Formation of fine-grained structure during high-temperature severe plastic deformation of high-strength aluminum alloy (overview)

O.S Sitdikov1
1Institute for Metals Superplasticity Problems, Khalturina 39, Ufa 450001, Russia
An overview of the author's works devoted to the study of features of plastic flow and "continuous" dynamic recrystallization, occurring in the as-cast high strength aluminum alloy 7475 (Al - Zn - Mg – Cu system) during multidirectional forging, carried out up to strain of ~9 at the temperatures of 250 - 490оС (~ 0.5 – 0.85Тm) and the strain rate 3x10-4 s-1, is represented. At the temperatures 250 - 400оС and relatively low strains, the plastic flow is characterized by the apparent value of the stress exponent n ~ 9, the main mechanism of the plastic deformation is dislocation gliding, controlled by dynamic recovery. The mechanism of grain refinement is related to formation of deformation bands, such as geometrically necessary boundaries and/or microshear bands, which develop in various directions and subdivide original grains. With increasing strain, the gradual increase in the number and misorientation of the band boundaries result in transformation of the latter to high-angle boundaries and evolution of fine-grained structure in the place of bands. The kinetics of the new grain formation is accelerated significantly with increasing the temperature of multidirectional forging. At 490оС, when n~3, grain boundary sliding starts to play an important role in grain refinement. It occurs non-uniformly in a coarse-grained microstructure, thereby causing the localization of plastic flow and formation of microshear bands even in the early stage of deformation. The grain boundary sliding provides also the rapid increase in misorientation of deformation-induced subboundaries during deformation and their transformation to new high-angle boundaries. The mechanisms of new grain formation at various temperatures of multidirectional forging are discussed in detail.
Received: 19 March 2015   Revised: 26 March 2015   Accepted: 27 March 2015
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