Using ECAP to achieve grain refinement, precipitate fragmentation and high strain rate superplasticity in a spray-cast aluminum alloy

An aluminum 7034 alloy, produced by spray casting and with an initial grain size of ∼2.1 μm, was processed by equal-channel angular pressing (ECAP) at 473 K to produce an ultrafine grain size of ∼0.3 μm. It is shown that the rod-like MgZn₂ precipitates present in the as-received alloy are broken into very small spherical particles during ECAP and these particles become distributed reasonably uniformly throughout the material. The presence of these fine MgZn₂ particles, combined with a distribution of fine Al₃Zr precipitates, is very effective in restricting grain growth so that submicrometer grains are retained at elevated temperatures up to at least ∼670 K. Tensile testing of the pressed material revealed high elongations to failure, including elongations of >1000% when testing at a temperature of 673 K at initial strain rates at and above 10 ^-2 s^-1. These results confirm the occurrence of high strain rate superplasticity in the spray-cast alloy.