The effect of temperature on strain-rate sensitivity in high strength Al–Mg alloy sheet

Comprehensive experimental work performed to assess the temperature and strain-rate affected deformation behaviour of commercial AlMg6 type, 1.0 mm thick annealed sheet. Elevated temperature tension test performed at temperatures ranged from 25 to 300 °C, and three cross-head rates, giving initial strain rates of ε̇1=6.7×10−4 s−1, ε̇2=6.7×10−3 s−1 and ε̇3=6.7×10−2 s−1. Steady state strain-rate sensitivity (SRS) parameters were calculated (m=d ln σ/d ln ε̇) for the strain-rate ratios of ε̇1/ε̇2 (1:10), ε̇2/ε̇3 (10:100). It was shown that in the tested AlMg6 alloy sheet the SRS is negative and decreasing type around the room temperature due acting the dynamic strain ageing (DSA). It becomes positive at higher temperatures when the DSA weakening. This transition temperature increases by strain rate. The monotonic increase of the SRS brought by temperature is assumed to be due the enhancement of dynamic recovery and it appeared to be independent on strain rate, except at 300 °C. At 300 °C decreasing the strain rate brings considerable increase of the SRS. The experienced increase of the terminal m values from ∼0.15 to ∼0.4 for the applied strain-rate ratios of ε̇1/ε̇2 (1:10) and ε̇2/ε̇3 (10:100), respectively, is assumed to be the result of changing the deformation mechanism from recovery to diffusion-controlled solute drag. However, the performed research has shown that the attained increase of the SRS was not followed with appropriate ductility improvement. Analysis of the strain localization (necking) has shown that increasing the strain rate from 6.7×10−4 to 6.7×10−2 s−1, the temperature brought softening can be compensated, and the process of strain localization (necking) shifted to higher strains.