The influence of minor additions of Y, Sc, and Zr on the microstructural evolution, superplastic behavior, and mechanical properties of AA6013 alloy

Abstract

Alloy AA6013 (Al-Mg-Si-based aluminum alloy) is of particular interest to the aerospace and automotive industries because of its attractive combinations of properties, however, accelerated grain growth at elevated temperatures limits its superplastic formability. The motivation for this study was to refine grain structure and control grain growth at elevated temperature in order to eliminate the lack of superplasticity in the alloy. For this reason, minor additions of Y, Sc, Zr were considered and their influence on the microstructural evolution, superplastic deformation behavior, and room temperature mechanical properties of a novel AA6013-type alloy was studied. A noticeable refinement of as-cast grain structure was observed due to the co-effect of Sc, Zr, Y additions. It was demonstrated that the Mg2Si phase and Y-bearing phases were formed during solidification of the studied alloys. The phases of silidification origin were fragmented during thermo-mechanical treatment led to the near uniform distribution of coarse particles of 0.5–0.8 µm size in the matrix. The coarse particles caused grain refinement during the superplastic deformation due to the particle stimulated nucleation (PSN) effect. In the alloy containing yttrium, during the decomposition of the aluminum-based solid solution supersaturated by Y, Zr, Sc, the following two precipitation mechanisms were involved: continuous precipitation resulting in the formation of nanoscale L12 phase dispersoids distributed homogeneously throughout the aluminum matrix and discontinuous precipitation that caused fan-shaped aggregations of the same phase near the high-angle grain boundaries. The nanoscale continuously-formed dispersoids retarded the recrystallization and dynamic grain growth during superplastic deformation due to Zenner pinning mechanism. Both PSN and Zenner pinning effects led to the grain refinement and achievement of superplasticity. The maximum elongation of 470% was observed under the test condition of T = 520 °C and ɛ.= 1 × 10−3 − 5 × 10−2 s −1. The room temperature tensile test revealed the following maximum characteristics of the sheet of the alloy with Y additions: yield strength of 330 MPa, ultimate tensile strength of 375 MPa, and elongation at fracture of 10%.