Grain refining with DDRX by isothermal MDF of Al-Zn-Mg-Cu alloy

Evolution of grain structure during isothermal multi-direction forging (MDF) of an Al-Zn-Mg-Cu alloy at 400°C and 450°C with strain rates from 10−3s−1 to 10−5s−1 was investigated with electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). The results demonstrated that the evolution of grain structure was dominated by discontinuous dynamic recrystallization (DDRX), during which the continuous forming of new recrystallized grains at grain boundaries was found. Characteristics of DDRX was enhanced with the decrease of MDF strain rate or temperature, which promoted the formation of recrystallized grains at grain boundaries. Besides, the increase of total strain enhanced such grain refinement effect until the grain structure reached the stable state, in which fine recrystallized grains replaced most of the initial coarse grains.