Microstructure Thermal Stability and Superplastic Behavior of Al-6%Mg-0.12%Sc-0.10%Zr-0.10%(Yb, Er, Hf) Ultrafine-Grained Alloys

Superplastic behavior of ultrafine-grained (UFG) Al-6Mg-0.12Sc-0.10Zr-0.1X alloys, where X = Yb (Alloy #1-Yb), Er (Alloy #2-Er), and Hf (Alloy #3-Hf), has been studied. The total content of Sc, Zr, Yb, Er, Hf in the alloys was 0.32 wt.% (0.117-0.118 at.%). The alloys used for benchmarking were Al-6Mg-0.12Sc-0.20Zr (Alloy #4-Zr) and Al-6Mg-0.22Sc-0.10Zr (Alloy #5-Sc). Their UFG microstructure was formed with ECAP. Two different types of deformation behavior during superplasticity were demonstrated. A simultaneous increase in yield stress and elongation to failure during superplastic deformation was discovered. High deformation temperatures were shown to cause a competition between dynamic (strain-induced) grain growth and dynamic recrystallization, leading to a finer grain microstructure. The values of strain hardening factor (n), strain rate sensitivity factor (m), and superplastic deformation threshold stress (Sp) were determined. The impact of the type and concentration of alloying elements on the deformation behavior and dynamic grain growth of Al-6%Mg alloys was investigated. It was established that the maximum elongation to failure in Alloy #1-Yb and Alloy #2-Er is observed at lower deformation temperatures than in Alloy #4-Zr and Alloy #5-Sc. The superplastic properties of Alloy #3-Hf are superior to those of Alloy #4-Zr and Alloy #5-Sc with high content of alloying elements (in at.%). Alloy #1-Yb manifests good elongation to failure (910%) at low temperatures (400 oC). The satisfiability of Hart's criterion for calculating uniform deformation value under superplastic conditions was verified. It was demonstrated that cavitation when pores are formed in large Al3X particles at high temperatures causes early failure of aluminum alloys.
Comment: 41 pages, 19 figures, 4 tables, 43 references