Effect of Grain Structure on the Mechanical Properties of Al-Zn-Mg-Sc Alloys with T6 State
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JMEPEG https://doi.org/10.1007/s11665-020-05120-1
Effect of Grain Structure on the Mechanical Properties of Al-Zn-Mg-Sc Alloys with T6 State Y.F. Hou, C.Y. Liu, and B. Zhang (Submitted March 10, 2020; in revised form May 31, 2020) Al-Zn-Mg-Sc alloys with different grain structures were fabricated using friction stir processing (FSP) and rolling with different routes. The hot rolling (HR) sample after T6 treatment exhibited stronger texture than the other three samples, and the T6 FSP samples exhibited the finest grains, with average grain size (AGS) of 5 lm. Cold or hot rolling before T6 treatment led to grain coarsening in the T6 FSP samples, and the AGS in the T6 FSP + HR reached 126 lm. All the samples exhibited a low density of dislocations and contained coarse grain boundary precipitates, Al3(Sc,Zr) phases, and a high density of g¢ phase. The T6 HR sample exhibited the highest strength mainly due to its strong texture and reasonable grains size, and the T6 FSP sample exhibited the highest ductility due to its fine equiaxed grain structure, which can promote the redistribution of stresses and prevent strain localization during the tension process. Keywords
Al alloy, grain structure, mechanical property
1. Introduction Al-Zn-Mg (7xxx Al) alloys are widely used in aeronautic, astronautic, rail transport, and automotive industries due to their excellent mechanical properties (Ref 1, 2). The grain structure is important to the mechanical properties of 7xxx Al alloys, and severe plastic deformation (SPD) methods, such as powder consolidation after cryomilling (Ref 3), high pressure torsion (Ref 4-6), cryogenic rolling (Ref 7), equal channel angular extrusion (Ref 8), and constrained groove pressing (Ref 9-12), are used to refine the grains of Al alloys. SPD-fabricated 7xxx Al alloys exhibit considerably higher strength and lower ductility than conventional 7xxx Al alloys. However, the unique mechanical properties of SPDfabricated 7xxx Al alloys cannot be attributed only to their fine grain structures. For example, SPD-fabricated Al alloys always deviate from the Hall–Petch law (Ref 13). SPD introduces high dislocation densities to 7xxx alloys, thereby increasing their strength. Dislocations weaken the ductility of alloys by reducing the work hardening rate (Ref 14), and SPD results in the transformation of equilibrium grain boundaries (GBs) into non-equilibrium GBs, promotes dynamic precipitation (Ref 6, 15, 16), and accelerates precipitation in 7xxx Al alloys (Ref 17). GB characteristics and precipitated phases can significantly affect the mechanical properties of 7xxx Al alloys (Ref 13, 18). Thus, the Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11665-020-05120-1) contains supplementary material, which is available to authorized users. Y.F. Hou and C.Y. Liu, Key Laboratory of New Processing Technology for Nonferrous Metal and Materials, Ministry of Education, Guilin University of Technology, Guilin 541004, China; and B. Zhang, State Key Laboratory of Metastable Mater
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