Microstructure and mechanical properties of plates warm extruded AZ80Mg alloy fabricated by multi-directional forging in advance

Abstract

Rectangular-shaped AZ80Mg alloy was multi-directionally forged at decreasing temperature conditions from 623 K down to 433 K at an initial strain rate of 3.0 × 10-3 s-1 to a cumulative strain of ∑Δε = 5.6 at maximum. By multi-directional forging, Mg alloys with having various average grain sizes from 0.7 μm to 26.0 μm were prepared. Then, the multi-directionally forged Mg alloys were further extruded using dies at temperatures between 453 K and 523 K at initial strain rates of 3.0 × 10-4 s-1 and 3.0 × 10-3 s-1 to form plates. Even at such relatively low temperatures, extrusion of the ultrafine-grained Mg alloys could be satisfactory carried out. At some conditions, the values of strain rate sensitivity on loading stress became more than 0.3 indicating occurrence of superplasticity. This result suggested that effect of grain-boundary sliding enabled the warm temperature extrusion of the ultrafine-grained Mg alloys. In such conditions, where lower strain rate, higher temperature and ultrafine-grained structure, slight gain coarsening was also observed. However, the decrease in the strength was quite limited or sometimes further strengthening in contrast took place. This should be caused possibly by combined effects of i) further grain refinement due to dynamic recrystallization, ii) sharp basal texture evolution, iii) dynamic age hardening and iv) work hardening. From the above warm extrusion tests, high strength Mg alloy plates could be successfully produced, therefore, possibility of extrusion of ultrafine-grained Mg alloys with specific high strength at room temperature by employing a mechanism of superplasticity was suggested.