Precipitation Processes in Hot-Rolled Al-Mn-Sc-Zr Alloy

The effect of hot rolling on mechanical and electrical properties, microstructure and recrystallization behaviour of the AlMnScZr alloy was studied. The mould-cast alloy and the alloy after hot rolling at 300 °C was studied during step-by-step quasilinear annealing from 200 °C up to 600 °C with heating rate 100 K/h followed by subsequent isothermal annealing at 600 °C/5 h. Precipitation reactions were studied by electrical resistometry, differential scanning calorimetry and hardness measurements. Transmission electron microscopy and electron backscatter diffraction examination of specimens quenched from temperatures of significant resistivity changes were used to identify microstructural processes responsible for these changes. Only occasional irregular sharp-edged polygonal particles of the AlMnFeSi system were found in the as-prepared state of the mould-cast alloy. The as-prepared state of the hot-rolled alloy was characterized by a dispersion of fine coherent Al3Sc and/or Al3(Sc,Zr) particles and furthermore a fine (sub) grain structure was observed. The hardening effect in the alloys is due to presence and/or precipitation of the Sc,Zr-containing particles with L12 structure. The distinct resistivity changes of the alloys are mainly caused by precipitation of Mn-containing particles. Two-stage development of the Al6Mn phase (in (sub) grain interiors and at (sub) grain boundaries) in the hot-rolled alloy was observed. The presence of Sc,Zr-and Mn-containing particles has an anti-recrystallization effect that prevents recrystallization minimally up to 600 °C and annealing of 1 hour in the hot-rolled alloy. The apparent activation energy for the Al3(Sc,Zr)-phase and Al6Mn-phase precipitation was also determined. The activation energy values obtained in the hot-rolled AlMnScZr alloy are comparable to those observed in the hot deformed AlMnScZr alloys prepared by powder metallurgy.