Acta Metall Sin  2020, Vol. 56 Issue (5): 723-735    DOI: 10.11900/0412.1961.2019.00292
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Deformation Mechanism and Dynamic Recrystallization of Mg-5.6Gd-0.8Zn Alloy During Multi-Directional Forging
ZHANG Yang, SHAO Jianbo, CHEN Tao, LIU Chuming, CHEN Zhiyong()
School of Materials Science and Engineering, Central South University, Changsha 410083, China
Abstract

Multi-directional forging (MDF) is an effective way to fabricate wrought magnesium alloy with ultrafine grains and random texture. Therefore, microstructure evolution and dynamic recrystallization (DRX) of magnesium alloys during MDF process have been widely investigated. Mg-Zn-RE alloys containing long-period stacking ordered (LPSO) phase have received considerable attention owing to their excellent mechanical properties. In addition, LPSO phase has great effects on the deformation mechanism and DRX behavior. Still, limited comprehensive studies can be found in the literature dealing with the microstructure evolution, deformation mechanism and DRX of magnesium alloys containing LPSO phase in MDF deformation. In this work, MDF was applied to a Mg-5.6Gd-0.8Zn (mass fraction, %) alloy containing LPSO phase. Microstructure characteristics, deformation mechanism and DRX behavior of the material in different passes were examined. Results show that there are several stages of the microstructure evolution. Twinning was activated only in a small part of grains in the early stage of deformation. As the forging direction changes, the number of twinned grains and the volume fraction of DRX grains increased. A mixed structure with coarse deformed grain and DRX grains was sustained till last forging pass, and the average size of DRX grains is about 4 μm with a random orientation. {$101ˉ2$} tensile twinning is the main deformation mechanism and the selection of twin variants was dominated by the Schmid law. Change in forging direction is beneficial to twinning stimulation in grains of different orientations. Kink and slipping deformation could effectively accommodate the plastic strain where the operation of twinning was hindered. Kink deformation resulted in lattice rotation predominately about the <$101ˉ0$> axis. DRX grains nucleated at different places during the forging process. Not only the grain boundaries and the twinned region, but also kink boundaries can induce the nucleation of DRX grains. Eventually, the twinned regions were transformed to a strip-like recrystallization structure. Under the combined influence of twinning and kinking, as well as DRX induced by twins, kink bands and grain boundaries, the initial coarse grains were significantly refined.

 ZTFLH: TG146.2
Fund: National Natural Science Foundation of China(51874367);National Natural Science Foundation of China(51574291)
Corresponding Authors:  CHEN Zhiyong     E-mail:  czysh@netease.com
 Fig.1  Schematics of multi-directional forging (a) and the position of the sample for microstructure examination (b) Fig.2  SEM images of as-cast (a), homogenized (b) and annealed (c, d) Mg-5.6Gd-0.8Zn alloy before multi-directional forging (The lamellar second phases are indicated by black arrows in Fig.2d, LPSO—long-period stacking ordered) Fig.3  STEM image and selected area electron diffraction (SAED) pattern (a) and magnification of the LPSO phase as marked in Fig.3a (b) (Beam direction is <$112ˉ0$>) Fig.4  OM images (a, c, e, g) and the corresponding magnifications of boxes (b, d, f, h) of annealed alloy after multi-directional forging of 1-pass (a, b), 2-pass (c, d), 3-pass (e, f) and 6-pass (g, h) (DRX—dynamic recrystallization) Fig.5  Twin-induced and kink-induced DRX(a) twinning in 1-pass (b) twinning and DRX in 2-pass(c) strip-like DRX structure in 6-pass (d) kink-induced DRX in 6-pass Fig.6  SEM images of multi-directional forged alloy after 1-pass (a), 2-pass (b), 3-pass (c) and 6-pass (d) Fig.7  Schematic of the DRX behavior during multi-directional forging(a) 1-pass (b) 2 and 3-pass (c) 6-pass Fig.8  Orientation maps of multi-directional forged alloy after 1-pass (a), 2-pass (b), 3-pass (c) and 6-pass (d) (Twins with smaller size can be observed in the grains with c-axes near parallel to the forging direction, as marked as ellipses in Fig.8a; with the changing of forging direction, untwined grains in the 1-pass are rotated to the favorable orientation for twinning (<$21ˉ1ˉ0$> parallel to CD), as indicated by ellipses in Fig.8b)Color online Fig.9  The orientation maps (a, d) and {0001} pole figures (b, e) of the twinned (a, b) and untwined (d, e) grains after 1-pass forging, and distributions of Schmid factor for {$101ˉ2$} tensile twinning (c) and basal slip (f) in {0001} pole figure (Due to the inhibition of LPSO phases, twinning propagation is arrested. While twining nucleation is promoted and multiple fine twins constitutes a coarse twin by twins-merging, as indicated by the ellipse in Fig.9a. The twin variants are marked as T1 and T2. The matrices of different grains are marked as A~E)Color online Fig.10  Analyses of twinned and kinked grains of the alloy in 2-passColor online(a) orientation map of twinned and kinked grains(b) misorientation angle distribution(c) orientation coloring map of kink region (Fig.10c hides the twined region of Fig.10a)(d) in-grain misorientation axes (IGMA) distribution ("pts" is the abbreviation of "points", and means the number of points with the defined misorientation(e) magnified view of black-lined region in Fig.10a(f) {0001}, {$101ˉ0$} and {$112ˉ0$} pole figures of grain in Fig.10e (Numbers 1~3 represent the matrix, kinked region and twined region respectively; the arrow in {$101ˉ0$} pole figure means one of the {$101ˉ0$} pole of the kink band and the matrix is coincided; the arrow in {$112ˉ0$} pole figure means one of the {$112ˉ0$} pole of the twin and the matrix is coincided) Fig.11  Orientation map of twined grain in 3-pass (a), {0001} pole figure of grain in Fig.11a (b), distribution of twinning variants' Schmid factor of M1 in {0001} pole figure during 2-pass (c), distribution of twinning variants' Schmid factor of M2 in {0001} pole figure during 3-pass (d), and schematic diagram of twin variants (e) (M1 and M2 represent the matrices, T1~T4 represent the actual twin variants of two grains in Fig.11a; V1~V6 represents the six possible twin variants acquired according to the orientation of the matrix)Color online