Formation and Stabilization of 18R Long-Period Stacking Order Phase Through Friction Stir Processing of Mg-Gd-Y-Zn Alloy
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The special structures of the long-period stacking order (LPSO) phases in Mg-RE-Zn alloys include various types expressed as 10H, 14H, 18R, and 24R, among which the 14H and 18R structures have been most commonly observed.[1] It has been shown that LPSO structures consist of Y/Zn-enriched building blocks holding a local face-centered cubic (f.c.c) stacking sequence on the close packed planes.[2] According to Zhu et al.,[2] the number of Mg atomic layers between the building blocks defines the type of LPSO phase. The LPSO phases in the a-Mg matrix are generally formed
H. SHAHSA and A. ZAREI-HANZAKI are with the Hot Deformation & Thermo-Mechanical Processing Laboratory of High Performance Engineering Materials, School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O Box 11155-4563, Tehran, Iran. Contact e-mail: [email protected] H.R. ABEDI is with the School of Metallurgy & Materials Engineering, Iran University of Science and Technology (IUST), Tehran, Iran. Contact e-mail:[email protected] J.H. CHO and SANG-HO HAN are with the Department for Microstructure Control and Processing, Korea Institute of Materials Science (KIMS), 797 ChangwonDaero Seongsangu, Changwon, Gyeongnam 51508, South Korea. Manuscript submitted May 27, 2020. Article published online September 23, 2020 METALLURGICAL AND MATERIALS TRANSACTIONS A
during solidification as the 18R-type structure[3] and transform into the 14H structure during heat treatment or deformation at the specified temperature range of 350 C to 500 C.[4,5] Kim et al.[6] conducted a study to analyze the structural evolution of the LPSO phase during heat treatment, and various types of metastable LPSO building blocks were observed in the a-Mg matrix. Up to now, however, most previous studies have focused on LPSO evolution and formation during heat treatment, and it is interesting to assess the capability of strain-induced formation of the LPSO phase through thermomechanical processing. In this regard, friction stir processing (FSP) as a novel severe plastic deformation (SPD) method can be effectively employed because of the combined effect of both thermal and mechanical aspects.[7–11] Although the effects of FSP on LPSO dissolution and transformation have been investigated before,[11,12] the formation micromechanism and arrangement of LPSO building blocks are still unclear. In the present study, a systematic atomic-scale structural analysis is conducted to study the formation and evolution of LPSO structures during the friction stir processing of an Mg-8.2Gd-3.6Y-1.6Zn0.5Zr (wt pct) alloy using high-resolution high-angle annular dark-field transmission electron microscopy (HAADF-TEM). Formation of various types of LPSO structures in the a-Mg grains and the involved mechanism will be discussed in detail. To this end, the proper workpiece in the form of a plate with dimensions of 9*25*100 mm was prepared from as-extruded Mg-8.2Gd-3.6Y-1.6Zn-0.5Zr (wt pct) alloy and was subjected to single-pass FSP at the rotation rate of 630 rpm and a transverse s
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