Changes in the diffusion properties of nonequilibrium grain boundaries upon recrystallization and superplastic deformation of submicrocrystalline metals and alloys

Effect of Sc, Hf, and Yb Additions on Superplasticity of a Fine-grained Al-0.4%Zr Alloy

“…The third factor contributing to higher superplastic characteristics of UFG alloys is the nonequilibrium state of their high-angle grain boundaries. As shown in [99,[110][111][112][113], during ECAP, HAGBs accumulate dislocation defects, leading to an increase in the free volume of grain boundaries and a decrease in the activation energy of superplastic deformation. During superplastic deformation of UFG alloys, their grain boundaries actively interact with lattice dislocations, thus triggering an increase in the free (excess) volume of high-angle grain boundaries [99,[111][112][113].…”

“…As shown in [99,[110][111][112][113], during ECAP, HAGBs accumulate dislocation defects, leading to an increase in the free volume of grain boundaries and a decrease in the activation energy of superplastic deformation. During superplastic deformation of UFG alloys, their grain boundaries actively interact with lattice dislocations, thus triggering an increase in the free (excess) volume of high-angle grain boundaries [99,[111][112][113]. Thus, ECAP provides all the necessary conditions conducive to the superplasticity effect in aluminum alloys.…”

“…It is usually assumed that  * = b, Deff = D0exp(Qeff/kT), where Qeff is effective activation energy of superplastic flow. Qeff is usually close to grain boundary diffusion activation energy Qb [109], but in the case of UFG metals, it often turns out to be less than Qb [99,111,112], and in the case of multiphase materials or in the case of diffusion creep, it can be greater than Qb [101].…”

Stress–strain relation during superplastic uniaxial tension at constant tension velocity

“…The proposed theories include grain boundary sliding (GBS) [6][7][8][9][10], grain rotation [11,12], grain rearrangement [13][14][15][16], dislocation activity [17][18][19], diffusional creep [20][21][22][23][24], and dynamic recrystallization [25][26][27][28][29]. The actual superplastic deformation should involve a combined mechanism and not a single one [13,14,[30][31][32][33]. The superplastic flow behavior is divided into three mechanism zones, namely, diffusion creep (Region I), grain boundary sliding (region II), dislocation creep (region III), according to the sigmoidal relationship between stress and strain rate [34].…”