Structure and hardness of 1570C aluminum alloy after room- and cryogenic temperature rolling and electrical pulsing

The effects of rolling temperature reduced to the cryogenic range and the energy of subsequent high-dense electric pulsing (HDEP) on the structure and hardness of a commercial alloy 1570C (Al-5.0Mg-0.18Mn-0.20Sc-0.08Zr-0.01Fe-0.01Si, wt.%) containing nanoscale Al3(Sc, Zr) precipitates were investigated. The homogenized ingot was isothermally rolled at room temperature and liquid nitrogen temperature (−196°C) with a total reduction of 75 – 80 %, followed by HDEP with a 100 µs pulse and integral current density Kj in the range of 0.12 – 1.34 ×104 A²s / mm4. It was found that room temperature rolling (RTR) and cryogenic rolling (CR) increased the hardness of alloy from 105 to 140 and 150 HV, respectively. RTR resulted in the formation of well-defined cell / deformation band structures with misorientations up to 5 –10° associated with microshear bands. Under CR, a weaker misoriented structure of the same type was formed. The latter was characterized by a much lower density of band boundaries whose misorientations rarely exceeded 5°, while the dislocation density and coherent domain size were more significantly altered by CR, indicating a higher structural non-equilibrium. This structural difference was due to a more homogeneous slip with a greater number of active slip systems dominating during cryogenic straining. In the subsequent HDEP, the alloy was softened in both work-hardened conditions due to static recovery and recrystallization. Therewith the dependence of hardness on pulse energy shifted to higher Kj values for the RTR specimens, indicating their greater thermal stability. For the CR alloy, recovery was more pronounced than recrystallization in the Kj range of 0.28 to 1.05×104 A²s / mm4, while at Kj =1.12×104 A²s / mm4, softening was mainly due to recrystallization. Meanwhile, for the alloy rolled at room temperature, recovery was still more prevalent in all the HDEP intervals mentioned. In its softening, the transformation of deformation bands into more balanced (sub)grains played a key role, while recrystallization started only when Kj reached 1.25 ×104 A2s / mm4.