Effects of Grain Size on Plastic Behaviour at Room Temperature of Eutectoid Zn-22%Al Alloys

Abstract

The plastic behaviour at room temperature is investigated on the superplastic Zn-22%Al alloys with grain sizes of 0.1∼4.9 μm. The optimum grain size giving the maximum value of total elongation existed for each strain rate.
The strain rate sensitivity m have the maximum value at grain size of about 0.6∼0.8 μm, and the strain rate sensitivity disappears above 4 μm (m\fallingdotseq0). The total elongation increases with increase in m-value at the same grain size.
The maximum tensile stress is reached at 2∼5% strain, and the grain boundary sliding of mother phase α′ is observed in these strain ranges. The local elongation after the occurrence of the maximum tensile stress depends mainly on the α-β grain boundary sliding and on the deformation of the grains.
In the specimen of optimum grain size, at low strain rate, the total elongation is large because the α-β grain boundary sliding is remarkable, but at high strain rate, the specimen is brittle because the α-β grain boundary sliding does not occur and the intercrystalline cracks of α′-boundary is observed. In the large grain size alloy, the deformation of grains is observed at any strain rate.