Grain refining of Zn-22 wt.% Al superplastic alloy

摘要

The superplastic deformation behaviour of two alloys of Zn-22 wt.% Al was studied in detail under identical conditions of grain size, temperature and initial strain rates. The second alloy has the same composition as the first alloy, but with theaddition of 0.2 wt.% Al-5Ti- 1B as a grain refiner. The two alloys were subjected to a special heat treatment process (solid solution heat treating, quenching and annealing) to obtain fine equi-axed grains. Thermomechanical treatment was carried out onthe equi-axed structure that was obtained to produce extra-fine grains. Tensile tests at superplastic temperature revealed that extremely large superplasticity of this alloy can be obtained mainly with extra fine grains which has an affect, on the otherhand, on the initial strain rate a', the grain size being decreased as the initial strain rate is increased. A greater superplastic elongation value can be obtained with higher strain rate and smaller grain size. Study of fracture surfaces of fourspecimens having maximum superplastic elongation revealed a granular fracture for this alloy.