Abstract
In the present work, the effect of hot rolling (at temperature 300 °C/60 min) on mechanical, thermal properties, microstructure and recrystallization behaviour of the commercial AlZnMgCuFe(ScZr) (7075-Sc,Zr) alloys was investigated. Microhardness together with differential scanning calorimetry measurements was compared to microstructure development that was observed by transmission and scanning electron microscopy and electron backscatter diffraction.
Microstructure observation proved two types of eutectic phase (T-phase and Mn,Fe,Si-containing phase) at (sub)grain boundaries in hot-rolled alloys. The grain size was ~ 20 μm in the AlZnMgCuFeScZr and ~ 1000 μm in the AlZnMgCuFe alloy.
Sc,Zr-addition in hot-rolled AlZnMgCuFe(ScZr) alloys significantly refined the grains. Primary multilayer Al3(Sc,Zr) particles precipitated during casting and subsequent cooling in the alloy with Sc,Zr-addition.
These particles were stable up to 450 °C. No changes of microhardness values (in accuracy of measurement) from room temperature up to 450 °C in the alloys were observed there.
Thus, the probable presence of these secondary Al3(Sc,Zr) particles is most likely the reason of hardening (ΔHV ALMOST EQUAL TO 20) of the AlZnMgCuFeScZr alloy. A little higher electrochemical potential was measured for AlZnMgCuFeScZr alloy (- 1144 mV) than for AlZnMgCuFe alloy (- 1171 mV).
One exothermic process was observed in hot-rolled alloys; apparent activation energy of this process was calculated as ~ 100 kJ mol-1. Additional precipitation and/or coarsening of S-phase together with precipitation of T-phase are probably connected with the exothermic process.
Sc,Zr-addition has a significant antirecrystallization effect. No recrystallization was observed in the hot-rolled AlZnMgCuFeScZr alloy, neither after annealing at 350 °C/10 h nor after annealing at 450 °C/10 h.
A partial recrystallization effect of the hot-rolled AlZnMgCuFe alloy is clearly registered after annealing at 350 °C/10 h and occurs very strongly after annealing at 450 °C/10 h.