Excellent room-temperature ductility and formability of rolled Mg–Gd–Zn alloy sheets
Research highlights
▶ The Mg–Gd–Zn sheets have an oval-shaped distribution of basal poles by the addition of Gd. ▶ The basal texture intensity of the sheets is effectively weakened by the addition of Gd. ▶ The sheets exhibit a large elongation-to-failure (nearly 50%) at room temperature. ▶ The sheets exhibit a uniform elongation (larger than 30%) at room temperature. ▶ The sheets exhibit a high Erichsen values (nearly 8) at room temperature.
Introduction
Mg alloys have high potential for improving the fuel efficiency and reducing the CO2 emission of vehicles because of their high specific strength and stiffness. There is an increasing demand for mass produced Mg alloy sheets with high performance for industrial applications. Unfortunately, the commercial magnesium alloys, e.g. AZ31 sheet, produced by the conventional rolling process usually have poor ductility and strong anisotropy at room temperature due to a pronounced basal texture [1], which limits their further press formability and industrial applications. Therefore, improving the low and/or room temperatures formability by changing or weakening the basal texture is important for promoting a wider use of Mg alloy sheets in industry.
Many different technologies, e.g. equal channel angular extrusion [2], [3], differential speed rolling [4], [5], torsion extrusion [6] and cyclic extrusion [7], have been used for processing magnesium alloys and have been proved to be effective in developing weaker or non-basal textures. The elongation-to-failure at room temperature can be enhanced significantly to ∼38% [7]. However, these processing technologies are not as efficient as rolling process in view of industrial application.
Recently, the addition of rare-earth metal, such as Ce [8], [9], Nd [9], Y [9], [10] and Gd [11], to Mg alloy has been found to be an effective way of weakening and changing the basal texture of wrought magnesium alloys. Some work [8], [9] attributes the texture weakening effect to the solute RE in Mg, and suggests that the addition of more RE should not be necessary for weakening the basal texture [11], [12], [13]; moreover, the excellent room temperature ductility and formability have indeed been observed in the Mg alloys with dilute (≤1 wt.%) RE elements [14], [15], [16]; the elongation-to-failure [16] and Erichsen value (IE) [14], [15] at room temperature in those work are more than 30% and 9, respectively. However, in some other work [11], [17], [18], where RE is also present as a microalloying additions (≤1 wt.%), the room temperature ductility and formability are only enhanced moderately, still far inferior to typical structural Al alloys. So, it is still not sure that microalloying Mg with RE could promise excellent sheet formability. In fact, the effect of RE elements on weakening the texture and improving the ductility and formability of Mg alloy depends not only on the content, but also obviously on the type of RE addition [12] and other alloying elements in Mg [11], [16], [19], which has been scarcely discussed in those references.
Gd is soluble in magnesium to ∼4 wt.% at 200 °C [20]. With Zn/Gd ratio in a certain range, there form a large amount of second-phase particles [21] in Mg matrix, which may serves as the sites of recrystallization, i.e. particle-stimulated nucleation (PSN), to weaken the texture [22]. Therefore, in Mg–Zn alloy, a relative high content of Gd, compared with dilute Gd addition [11], [12], [16], may act more effectively on weakening and changing the basal texture in Mg sheet, and benefit its ductility and formability at room temperature more. In this paper, we add Gd as a major alloying element (the content of Gd higher than Zn) in Mg–Zn alloy and study the texture, ductility, strain-hardening behavior, and stretch formability of two rolled Mg–Gd–Zn alloy sheets with 2% and 3% Gd at room temperature. High room-temperature ductility and easy formability in these alloy sheets will be presented.
Section snippets
Experimental procedures
Two magnesium alloys denoted as GZ21 and GZ31 were examined in the present study. The chemical compositions are listed in Table 1. They were prepared with pure Mg (99.9%), Zn (99.9%) and Gd (99.5%) by melting under the protection of a mixed SF6 (1, vol.%) and CO2 (99, vol.%) atmosphere. Ingots with a dimension of 75 mm × 200 mm × 200 mm were prepared by pouring the melt into a preheated steel mold. They were homogenized at 500 °C for 10 h, then quenched in water, and subsequently machined to slabs with
Microstructure and texture
The microstructures of the rolled GZ21 and GZ31 alloys in the RD–TD plane are shown in Fig. 1. Both alloys have equiaxed grain structures with a few twins. The average grain sizes of the two alloys are 16 μm and 12 μm, respectively. Fig. 2 is the SEM images of the GZ31 sheet and corresponding EDX analysis results. In the SEM images, there are many fine particles smaller than 2 μm and some larger particles ∼10 μm (Fig. 2a), which are homogeneously distributed in the matrix. A similar phase
Texture
It is known that the basal plane of rolled AZ31 alloy sheet is intensively distributed parallel to the RD–TD plane, which corresponds to a so-called basal-type texture. The other important feature of the AZ31 sheet texture is a broader intensity spread of the basal poles from the ND toward the RD than to the TD [5]. This type of texture has often been referred to as the typical texture of rolled or rather tempered Mg alloy sheets [1], [5], [29]. Some researchers have found that a combination of
Conclusions
The rolled Mg–Gd–Zn alloys with excellent ductility and formability at room temperature are developed, and the results are summarized as follows:
- 1.
The rolled Mg–Gd–Zn alloys have fine recrystallized microstructures with a large amount of tiny particles homogeneously distributed in the matrix.
- 2.
The Mg–Gd–Zn sheets have an oval-shaped distribution of basal poles at angles of about 30° to normal direction (ND) of the sheets, and it seems that the basal texture intensity is effectively weakened by the
Acknowledgements
This work was funded by the National Basic Research Program of China (973 Program) and National Natural Science Foundation of China (NSFC) through projects No. 2007CB613704 and No. 50874100, respectively.
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