Correlation between dynamic recrystallization and formation of rare earth texture in a Mg-Zn-Gd magnesium alloy during extrusion

www.nature.com/scientificreports OPEN Received: 30 April 2018 Accepted: 2 October 2018 Published: xx xx xxxx Correlation between dynamic recrystallization and formation of rare earth texture in a Mg-Zn-Gd magnesium alloy during extrusion M. G. Jiang1,2, C. Xu3, H. Yan2, S. H. Lu2, T. Nakata4, C. S. Lao1, R. S. Chen2, S. Kamado4 & E. H. Han2 The trace addition of rare earth (RE) elements in Mg alloys can modify the extrusion texture, leading to the formation of RE texture and thus improved formability. The interrupted extrusion experiment as well as electron back-scatter diffraction (EBSD) characterization was conducted in Mg-1.5Zn-0.5Gd (wt.%) alloy to unveil the dominant dynamic recrystallization (DRX) mechanism and its correlation with the formation of RE texture during extrusion. The results indicate that continuous DRX (CDRX) dominated the microstructural development. Fresh DRXed grains with 30° [0001] grain boundaries preferentially nucleated in unDRXed grains with [1010] basal fiber orientation via CDRX, showing preferred selection of [2110] basal fiber orientation rather than RE texture orientation. Consequently, CDRX contributed to the weakening of [1010] basal fiber texture and had a more significant effect on the formation of [2110] basal fiber component than that of RE texture component. Besides, the preferred growth of recrystallized grains with RE texture orientation was confirmed to occur during static annealing after extrusion, which is inferred as the key reason for the formation of RE texture in dilute Mg-RE alloys. Commercial Mg alloys usually develop a strong basal fiber texture during hot extrusion process, which accounts for the unsatisfactory formability and ductility at room temperature. The trace addition of rare earth (RE) elements has been recently claimed as an effective approach to modify the extrusion texture of Mg alloys, leading to the formation of a RE texture component with <1121> directions parallel to the extrusion direction (ED)1–5, which contributes to significant improvement in ductility along the ED1,2,4. Mg alloys are usually subjected to hot deformation for improving workability, during which dynamic recrystallization (DRX) often takes place. The final microstructure and mechanical properties of the alloys are closely related to the evolution of DRX process. There are two mechanisms mainly proposed that account for DRX in Mg alloy6–9: continuous DRX (CDRX) and discontinuous DRX (DDRX). The CDRX is featured by the development of low angle grain boundaries (LAGBs) and their progressive rotation into high angle grain boundaries (HAGBs), and thus new grains, which is different from the DDRX involving classic nucleation and subsequent growth of new DRXed grains. To date, the research on DRX behavior and its effect on texture formation during extrusion in RE-free Mg alloys has been well established10–12, but this is not the case for dilute Mg-RE alloys. Some recent studies have already been devoted to the issue on the correlation with DRX and formation of RE texture in dilute Mg-RE extrusions13–17. Hadorn et al.13,14 and Robson15 hypothesized that CDRX is the key reason for the texture weakening and consequential formation of RE texture in dilute Mg-RE alloy during extrusion based on the evidence of RE solute segregation at GBs that is expected to hinder the motion of GBs and thus suppress the conventional DDRX process, but the authors did not further evidence experimentally whether the CDRX occurs and how such DRX mechanism affects the texture development during extrusion. Recently, 1 College of Mechatronics and Control Engineering, Shenzhen University, 3688 Nanhai Ave, Shenzhen, 518060, China. 2The Group of Magnesium Alloys and Their Applications, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang, 110016, China. 3School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China. 4Department of Mechanical Engineering, Nagaoka University of Technology, Nagaoka, 940-2188, Japan. Correspondence and requests for materials should be addressed to H.Y. (email: hyan@ imr.ac.cn) or R.S.C. (email: ) SCientifiC REPOrts | (2018) 8:16800 | DOI:10.1038/s41598-018-35170-4 1 www.nature.com/scientificreports/ Figure 1. Microstructure and RE texture of the extruded alloy: (a) inverse pole figure map, (b) (0001) pole figure and (c) inverse pole figure. Imandoust et al.16 studied the effect of RE element on the recrystallization textures in Mg-Ce and Mg-Gd binary alloys and reported that CDRX facilitated the transformation of a sharp <1010> basal fiber texture into a randomized texture. In contrast, their following study17 in the extruded Mg-Zn-Al-Y-MM (MM: Mischmetal) alloys revealed that CDRX sharpened the <1010> basal fiber texture and DDRX was shown to be the predominant mechanism for texture modification. Despite these efforts, there are still some differences of views on the DRX behavior in dilute Mg-RE alloys during extrusion and its corresponding contribution to the texture development. Besides, it should be noted that interrupted extrusion experiment is necessary to unveil the DRX behavior during extrusion, as this method is effective to reflect actual microstructural evolution and avoid the interference of DRXed grain growth during extrusion10–12,18,19, which was not adopted in Imandoust et al.’s studies16,17. In our previous studies4,20–22, a series of Mg-Zn-Gd alloys have been developed as high-ductile wrought Mg alloys and it is found that a RE texture component formed in Mg-1.5Zn-0.5Gd (wt.%) alloy after extrusion under appropriate conditions4. In this study, therefore, interrupted extrusion experiment was designed for this Mg-Zn-Gd alloy, and electron back-scatter diffraction (EBSD) characterization was conducted to disclose the dominant DRX mechanism during extrusion and achieve a comprehensive understanding of the correlation between the DRX and the formation of RE texture for providing insightful knowledge into tailoring the texture formation and thus better designing new wrought Mg-RE alloys with high performance. Results Microstructure and RE texture of the extruded alloy. The inverse pole figure map (Fig. 1a) shows a fully recrystallized microstructure with DRXed grain size of 12.0 μm. The (0001) pole figure (Fig. 1b) exhibits a weak extrusion texture with maximum intensity of 3.4 and basal poles showing a large angle distribution of ±60° away from the transverse direction (TD) to the ED (TD indicates the direction perpendicular to the ED). In the corresponding inverse pole figure (Fig. 1c), two texture components were evident: one was a [2110] basal fiber component with basal planes and [2110] directions parallel to the ED and the other was a non-basal component locating between [2114]//ED and [2112]//ED, i.e., RE texture component. This suggests that the RE texture component had already formed just after passage through the die. Microstructural evolutio (...truncated)