Formation Mechanism of Ultrafine-Grained Structure by ECAP

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1. It was possible to obtain grain sizes of 〜μm for pure A1, 〜0.5μm for A1-1%Mg and 〜0.2 μm for A1-3%Mg using equal-channel angular pressing. Addition of Mg was effective to grain refinement of A1. The refinement was achieved because of the reduction in dislocation mobility and to the consequent lower rates of recovery in the solid solution alloys.2. The direct measurements of sample temperature during ECA pressing revealed that the temperature increase is 〜30° for pure A1 and 〜70° for an A1-3%Mg alloy. There was a linear correlation between the temperature increase and the applied load. However, such temperature increases did not affect the fine grain structure during ECA pressing.3. The microstructure did not change irrespective of the pressing speeds within the range of 0.18 mm/min to 18 mm/mini.4. The addition of 0.2%Sc to an A1-3%Mg alloy retained the ECA-pressed small grain size up to a temperature of 450℃. This alloy exhibited the superplastich elongation more than 1000% at a relatively lower temperature of 300℃ with a high strain rate of 3.3x10ィイD1-2ィエD1s-ィイD11ィエD1.5. ECA pressing at elevated temperatures showed that subgrain structures are developed at and above the temperature of 200℃ for pure A1 and A1-3%Mg. However, fine grain structures with high angle grain boundaries were obtained even at a temperature of 300℃. The presence of thermally stable particles was important for the retention of the fine grain structures.