As a result of computer experiments carried out in the framework of the method of granular dynamics, for model systems that correspond to nanoscale aluminum oxide powders with a weak and strong tendency to agglomeration, energy costs are calculated for compaction under various conditions: comprehensive compression, two - and uniaxial compression, shear compression. It is found that the work of pressing depends significantly on the conditions of the process and is not an unambiguous function of the powder density. This suggests that nanopowders in addition to the family of loading surfaces must be independently characterized by a family of iso-levels of "plastic potential", which correspond to certain values of the pressing. The data are calculated numerically for the systems under study and approximated by analytical expressions. It is established that the known associated rule of plastic flow is not applicable to nanoscale oxide powders. Instead, another criterion is proposed, which allows to predict the nature of deformation processes in the system, according to which the type of strain tensor is determined not only by the direction of the gradient of the plastic potential (the associated law), but also by the type of stress tensor in the process of compaction.
A software package was developed and computer experiments were carried out to simulate the processes of compacting nanoscale powders taking into account certain distributions of powder particles by size. In two-dimensional geometry, model systems with bimodal particle distribution are analyzed, where the variable parameter is the ratio of the diameters of two different types of particles, and systems with a lognormal distribution characteristic of oxide nanopowders obtained in the IEP, UB of RAS.
A package of programs for simulation of dynamic processes of magnetic pulse pressing in the framework of granular dynamics has been developed. The theoretical analysis, allowed to enter in the numerical scheme of the relaxation mechanisms of interparticle interaction. The corresponding additional characteristics have the form of viscous forces, and are included in all contact interactions (normal and tangential forces, torsional and rolling torques). Analytical relations for viscous forces arising in the contact area of two spherical elastic granules during their mutual displacement are obtained. The relaxation times of translational, shear and rotational modes of individual particles in the powder structure are estimated. The coefficient of shear viscosity of the particle material (aluminum oxide) is estimated from the known data on the attenuation of ultrasound, which gives values of the order of 10-3 Pa s. The characteristic decay time of the two solitary particles is about 3 ns. The attenuation time of collective oscillations in the powder system may be 2 – 4 orders of magnitude greater, i.e. comparable to the characteristic times of magnetic pulse processes (10 – 100 µs). The simulation shows a weak influence of the powder compact loading rate on the position of the compaction curves in the range of relative strain rates 104 – 107 per second.
