Continuous measurement of m-parameter for Hart analysis of superplastic behaviour of ultra-fine grained AZ31 Mg alloy

J. Stráský1, J. Stráská1, M. Janeček1*
1Charles University in Prague Ke Karlovu 5 121 16 Praha 2
The ultra-fine grained materials (UFG) often show superplastic behaviour at elevated temperatures due to small grain size. On the other hand, UFG microstructure is often not stable at increased temperatures and undergoes recovery and recrystallization. In this paper, we investigate the superplastic properties of UFG AZ31 Mg alloy prepared by equal channel angular pressing. The computer controlled tensile tests were performed in a temperature range of 175°C−250°C and at strain rate around 10-4 s-1. The superplastic behaviour and strain hardening is related to temperature, strain-rate and microstructure stability. Methodology of continuous measurement of m-parameter during the tensile test to achieve the dependence of m-parameter on true strain is presented. The measurement is based on alternating two slightly different true strain rates, keeping the overall true strain-rate constant. The m-parameter decreases with true strain for all studied conditions from approx. 0.5 to 0.3 and the decrease is fastest for the highest considered temperature (250°C), which is attributed to recovery and recrystallization. The sample deformed at 250°C strain hardens over significantly shorter interval of true strain which also suggests microstructure changes. Limits of plastic instability according to Hart analysis were computed for all conditions. Hart analysis of stability of plastic deformation seems not to be sufficient for description of elongation until fracture for studied material.
Accepted: 14 April 2015
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