Microstructure and Mechanical Properties of Magnesium ZRE1 (Mg-Zn-Zr) Alloy with Rare Earth Element (Samarium) Addition

The influences of rare earth (RE) samarium (Sm) added contents to ZRE1 (Mg-Zn-Zr) magnesium (Mg) cast alloy over strength properties to be investigated. Sm at 0.5, 1.0, 1.5, and 2.0 wt.% were added separately as an alloying element to ZRE1 alloy. Optical Microscope (OM), X-ray powder diffraction (XRD), and scanning electron microscope Energy-dispersive X-ray (SEM/EDS) were used to investigate the microstructure of alloy, while mechanical properties investigated include Ultimate Tensile Strength (UTS)and Micro-Hardness (MH) tests. The result revealed that as the Sm level reached 1.5 wt.%, the grain size decreased by 20.9 %. Additionally, UTS and Yield Strength (YS) showed improvements of 8.5 % and 7.9 %, respectively, with the addition of 1.5 wt.% of Sm. In addition, elongation and hardness have been improved by 32.3 % and 10.9 % respectively at 1.5 wt.% Sm addition. Mg-Zn-Ce-Sm was formed as a new phase upon the addition of Sm and was detected via XRD analysis. The addition of Sm to the ZRE1 alloy had significant effects on the refinement of the material's microstructure, leading to an increase in its mechanical and physical properties. Therefore, the new ZRE1-RE magnesium alloy was developed.

Magnesium ZRE1 alloy; Microstructure and mechanical properties; Metal forming and materials refinement

ZRE1 is one of many Mg alloys available in the market nowadays. ZRE1 is an Mg alloy containing RE metal with the addition of zinc and zirconium, forming Mg-RE-Zn-Zr (Ahmad et al., 2017a). The detailed chemical composition of the ZRE1 alloy is shown in Table 1. ZRE1 exhibits superior high-temperature creep resistance (Ferro, Saccone, and Delfino, 2013) and resistance to stress relaxation compared to that of benchmark alloy AE42 (Rzychon, Kielbus, and Szala, 2008). The presence of RE in ZRE1 makes the alloy free of microporosity and holds up to hot cracking, which at the same time offers good weldability (Ferro, Saccone, and Delfino, 2013). Currently, ZRE1 is used in the aerospace industry for intermediate casings on Tay engines, gearboxes of RB211 engines, and gearboxes of Tay engines (Materialstechnology, 2019).

Table 1 Chemical composition of the ZRE1 alloy in wt.% (Rzychon, Kielbus, and Szala, 2008)

Research by (Ahmad et al., 2017a) explored the effect of Praseodymium (Pr) addition on the microstructure and hardness of cast ZRE1 Mg alloy. They reported that 1 wt.% Pr addition on ZRE1 cast alloy reduces the grain size of ZRE1 alloy by around 37%. Meanwhile, the addition of Pr improves the hardness value of ZRE1 by 24 %. This is due to the effect of grain refinement and the effect of the second phase (Mg-Zn)12 RE and intermetallic compound (Mg-Zn-Ce-Pr). In their study, Ahmad et al. (2016) extended their research to examine the effects of Gadolinium on the microstructure and hardness of ZRE1 cast alloy. They found out that the addition of 3 wt.% heavy RE Gadolinium (Gd) reduce grain size by 28% and increased the volume fraction of eutectic secondary phase while at the same time improving the hardness by 34%.

Samarium (Sm) is one of the RE elements with huge potential to form Mg alloy. Researchers have been applied Sm as an alloying element for various Mg alloys for a long time (Guan et al., 2018). The addition of Sm to Mg alloy could reduce grain size, improve corrosion resistance, enhance yield strength and ultimate tensile strength, improve toughness, and at the same time, better elongation to Mg alloys, as shown in Table 2. However, to the best of our knowledge, researchers still have not given any attention to the influence of Sm addition on microstructures and mechanical properties of as-cast ZRE1 alloy. Sm is an orthorhombic structure with good solid-solution and precipitation strengthening in Mg alloys (Li, Xu, and Tong, 2019). Additionally, Sm costs are lower compared to Nd, Y, and Gd in the market (Lucas et al., 2014). Based on the Rare Earth Metal price, Sm is the cheapest RE of all, only beaten by Ce and La in price (Institute of Rare Earths and Strategic Metals eV, 2020). Additionally, Sm is the seventh most abundant RE in earth’s crust, with 7.1 ppm. Among all earth’s elements, Sm is the 40th most abundant and widely available than silver, tin, and gold (Enghag, 2004).

Hence, researchers have conducted studies on the effect of Sm addition to various magnesium systems such as Mg-6.0Zn-0.5Zr alloy (ZK60) (Guan et al., 2018), Mg-11Gd-2Y-0.6Al (Chen et al., 2019), Mg-Al-Zn (AZ61) (Anawati, Asoh, and Ono, 2018; Chen et al., 2015) Al-Zr-Ce (Kirman, Zulfia, and Suharno, 2016) and Mg-5Y-2Nd-0.5Zr (Yunwei et al., 2018). It is found the application of 1.5 wt.% Sm additive in Mg- 6.0Zn-0.5Zr alloy (ZK60) refined the grains of the as-cast sample, while the extruded sample shows even finer (Guan et al., 2018). At the same time, Sm addition further improves the strength of ZK60 alloy due to fine grain strengthening and precipitation strengthening. This outcome is consistent with Chen et al. (2019) findings, where they demonstrated a refined grain structure and improved tensile strength in the microstructure and mechanical properties of the Mg-Gd-Y-Sm-Al alloy. This corresponds to a study on the effects of samarium addition to the AZ61 (Mg-Al-Zn) magnesium alloy, resulting not only in a refined microstructure but also in improved ultimate tensile strength and elongation. However, excessive Sm addition could cause the coarsening of grain, leading to the decline of strength and plasticity (Rady et al., 2019; Al-Alimi, Lajis, and Shamsudin, 2017).

Table 2 Previous study on RE addition to Mg alloy

      An Mg base alloy (ZRE1 Mg cast) was supplied by ILM Ventures Ltd., Kuala Lumpur, Malaysia in ingot cast. The base alloy was melted in an electrical resistance furnace with a steel crucible under a cover-gas mixture of 99 vol% Argon (Ar) and 1 vol% of Sulfur hexafluoride (SF6), both from Linde and supplied by Linde Malaysia Sdn Bhd, Kuala Lumpur, Malaysia. Sm from Merck, Kuala Lumpur, Malaysia was added in contents of 0.5 wt.%, 1.0 wt.%, 1.5 wt.% and 2.0 wt.% separately as small pieces after the base alloy melted at approximately 730°C. After the addition, the melt was stirred for approximately 30 minutes to ensure stabilization, homogeneous composition, and dissolution of the alloying elements. The molten metal was then poured into a permanent steel mold preheated to 400°C. The cast was then left to cool down to room temperature before being removed from the mold. The graphical abstract of the research methodology and the melt-stir casting setup are shown in Figure 1 and Figure 2, respectively.

Figure 1 Graphical Abstract of research methodology

        XRD analysis was performed on Figure 3 and the selected specimens to identify the phases present in the ZRE1-xSm alloys; the results are shown in 1. It can be seen that the base alloy consisted of phase and Mg-Zn-Ce secondary intermetallic phase. The  phase was found in a hexagonal structure, while the secondary phase was a tetragonal structure. The position of peaks originating from Mg are exactly equivalent to Mg standard peaks as reported by (Sheggaf et al., 2017; Wang et al., 2014).  phase was found to possess a hexagonal structure with crystallographic parameters of (a = 3.20936; b = 3.20936; c = 5.21120)  while the Mg-Zn-Ce phase was tetragonal with (a = 3.83100; b = 3.83100; c = 3.83100)  crystallographic parameters. Similar phases were obtained by other Mg-Zn-Zr with RE addition alloys (Sheggaf et al., 2017; Liu et al., 2016).

Figure 3 XRD pattern of ZRE1-xSM alloys