Superplasticity of 5083 alloys with Zr and Mn additions produced by reciprocating extrusion

In this study, 5083 aluminum alloys modified with grain refiner, 0.25% Zr and 0.46% Mn, were processed by reciprocating extrusion to yield high-strain-rate superplasticity above 400 °C and superior room-temperature mechanical properties. Without any prior homogenization treatment, 10 extrusion passes could give the cast billets an equiaxed grain structure with a grain size of about 4.5 μm and a subgrain size about 0.2 μm, and a uniform distribution of fine inclusions and dispersoids in the matrix. The fine-grained structure was stable up to 525 °C, giving the alloy a high-strain-rate and low-stress superplasticity over a wide operating temperature of 400–500 °C. In the tensile test at 500 °C, a maximum elongation of 1013% and a low flow stress of 7.7 MPa at 5 × 10−2 s−1 were achieved. The apparent and true activation energies for low temperatures (300–400 °C) without high-strain-rate superplasticity were 220.6 and 208 kJ/mol, respectively, whereas those at high temperatures (400–500 °C) were 88.4 and 98.7 kJ/mol, respectively. Further analysis confirms that grain boundary sliding is the dominant mechanism over the high-strain-rate region from 1 × 10−2 to 5 × 10−1 s−1 at 500 °C, and power-law breakdown mechanism occurs over the strain rate from 5 × 10−4 to 1 × 10−2 s−1 at 300 °C. The high-strain-rate superplasticity was more strongly enhanced by Zr addition than addition of Cr and Mn. Two enhancing mechanisms for the maximum superplastic elongation and the optimum strain rate are proposed. This study concludes that the effectiveness of Zr is caused by the fineness and the coherency of Zr-rich dispersoids in the matrix.