Hot deformation behaviour of Mg-3Al alloy-A study using processing map

Abstract

The hot deformation behaviour of Mg–3Al alloy has been studied using the processing-map technique. Compression tests were conducted in the temperature range 250–550°C and strain rate range 3×10⁻⁴ to 10² s⁻¹ and the flow stress data obtained from the tests were used to develop the processing map. The various domains in the map corresponding to different dissipative characteristics have been identified as follows: (i) grain boundary sliding (GBS) domain accommodated by slip controlled by grain boundary diffusion at slow strain-rates (<10⁻³ s⁻¹) in the temperature range from 350 to 450°C, (ii) two different dynamic recrystallization (DRX) domains with a peak efficiency of 42% at 550°C/10⁻¹ s⁻¹ and 425°C/10² s⁻¹ governed by stress-assisted cross-slip and thermally activated climb as the respective rate controlling mechanisms and (iii) dynamic recovery (DRV) domain below 300°C in the intermediate strain rate range from 3×10⁻² to 3×10⁻¹ s⁻¹. The regimes of flow instability have also been delineated in the processing map using an instability criterion. Adiabatic shear banding at higher strain rates (>10¹ s⁻¹) and solute drag by substitutional Al atoms at intermediate strain rates (3×10^-2 to 3×10⁻¹ s⁻¹) in the temperature range (350–450°C) are responsible for flow instability. The relevance of these mechanisms with reference to hot working practice of the material has been indicated. The processing maps of Mg-3Al alloy and as-cast Mg have been compared qualitatively to elucidate the effect of alloying with aluminum on the deformation behaviour of magnesium.