The role of grain boundary sliding and intragranular deformation mechanisms for a steady stage of superplastic flow for Al–Mg-based alloys

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 833; p. 142524
• Main Authors: Mikhaylovskaya, A.V., Yakovtseva, O.A., Irzhak, A.V.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 26.01.2022; Elsevier BV
• Subjects: Aluminum; Aluminum alloy; Creep (materials); Deformation; Deformation analysis; Deformation mechanisms; Depletion; Diffusional creep; Dislocation structure; Elongation; Evolution; Forming techniques; Grain boundaries; Grain boundary sliding; Grain growth; Heat treating; Magnesium base alloys; Microstructural evolution; Microstructure; Plastic deformation; Slip; Strain rate sensitivity; Striations; Superplastic deformation; Superplastic forming; Superplasticity; Surface structure; Superplasticity; Diffusional creep; Microstructural evolution; Deformation mechanisms; Dislocation structure; Grain boundary sliding; Aluminum alloy
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2021.142524

The superplastic forming technique, which realized extremely high plastic deformation, is used for producing complex shaped lightweight constructions. Physical models and experiments with surface microstructure evolution indicate that superplastic deformation predominantly occurs owing to grain boundary sliding (GBS) that accommodated by dislocation slip/creep and diffusional creep. Unusually weak GBS and increased contributions of accommodation mechanisms during the initial stage of superplastic deformation are observed for the fine-grained commercial Al–Mg based alloys. In this study, microstructural evolution during the deformation at a temperature of 0.97Ti.m.with a constant strain rate of 4 × 10−3 s−1 was investigated by scanning and transmission electron microscopy for Al–Mg based alloys with a different Mg content. A strain-induced evolutions of the grain and dislocation structure and the surface structure with FIB-milled grids during elevated-temperature deformation were analyzed. A decrease in the strain rate sensitivity m-coefficient from 0.6 to 0.4, dynamic grain growth with significant grains elongation to the tensile direction accompanied by a pronounced dislocation activity, subgrains formation, and the development of the precipitated depleted zones were observed during deformation. The intergranular and intragranular strains were measured to estimate the contributions of superplastic deformation mechanisms. An increase in solute Mg from 4.8wt% to 6.5–7.6wt% inhibited dynamic grain growth and increased a mean GBS contribution from ~24% to ~40%. “Striation” zones developed near the transverse grain boundaries, and precipitation-depleted zones accumulated up to 50% of strain for the alloy with 4.8wt%Mg and 20–35% for higher Mg alloys. Grain body deformation via dislocation clip/creep provided 20–30% of total strain. The results confirmed the critical nature of the diffusional creep and dislocation slip/creep mechanisms for superplastic deformation of the studied alloys.