Mechanism of Grain Refinement in Aluminium in the Process of Hydrostatic Extrusion

Technically pure aluminium was subjected to three passes by the process of hydrostatic extrusion (HE) to a true strain of 3.8. TEM observations of the microstructure revealed that the first pass of the process refined the grains to 0.6 μm, but the mean grain size remains unaffected by further strain. The microstructural evolution over the strain range from 1 to 4 proceeds through the recovery of grain interiors. Significant changes have been found in grain boundary characteristics. The fraction of high angle grain boundaries profoundly increases as the true strain increases. Various mechanisms for their formation and their relevance to HE are discussed