Effect of Sn addition on the deformation behavior and microstructural evolution of Mg-Gd-Y-Zr alloy during hot compression
Introduction
Magnesium (Mg) and its alloys have the advantages of a low density, high specific strength, high stiffness, and good thermal conductivity and processability. As the lightest metal structure material, it has been widely used in the automotive, electronics, and medical equipment industries [[1], [2], [3], [4], [5], [6]]. Most Mg alloy structural parts are produced by die casting and are mainly limited to small volume parts, so the potential advantages of Mg alloys have not been fully exploited [7,8]. Compared with cast Mg alloys, wrought Mg alloys have more development potential. Large structural parts with various sizes can be obtained by deformation and generally have higher strength and toughness than cast Mg alloys [9,10]. Results [11] show that potential nonbasal slip systems (such as prismatic slip and pyramid slip) can be activated by thermal activation due to the enhanced atomic activity at 225 °C, thus greatly improving the plastic deformation ability. Therefore, Mg alloys are usually deformed at high temperature, during which dynamic recrystallization (DRX) and dynamic precipitation occur simultaneously [12,13]. DRX occurs in pure Mg and its alloys during hot deformation, resulting in significant grain refinement [[14], [15], [16]].
Rare earth elements (REs) are an effective strengthening element in Mg alloys. Mg-RE alloys have excellent mechanical properties realized through solution strengthening and precipitation strengthening [17]. Multielement alloying is a necessary way to improve the comprehensive properties of Mg alloys. Gd, Y, Ce, Nd, La and Sm are commonly used RE elements. In addition, some researchers have added Zn, Ca, Sr, Sn and Si to Mg-RE alloys to further improve the comprehensive properties of Mg alloys [18,19]. The newly developed Mg-Gd-Y alloy can show excellent heat resistance after aging, which is mainly due to the precipitation of the βꞌ phase. At the same time, the properties of the Mg-Gd-Y-Zr alloy are further improved by using Zr as the grain refiner. The room temperature and high temperature strength of Mg-Gd-Y-Zr alloys is much higher than that of industrial Mg alloys [[20], [21], [22], [23], [24]] [[20], [21], [22], [23], [24]] [[20], [21], [22], [23], [24]]. Mg-Gd-Y-Zr alloys are currently the most promising high-strength Mg alloys, and research on their microstructure and properties is relatively mature [25,26]. The results of He [27] showed that the mechanical properties of Mg-Gd-Y alloys can be further improved by combining heat treatment with hot deformation. The strength of such alloys is 370 MPa after aging treatment and 491 MPa after hot extrusion. However, rare earth (RE) resources are scarce and difficult to extract, which leads to high prices, and a large amount of mining will worsen the environment. Therefore, people began to consider the use of cheap alloy elements to replace RE elements to reduce the cost and further improve the performance.
Since 2000, increasing attention has been given to the application of Sn in Mg alloys. According to the phase diagram of the Mg–Sn binary alloy, the eutectic transition temperature occurs at 561.2 °C, and the saturated solid solubility of Sn in the Mg matrix is 14.48 wt% (unless otherwise noted, wt% is used as the component unit in this article). With decreasing temperature, saturated solid solubility decreases sharply, falling to 4.4% and 0.45% at 400 °C and 200 °C, respectively. Therefore, Sn shows a typical aging strengthening ability. Sn can form a Mg2Sn phase with an FCC crystal structure in Mg, which has a very high melting point (776 °C) and shows great potential to improve the high-temperature mechanical properties of Mg alloys [28,29]. Sn has been widely used in Mg-Al [2,[30], [31], [32]] and Mg–Zn [33,34] alloys. Wang [2,31,32] found that when the Sn content is less than 3%, Mg–Sn–Al alloy shows excellent plasticity, with a rolling deformation of approximately 80%. Although the Mg–Sn phase precipitated parallel to the basal plane has little effect on the strength of the matrix, the composite addition of Al and Sn can significantly improve the plasticity. Sasaki [30,34] successfully prepared a high-strength Mg–Sn–Al–Zn alloy with a tensile strength of 354 MPa, yield strength of 308 MPa and elongation of 12%. However, there are few reports about adding Sn as an alloying element to Mg-RE alloys. Wei [35] and Zhao [36] found that a small amount of La/Y and Sn compound addition can significantly improve the high-temperature creep resistance of the Mg alloy. Lim [37] reported that the addition of Sn to Mg-MM (mixed rare earth) alloys results in the formation of a small rod-like phase at the grain boundaries of as-cast alloys. Although the structure of the phase was not identified, the rolling ability of the alloy was improved with the addition of Sn.
It is well established that the plasticity of Mg-Gd-Y-Zr alloys is poor. Combined with previous studies, we notice that Sn has great potential to improve plasticity. In our previous study, we successfully prepared an as-cast Mg-Gd-Y(-Sn)-Zr alloy with high strength and high plasticity by adding the alloying element Sn [38]. To further improve the mechanical properties of the alloy, we consider the comprehensive strengthening method of heat treatment and hot deformation. Previous research on Mg-Gd-Y-Zr alloys has mainly focused on the aging precipitation behavior, but there have been few reports for the hot deformation behavior. Xiao [39] reported that the dynamic precipitation of β-Mg5(Gd, Y) in Mg-Gd-Y-Zr alloys during hot deformation is relatively sensitive to the deformation temperature and is mainly distributed at the grain boundaries. However, the interaction between dynamic precipitation and dynamic recrystallization is still unclear. In this paper, the dynamic precipitation characteristics of Mg-Gd-Y-(Sn)–Zr alloys during hot compression and their effect on DRX, as well as the effect of Sn on the hot deformation behavior of Mg-Gd-Y alloys, are systematically studied. It is expected that a new type of Mg alloy with high strength and heat resistance will be developed.
Section snippets
Experimental materials and procedures
A Mg-Gd-Y-Zr alloy was used as the matrix alloy, with Sn as the alloying element. The actual composition of the alloy was determined by inductively coupled plasma spectrometry (ICP-OES), as shown in Table 1. A ZGJL0.01-4C-4 induction furnace was used to melt the experimental Mg-Gd-Y-(Sn)–Zr alloy. The raw materials were industrial pure Mg (purity 99.5%), master alloys Mg-30Gd, Mg–30Y and Mg–30Zr (wt.%), and analytical purity Sn. The raw materials were weighed according to the mass percentage
True stress–true strain curves
Fig. 2 shows the true stress-true strain curves obtained for Mg-Gd-Y-Zr and Mg-Gd-Y-Sn-Zr alloys under different deformation conditions. The flow stress curve mainly consists of two parts: work hardening and dynamic softening. At the initial stage of deformation, with increasing strain, the flow stress increases rapidly and reaches the peak value, which is mainly due to the work hardening dominating at the initial stage of deformation. In the work hardening stage, the slip surface and lattice
Discussion
The results reveal the effect of Sn on the macroproperties (flow stress) and microstructure of Mg-Gd-Y-Zr alloys. Hence, we raise the question of whether this is related to dynamic precipitation. With these doubts, the Mg-Gd-Y(-Sn)-Zr alloy precipitates were characterized by TEM, and the Schmid factor was analyzed by EBSD to further reveal the mechanism of Sn in the hot deformation behavior of Mg-Gd-Y(-Sn)-Zr alloy.
Conclusion
We used a Gleeble-1500 simulation testing machine to conduct a hot compression test on Mg-Gd-Y-Zr and Mg-Gd-Y-Sn-Zr alloys under the conditions of a deformation temperature of 350 °C–500 °C and a strain rate of 0.002 s−1–1 s−1, and analyzed and discussed the influence of the addition of Sn on the microstructure and deformation behavior. The following conclusions can be drawn:
- 1)
When deforming at a lower temperature (350°C–400 °C), the addition of Sn promotes dynamic precipitation, and the
CRediT authorship contribution statement
Qian Zhang: Investigation, Writing – original draft. Quanan Li: Writing – review & editing. Xiaoya Chen: Writing – review & editing. Jian Bao: Investigation. Ziyi Chen: Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This project is sponsored by the National Natural Science Foundation of China (Nos. 51571084).
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