The Effect of Alloying on the Structure Formation during Compaction of Microcrystalline Aluminum-Silicon Alloys
The microcrystalline structure of aluminum-silicon alloys is obtained when high cooling rates (more than 104 K.s-1) are applied, which results in a highly non-equilibrum state in the form of suppersaturate solid solution. A product is obtained in the form of ribbons which are less than 100 μm thick. These fine ribbons are usually subjected to consolidation by cold isostatic compaction followed by hot extrusion at relatively high temperatures (above 400°C), during which phase transformations (decomposition of the supersatursted solid solution) and coarsening of the structure occur and this results in deterioration of the properties. The purpose of recent work is to study the structure formation at lower temperatures. These data will allow the development of technologies that retain as much as possible the finegrained two phase structureas after the applied heat treatment.The microstructures of the alloys are examined with a Reichert MeF2 optical microscope and the average area of the silicon particles (S, μm2) is determined as a measure of the structure dispersion. Particular stages in structural change are determined, both by X-ray analysis of crystal lattice parameters of the alluminium solid solution, and by the Perkin-Elmer DSC-2 Differential Scanning Calorimeter, with transient heating. X-ray tests are performed with a powder diffractometer DRON-3 (CuKαfiltered emission, scintillation registrtion, continuous recording on a chart band).The lattice parameter variatiosn are used to examine the kinetics of structural changes in the microcrystalline state. The resulting curve shape suggests that the lattice parameter follows a parabolic dependence. A value of 94.6 kJ.mol-1is obtained for activation energy of the decomposition of the solid solution at lower temperatures which was explained with acceleration of the Si diffusion process, due to the defects in the structure of the aluminum matrix.
In the case of high temperature annealing at 400-500°C the activation energy of the process is 135kJ.mol-1 which was explained with the decomposition of the supersaturated solid solution. Coarsening process can thus be devided into two stages. During the first stage the particles reach size of several tens of nm. During the second stage, the average size of the silicon phase is in the micronial area. The temperature efect requires special measures to reduce the microstructure coarsening. One of the possible ways is via an additional alloying which is object of this investigation.
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