Deformation Mechanism of Mg-Gd-Y-Zn-Zr Alloy Containing Long-Period Stacking Ordered Phases During Hot Rolling
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INTRODUCTION
MAGNESIUM and magnesium alloys show great promise as lightweight structural materials in automotive, electronics industries and modern aerospace owing to the low density.[1] However, their practical applications are greatly impeded by the low strength and poor formability. Great efforts have thus been devoted to improving the mechanical properties of magnesium alloys.[2–6] In 2001, the Mg97Zn1Y2 (at. pct) alloy containing LPSO phases with a tensile yield strength of ~ 600 MPa and elongation of ~ 5 pct at room temperature was produced by Kawamura et al. via using rapidly solidified powder metallurgy processing.[2] The existence of LPSO phases is considered to be beneficial for the strength, and the structure and strengthening mechanism of LPSO phases have received great attention.[7–11] It has been identified that 14H and 18R are two common types of the LPSO phases in Mg-Zn-RE alloys.[7] According to the research of Zhu et al.,[8]
JIANBO SHAO, ZHIYONG CHEN, TAO CHEN, and CHUMING LIU are with the School of Materials Science and Engineering, Central South University, Changsha 410083, China. Contact e-mail: [email protected] Manuscript submitted August 20, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
14H and 18R LPSO phases have the same structural unit with ABCA stacking sequence. 14H LPSO phase is made up of two structural units and the two adjacent units are separated by three {0001} planes of Mg matrix. As for 18R, three structural units are separated by two {0001} planes of Mg matrix. In wrought magnesium alloys, LPSO phases were reported to present at the grain boundary in the form of bulk-shape or to precipitate along the basal plane of the matrix in the needle-like shape.[12,13] The effect of LPSO phases on the deformation behavior of the matrix was widely studied. Matsuda et al. found a number of straight hai dislocations in the grain without LPSO phases and hc + ai dislocations in grains with LPSO phases, indicating that LPSO phases suppress the activation of basal hai slip and promote the activation of non-basal slip.[14] Kim et al. pointed out that the elastic modulus of LPSO phases is higher than that of the matrix, and the elastic modulus mismatch between Mg matrix and LPSO phases is the main cause for the activation of non-basal dislocations.[15] In addition to the inhibition of basal hai slip, f10 12g twinning, which is another common deformation mode in magnesium alloys, was also reported to be strongly restricted by LPSO phases. As reported by Matsuda et al., the densely developed LPSO phases can remarkably suppress the growth of f1012g twinning.[16] Shao et al. pointed out that LPSO phases with a thickness
over 12 nm are not twinned, and LPSO phases can inhibit the twinning of the matrix between two LPSO phases.[17,18] Little or no twinning in magnesium alloys containing LPSO phases after deformation was extensively reported.[4,15,19–21] However, Garces et al. considered that f10 12g twinning is still the dominant deformation mechanism for magnesium alloys containing lamellar LPSO
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