Microstructure and texture evolution during hot rolling and subsequent annealing of Mg–1Gd alloy
Introduction
Strong texture with the c-axis of grains perpendicular to the extrusion direction or rolling plane generally develops in conventional wrought Mg alloys, resulting in limited formability, poor ductility and strong anisotropy in their mechanical properties. One of the most effective approaches to improve the ductility of these Mg alloys is weakening the recrystallization texture. Recently, it has been found that the recrystallization texture following hot processing and annealing can be significantly weakened by the addition of rare earth (RE) elements, such as Nd, Ce, Gd and Y [1], [2], [3], [4], [5], [6], and therefore, leading to improved formability and ductility. However, the mechanisms by which RE elements weaken the texture remain unclear, although several mechanisms have been proposed, such as shear bands nucleation [7], [8], particle-stimulated nucleation (PSN) [9], dynamic strain aging (DSA) [3], [10] and solute segregation at grain boundaries [11].
In our previous studies, we found that DRX of Mg alloys can be inhibited during hot deformation by the addition of RE elements. Under this condition, static recrystallization (SRX) between passes during multipass rolling plays an important role in the microstructure and texture evolution of Mg–RE alloys. Therefore, an understanding of microstructure and texture evolution during annealing treatment is essential in order to optimize the rolling and annealing parameters.
The previous studies have already shown that the texture of Mg–RE alloys is significantly weakened after post-deformation annealing compared to the as-rolled materials [5], [12]. However, the mechanisms for such weaker texture development during static recrystallization (SRX) are not clearly defined. Farzadfar et al. [13] found that upon isothermal annealing of a single-pass rolled Mg–2.9 wt% Y alloy, the texture is weakened, and ascribed it to SRX in basal parent grains. However, Wu et al. [14] found that the tensile twins observed in hot-rolled Mg–3Gd–Zn alloy might serve as favorable sites for SRX, and the recrystallized grains were oriented randomly, resulting in a weak texture.
The aim of this study is to investigate the effect of 1 wt% Gd addition on the microstructure, texture evolution and mechanical properties of Mg sheet by multipass rolling and annealing treatment, and elucidate the mechanisms by which the texture is weakened. In order to examine the effect of SRX on the microstructure and texture, the Mg–1Gd alloy is rolled at 300 °C and annealed at 400 °C to suppress DRX and promote SRX. Another goal of this study is to explore whether Mg–RE alloys can be rolled at a low temperature and annealed at a relatively high temperature, which would provide useful guidance to design a rolling processing route for Mg–RE alloys. Gd is used for such a study because of its high solubility in order to avoid Mg–RE particle effects.
Section snippets
Experimental
Mg–1 wt% Gd (referred as Mg–1Gd hereafter) alloy was prepared from high purity Mg and Mg–25 wt% Gd master alloy, and was produced by an electric furnace under a mixed protection gas atmosphere of SF6/CO2 and casting into a preheated steel mold. Then, the as-cast ingots were sectioned into slabs with a dimension of 110×60×10 mm3 for rolling experiments. Pure Mg was selected as a reference alloy for comparison.
The rolling experiments were performed on a rolling mill with a roller diameter of 320 mm.
Microstructure and texture evolution during hot rolling and subsequent annealing
Fig. 1a and b shows the macrostructures of the as-cast samples of pure Mg and Mg–1Gd alloy, respectively. Both samples show very large columnar grains with some millimeters in length and width, which indicates that the 1 wt% addition of Gd does not result in a grain refinement of the cast microstructure. This is in agreement with a previous study by Stanford et al. [6].
The optical microstructures of pure Mg and Mg–1Gd alloy in the as-rolled condition with a total thickness reduction of 80% are
Texture weakening mechanism by Gd addition
In the present work, DRX is inhibited during hot rolling at a low temperature of 300 °C in Mg–1Gd alloy. Thus, SRX is the most important mechanism during annealing at 400 °C. Therefore, it seems reasonable to suggest that the texture weakening is related to the SRX.
The recrystallization behavior of pure Mg, which is similar to conventional Mg–Al based alloys such as AZ31, has already been discussed in the literature. During the annealing process of the as-rolled pure Mg sample, nucleation occurs
Conclusions
The microstructure, texture and tensile ductility of Mg–1Gd alloy and pure Mg sheet were investigated following rolling and subsequent isothermal annealing. The addition of Gd weakens the basal texture in both the as-hot rolled and annealed conditions. This is related to the enhanced activity of pyramidal 〈c+a〉-slip during hot rolling by Gd addition and the suppressed grain boundaries migration due to solute segregation of Gd at grain boundary during annealing. Pure Mg sheet shows a strong
Acknowledgments
The authors gratefully acknowledge the support of the National High-Tech R&D Program of China (Grant no. 2011BAE22B06), the National Natural Sciences Foundation of China (Grant nos. 50901044 and 51271118) and the Shanghai Rising-Star Program (B type) (Grant no. 12QB1403300).
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