Effects of Trace Al Addition on the Microstructure, Hot Deformation Behavior and Mechanical Properties of High-Strength
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INTRODUCTION
AS one of the lightest structural materials, magnesium (Mg) alloys have great potential in the automotive industry owing to the low density and high specific strength.[1] However, most of the commercial Mg alloys currently are still inferior to traditional Al alloys in terms of strength and formability. Over the past decades, Mg-rare earth (RE) alloys have attracted much attention from researchers because of their weak basal texture, relatively good corrosion resistance and high strength that mainly benefits from significant solute and precipitate strengthening.[2–5] Among them, the Mg-RE-Zn alloys with heavy rare earths, especially the Mg-Gd-Zn base alloys, are a hotspot since the formation of LPSO phases can bring effective reinforcement.[6–14] For instance, the ultimate tensile strengths of the Mg-10Gd-6Y-2Zn-0.6Zr (wt pct, hereafter in weight percentage) alloys prepared by conventional extrusion and peak-aged treatment were 360 MPa and 432 MPa, XIANG LI, WENLONG XIAO, SHAOYUAN LYU, MAOWEN LIU, and CHAOLI MA are with Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, School of Materials Science and Engineering, Beihang University, Beijing 100191, China. Contact e-mail: [email protected] Manuscript submitted January 10, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
respectively.[8] The Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt pct, hereafter in weight percentage) alloy sheet fabricated by large-strain hot rolling and a subsequent aging process exhibited an ultimate tensile strength of 517 MPa and good elongation of 4.5 pct.[9] Compared with the as-cast alloys, wrought Mg alloys show greater mechanical properties due to microstructural refinement and work hardening.[15,16] However, high-strength Mg-RE alloys normally exhibit insufficient hot workability. At present, the influence of microconstituents, especially the LPSO phases, on the hot deformation behavior of Mg-RE-Zn alloys has yet to be deeply understood. Lv et al.[17] investigated the effect of 14H LPSO phase on the dynamic recrystallization (DRX) behavior of Mg-2.0Zn-0.3Zr-5.8Y alloy and found that the existence of lamellar 14H LPSO phase deferred the dynamic recrystallization and hindered the growth of DRXed grains. Liu et al.[18] found that 18R LPSO phase could promote DRX through a particle-stimulated nucleation (PSN) mechanism, and 14H LPSO phase could accelerate DRX when deformed to a certain extent. On˜orbe et al.[19] reported that the original coarse LPSO particles, with an 18R structure, were transformed into 14H long plates because of the diffusion of Zn and Y atoms when the alloy deformed at high temperature. Our recent work indicated that the original microstructure also had an evident effect on the hot deformation of Mg-Gd-Zn alloy.[20] Both LPSO
phase and Zn solute in the a-Mg matrix narrowed the flow stability regions of the processing map and impeded the DRX, especially when Zn solute was mainly in an a-Mg matrix. In this work, a trace of Al addition was proposed to modify the microstructure of high-streng
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