| Magnesium sheet for automotive applications is very attractive due to its light weight. The poor formability of magnesium and its alloys at room temperature, however, has limited the applications of these alloys. For this reason, at present, magnesium must be formed at elevated temperatures. This study investigates the hot deformation and fracture characteristics of Mg-1wt% Zn alloys containing a range of Al and Mn levels. Hot-rolled specimens were tensile tested over a range of strain rates and temperatures. Strain rate versus flow stress diagrams plotted on log-log scales revealed a transition in deformation mechanisms as a change in slope (the 'stress exponent'). Specifically, non-uniform deformation (i.e. necking) is observed at high strain rates, while uniform deformation is observed at low rates. This transition is accompanied by a change in fracture mechanism from dimpled rupture at high strain rates to cavitation and cavity interlinkage at low strain rates. Specimens which had a stress exponent of ∼2 and which failed through uniform deformation showing interlinked cavities have been associated with the grain boundary sliding (GBS) deformation mechanism. Specimens which had a stress exponent of ∼5 and which failed through necking showing a dimpled fracture surface have been associated with the dislocation creep deformation mechanism. Increasing aluminum appears to somewhat favour the GBS regime as indicated by a slightly decreasing stress exponent. Manganese also appears to favor the GBS regime, since the onset of cavitation appears at higher strain rates compared to alloys without Mn. |
