Solidification, Microstructure, and Mechanical Properties of Mg-Nd-Gd-Zn-Zr Magnesium Alloy with 1.5 Samarium

The influence of samarium (Sm) content on the solidification characteristics, microstructure, and mechanical properties of Mg–2.8wt%Nd–1.5wt%Gd–0.5wt%Zn–0.5wt%Zr magnesium alloy was studied. The cooling curves and microstructure analysis results showed that Sm rare earth element refined the grains of the alloys, where the solidification time of α-Mg phase of decreased as addition of Sm, which reflected to the microstructure of alloy and the grains became refined, also Sm combined with the initial phase of the intermetallic base alloy and crystallized along the grain boundaries. In addition, Mg41Sm5, Mg3Zn6Sm and (Mg, Zn)3Sm new intermetallic phases were formed as addition of Sm. Both grain refinement and formed intermetallic phases led to the improvement in hardness and tensile strength.


Introduction
Reduced weight of products can be attained by many approaches such as redesign, removal of unnecessary parts, reducing thickness and selection of light materials. For such reasons, much effort has been spent on developing lighter materials, such as composites, and Al and Mg alloys for vehicles, and aerospace industrials (Ferro, Saccone et al. 2013). Magnesium is the 6th most plentiful component in the world's covering and is the lightest of every single auxiliary metal with a high specific stiffness (Ali, Qiu et al. 2015). The focus on Mg-RE (RE such as Sm, Yt, Pr, Gd, etc.) alloys is attributed to the significant strength at the normal and relatively high temperatures , and also because of their practical creep resistance (Easton, Gibson et al. 2018). Recent studies highlighted that some rare earth (RE) metals, such as Yttrium (Y), Neodymium (Nd) and Samarium (Sm) are very effective elements to improve the mechanical properties of Mg alloys. Mechanical properties of magnesium can be enhanced by adding different contents of element-alloy which results improvements in increased tensile strength, creep resistance, thermal stability, or corrosion resistance. Thermal analysis techniques are performed to investigate the solidification characteristics of metals and their alloys. Recently, in industrial applications, computer-aided cooling curve analysis (CA-CCTA) is common method, and has been successfully applied to several investigations on the solidification sequences of magnesium alloys (Farahany, Bakhsheshi-Rad et al. 2012), (Lotfabadi, Idris et al. 2013). In this paper, the solidification behavior, microstructure, and mechanical properties of Mg-Nd-Gd-Zn-Zr magnesium alloy with samarium additions are investigated.

Experimental
In this paper samarium (Sm) rare earth element with (1.5wt.%) added into the Mg-2.8wt%Nd-1.5wt%Gd-0.5wt% Zn-0.5 wt% Zr magnesium alloy. The base alloy was melted at a temperature of 750 °C in an induction furnace using Ar and 2% of SF6 mixed gas. The solidification characteristics of cooling curves of the investigated alloys were determined via CA-CCTA, where two K-type thermocouples are used, one of them was placed in the center of the mould and the other one is closed to the wall. The solidified samples were cut from the central section of ingots and then prepared according to the standard of ASTM E3 to be used for metallographic examination. The optical microscope (OM) observed the microstructure of the alloy, and a scanning electron microscope (SEM). The samples of tensile test were produced using the permanent casting process with steel mould, and each casting provided three tensile bars. The bars of final test were cut and machined using a CNC machine according to the standard dimensions of ASTM B557, and the reported values are the average of at least three measurements. Hardness test is performed using Vickers hardness (Hv) on the same samples that are used for microstructure analysis according to the standard of ASTM E92, with the 5 Kg, applied load and 10-second load holding time, and the reported values of hardness are the average of seven readings for any sample.

Solidification Characteristics
Fig . 1 shows the cooling curves that extracted during solidification process of the investigated alloys. The primary stable dendrites start to solidify from the melt at nucleation temperature (TNα-Mg), which decreased to 641.3 °C as 1.5 wt. % of Sm added due to forming of third phase which detected via XRD. The above results showed that a part of Sm dissolves into α-Mg matrix during the solidification of the alloy. Moreover, high melting point compounds Mg41Sm5 act as a bar to the growth of α-Mg grain, which lead to decreasing the nucleation temperature of α-Mg phase (Zhou, Li et al. 2015). The solidus temperature decrements as result of decrease the nucleation temperature of α-Mg phase and simultaneously widened the solidification range (Dobrzański, Król et al. 2010). Addition of Sm, caused to segregate the second phase gradually, and reduce the nucleation particles of α-Mg (Chen, Jin et al. 2015). Furthermore, forming a new intermetallic compound (Mg3Zn6Sm ternary phase) which detected by XRD at the grain boundaries of microstructure of treated alloy that led to decrease of the nucleation and growth temperature of primary magnesium dendrite. Addition of Sm results in it enrichment into front of the solidification interface, then the enrichment Sm effectively prevents the other alloy elements from diffusing to the matrix α-Mg, which led to significant increase in quantity of the eutectic phases (Zhang, Huang et al. 2017).
As addition of Sm, the solidification time α-Mg phase (Δtα-Mg= tGα-Mg-tNα-Mg) decreased, which leads to grain refinement of the alloys. It has been reported that addition of rare earth (RE) into Mg-Zn alloys had an important effect on changing the solid solubility of the Zn in the matrix as a result of decrement in the solidus curve, which shortened the time of nucleation and reduced the grain size ( , and according to the metallographic theory, Sm element usually acts as heterogeneous crystalline core which lead to the refinement of grains, and shortens the nucleation (Xu, Feng et al. 2017). The solidification temperatures results showed a high change, which receded from 124.8 to 119.6 °C as the base alloy treated with Sm. The solidification temperature clearly decreased and diffusion of Sm atoms become harder, nucleation of new phase formed at low temperature thereby widen the solidification temperature and solidification time in the present investigated alloy.

Microstructure Analysis
Optical micrographs are shown in Figure 2. As a result, the microstructures of alloys consist of α-Mg matrix and second phase crystallized along the grain boundaries, which showed a kind of massive morphology with dark contrast. In addition, microstructure of base alloy appears to have a greater density of fine second phase particles within the grain as well as a few coarse precipitates at the grain boundaries. As shown from SEM micrographs, the microstructure of untreated alloy consists of α-Mg grains marked A, surrounded by second phase crystallized along the grain boundaries as a kind of massive morphology (Figure 3a) marked B. Moreover, as a result of EDS spectrum for regions A and B, the intermetallic secondary phase was composed of magnesium, zinc, gadolinium and neodymium, (Marked B) and the matrix contains magnesium and a small amount of zirconium, (Marked A). SEM micrographs of the alloy treated with Sm showed that its microstructure was composed of primary α-Mg phase and intermetallic eutectic phase distributed along grain boundaries as well as a few isolated particles inside the grains also a third phase coexisted with the secondary phase. Two different regions and morphology for intermetallic compounds, which crystallized along the grain boundaries; light grey region (Figure 3-b marked D) and dark grey region (Figure 3-b marked C). The light grey region consists of Mg-Zn-Nd-Sm, and the dark grey phase consists of Mg-Nd-Gd-Zn-Sm, which formed as a third phase of the alloy. SEM images also showed that the microstructure refined as addition of Sm and the grain boundaries become extremely distinct. In addition, the eutectic phase showed as a needle and granular morphology distributed randomly, Figure 3

Mechanical Properties
As shown in Table 1, addition of Sm modified the values of Ultimate Tensile Strength(UTS).The UTS for base alloy with the value of 109 MPa. However, with 1.5 wt. %Sm, the UTS increased sharply to 127.56 MPa. The results of this study are in good agreement with that obtained from the relationship between tensile properties and grain size of the Mg-6Zn-0.4Zr magnesium alloy (Zhang, Huang et al. 2017). The improvement in tensile properties is attributed mainly to refined grains and the formation of Al 2 Sm intermetallic compounds (Chen, Jin et al. 2015). The YS increased sharply with the addition of 1.5 wt. % Sm, obtaining 97.6 MPa compared to 82.09 MPa for the base alloy. The yield strength is dependent on RE content and increases with more RE, because of secondary and ternary phases that appears when RE add to magnesium alloy, which is due to decreasing of grain size. Sm rare earth element has a high solid solubility into the magnesium alloy, therefore, the solid solution strengthening has an important role in improving the mechanical properties of the Mg alloys treated with Sm (Li, Liu et al. 2014). Addition of Sm led tosharply increases the elongation (El) of alloy, which were 5.934 and 8.176 % for 0 and1.5 wt.% Sm respectively, which attributed to the effect of the formed intermetallic phases.
Previous studies showed that the elongation of Mg alloys were increased as addition of Sm (ZHANG, HUANG et al. 2017), the intermetallic phases can provide very effective hindrances to grain boundary sliding and dislocation motion during deformation process, which leads to improved tensile properties ( The hardness value increased with addition of Sm, which obtained 60.4 Hv compared with 48.33 Hv for the untreated alloy, the improvement in hardness attributed to the presence of comparatively harder (Mg-Zn-RE) formed phases, as well as increasing its volume fraction. Moreover, the reduction in grain size (decremented by about 160 μm) as addition of Sm can also led to increase hardness value. It has been found that the addition of Sm to Mg alloys leads to increase hardness values with increasing Sm content due to the increase of volume fraction of the solute-rich precipitates (

Conclusions
The effects of added Sm on the solidification characteristics, microstructure, and mechanical properties of Mg-2.8 wt. % Nd-1.5 wt. % Gd-0.5 wt. % Zn-0.5 wt. % Zr magnesium alloy were investigated. The results are summarized as follows: (i) The computer-aided cooling curve thermal analysis and microstructure results showed that Sm rare earth improve solidification time and grain refinement, where the grain size of treated alloy decreased by 160 μm as decrease the solidification time by 47 s. (ii) SEM results showed that Sm combined with the initial phase of the intermetallic base alloy, which crystallized along the grain boundaries, and M g41 S m5 , M g3 Z n6 Sm and (Mg, Zn )3 Sm were obtained as new phases. (iii) The mechanical properties improved significantly as addition of Sm, were UTS and YS increased by 14.6 and 15.8 % respectively as Sm content reached 1.5 wt. %, furthermore, the hardness value improved greatly by 19.9 % as well.

Acknowledgements
The authors would like to thank Universiti Tun Hussein Onn Malaysia (UTHM) for the financial support.