Microstructural characteristics and superplastic ductility in a Zn-22% Al alloy with submicrometer grain size

Submicrometer grain sizes were introduced into a Zn-22% Al eutectoid alloy using two different procedures: torsion straining and equal-channel angular (ECA) pressing. Microstructural examination showed that torsion straining gives an essentially equiaxed grain configuration with some mixing of the two separate phases. After ECA pressing to a strain of ∼8 at a temperature of 373 K, there is a submicrometer grain size but with agglomerates of ultrafine Al-rich and Zn-rich grains which are formed by the separate division of the original grains into smaller submicrometer grains with only very limited mixing of the two phases. Tensile testing of the ECA pressed material gave neck-free superplastic flow but, except only at 473 K at the fastest strain rate of ∼10−1 s−1 where the elongation was unusually high, the elongations to failure were similar to those reported earlier for a commercial alloy with a grain size of ∼2.5 μm. The results demonstrate the need to develop an ECA pressing procedure which avoids the formation of agglomerates of the two phases.