shockley partials, however, when dislocation intersection happens, the majority are still Shockley partials, while the rest consists of Frank partials, perfect fcc dislocations and other dislocation ingredients. Voids collapse at the strain of 11.8%. No twins are found in the present simulation due to the high stacking-fault energy of aluminum. Prismatic dislocation loop emission is observed in this simulation.
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