Ultra-Fine Grain Structures Of Model Al-Mg-Si Alloys Produced By Hydrostatic Extrusion

Microstructure and mechanical properties were studied in model Al - Mg - Si alloys (Al - 1 % Mg - 0.8 % Si and Al - 0.5 % Mg - 0.3 % Si - wt %) deformed by hydrostatic extrusion (HE) to strains of 1.4 and 3.8. In these alloys the different percentage of two hardening second - phase precipitates (Mg2Si and Si) were observed. The microstructure was characterized by transmission electron microscopy and optical microscopy. The microstructure of the alloys in the initial state was built of coarse grains of an average diameter of similar to 30 mu m. The refined microstructure was examined qualitatively and quantitatively using the stereological method and a computer image analysis. The deformation-processed structures evolved very rapidly, forming ultrafine grained (UFG) materials with grains of about 0.4 mu m. In addition, the grain refinement in the HE - treated materials has a substantial effect on their properties, such as the mechanical strength and micro-hardness which increase significantly. It has been found that, after epsilon=3.8 in the Al - 1 % Mg - 0.8 % Si alloy, the micro-hardness increases approximately twofold. The yield stress is more than four times higher in the UFG alloys, in comparison to the initial state. Similar results were identified in the Al - 0.5 % Mg - 0.3 % Si. This is due to the very rapid refinement of the microstructure during the deformation and presence of second - phase particles.