Equal channel angular extrusion has been used to deform an Al ± 3 wt-%Mg alloy to an effective strain of 10, resulting in a 0 . 2 mm grain size. In the as deformed condition the yield strength was increased to y500 MPa. During annealing the grain structure coarsened uniformly and the yield stress was found to follow the Hall ± Petch relationship, even in the submicron range. There was an abrupt transition in elongation at a grain size of y0 . 5 mm. Samples with smaller grain sizes showed no uniform elongation and limited ductility. For slightly greater grain sizes there was only a relatively small reduction in elongation, compared to a coarse grained material, while the yield stress was still increased by a factor of over four. Reducing the grain size to the submicron range led to far higher Lu Èders strains than are normally observed in Al ± Mg alloys.MST/4770
Superplastic testing of the aluminium-lithium alloy 8090 has been performed in the temperature range 485°C to 550°C. Up to 530°C, raising the test temperature has the effect of increasing the m-value and decreasing the flow stress. Changes in superplastic forming characteristics and microstructure have been followed during straining using a variety of techniques. Jump-strain rate testing reveals that in the optimum temperature range (515°C to 525°C).there is a steady decrease in m-value. Cavitation also occurs, but can be suppressed effectively by applying a hydrostatic pressure during testing. Raising the test temperature above this optimum range to 550°C produces a significant decrease in the m-value and a more rapid fall-off with strain. These observations are explained in terms of recrystallisation and grain growth, and the effectiveness of precipitates in pinning grain boundaries at the various temperatures.
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