A. CHALAY-AMOLY - School of Metallurgical & Materials Engineering, University of Tehran, North Kargar, Tehran, Iran
AZARE HANZAKI - Department of Metallurgical and Materials Engineering, University of Tehran, Iran
S. KHODDAM - Mechanical and Aerospace Engineering Department, Monash University, Clayton, Victoria, Australia
M. FATEMI-VARZANEH - Departmen ofMetallurgical and Materials Engineering, University of Tehran, Iran

AZ91 (Mg-9Al-1Zn) cast alloy was severely deformed through backward extrusion processing at 450°C. The microstructure evolutions have been precisely investigated using optical microscopy and scanning electron microscope. A bimodal grain size distribution has been achieved where fine grains of 1-2 um mean grain size embedded within the elongated coarse grains. Different recrystallization mechanisms are found to occur during back extrusion of the experimental alloy. The new grains preferentially nucleated at the prior grain boundaries. The recrystallized grains are observed within twins and shear bands. The latter is considered as a result of extended dynamic recovery inside the bands which may lead to the formation of new grain boundaries. Moreover, the stimulated nucleation of new grains has been traced at the vicinity of y type precipitates. It is suggested that the fragmented precipitates into the finer particles may have remarkable pinning effect on the growth of new grains.

AZ91 alloy; backward extrusion; dynamic recrystallization; shear band; particle stimulated nucleation