Effect of pressing temperature on fine-grained structure formation in 7475 aluminum alloy during ECAP

The effect of pressing temperature on microstructural development under ECAP conditions was studied in an as-cast 7475 aluminum alloy at temperatures from 523 to 673 K. The samples were pressed by using route A up to strains of 12. New equiaxial grains with high angle boundaries (HABS) start to be formed after three passes and then gradually developed with repeated ECAP, finally, followed by fully development of a new fine-grained structure. The average (sub)grain size tends to increase and the aspect ratio decreases with increasing pressing temperature. The misorientation distribution of strain-induced dislocation boundaries shows a single peak type at low angle misorientations until second pass and then gradually shifts to higher angles with increasing of strain. Increase in pressing temperature retards the transformation kinetics from low angle boundaries to high angle ones, while the fraction of the latter as well as the average misorientation approach a same saturation value irrespective temperature in high strain. The mechanism of ultra-fine grain formation during ECAP is discussed.