Figure 1 is the XRD patterns of the as-cast AZ80–2Sn-x mass %Bi (x=0, 0.5, 1, 1.5) alloys. As shown in Figure 1(a), the as-cast AZ80–2Sn alloy is mainly consisted of α-Mg phase, β-Mg₁₇Al₁₂ phase and Mg₂Sn phase, while additional peaks of Mg₃Bi₂ phase emerge with the addition of Bi (Figure 1(b)–(d)). Figures 2 and 3 show the microstructure of the as-cast AZ80-2Sn alloys with different Bi contents. The as-cast AZ80-2Sn alloy consisted of primary α-Mg matrix, Mg₂Sn phase and divorced eutectic β-Mg₁₇Al₁₂ phase, which is semi-continuous and distributes in the form of network at grain boundaries (Figures 2(a) and 3(a)). The average grain size of the as-cast AZ80-2Sn alloy is about 81 μm. However, the amount of semi-continuous eutectic phase decreases and the new fine disperse phase occurs at grain boundaries with addition of 0.5 mass % Bi (Figures 2(b) and 3(c)). XRD and EDS analysis indicate that the new phase is Mg₃Bi₂ phase. The grains of as-cast AZ80-2Sn-0.5 %Bi alloy get refined and the average grain size decreases to about 72 μm. The grains get further refined with an increase in Bi content, as shown in Figure 2((c) and (d)) and Figure 3((e) and (g)). The average grain size decreases to about 61 μm and more discontinuous β-Mg₁₇Al₁₂ phases occur with addition of 1 mass % Bi. When the contents of Bi increases to 1.5 mass %, the average grain size decreases to about 55 μm, while flaky-shaped Mg₃Bi₂ phases distribute at grain boundaries and semi-continuous reticulated β-Mg₁₇Al₁₂ phases increase.

Figure 1:

The XRD patterns of the as-cast AZ80–2Sn alloys with different Bi additions: (a) 0 mass %Bi; (b) 0.5 mass %Bi; (c) 1 mass %Bi; (d) 1.5 mass %Bi.

Figure 2:

Optical micrographs of the as-cast AZ80–2Sn alloys with different Bi additions: (a) 0 mass %Bi; (b) 0.5 mass %Bi; (c) 1 mass %Bi; (d) 1.5 mass %Bi.

Figure 3:

SEM morphology of as-cast AZ80–2Sn alloys with different Bi additions: (a) 0 %Bi; (c) 0.5 %Bi; (e) 1 %Bi; (g) 1.5 %Bi;(b) EDS of A; (d) EDS of B; (f) EDS of C; (h) EDS of D.

The grains of alloys get refined with the addition of Bi. This may be the result of newly formed Mg₃Bi₂ phase, which has a structure of hexagonal closed-packed (a=0.4666 nm, c=0.7401 nm) [11]. Based on the Bramfitt two-dimensional lattice mismatch degree theory, when the lattice mismatch δ<6 %, it is most effective to promote nucleation as the nuclei during heterogeneous nucleation. The orientation relationship between Mg₃Bi₂ and α-Mg matrix can be described as [0001] _Mg3Bi2//[110] _α-Mg and (010) _Mg3Bi2//(010) _α-Mg. As the lattice mismatch between {010} _Mg3Bi2 and {010}_α-Mg is only 3 % in the [010] _α-Mg direction [11], Mg₃Bi₂ phase can effectively promote nucleation of α-Mg as the nuclei. Furthermore, Mg₃Bi₂ has a higher melting point (T_m=821 °C), it can nucleate firstly during solidification and then some of Mg₃Bi₂ can act as the nucleation sites for α-Mg, stimulating nucleation of α-Mg; while the other Mg₃Bi₂ may gather around the front of the newly formed α-Mg phase, preventing the further growth of the grain [16]. In this case, the grains of the alloys were refined with addition of Bi.

The optical micrograph (OM) and back scattered electron (BSE) images of as-extruded AZ80–2Sn alloys with different Bi contents are plotted in Figures 4 and 5. As shown in Figure 4, the grain size of as-extruded alloys is decreased compared with those of as-cast alloys, this is because the grains were broken seriously by the three-dimensional stress and got refined through dynamic recrystallization (DRX). There are some black areas in Figure 4(a), in which some are the broken Mg₁₇Al₁₂ phases and the others are the recrystallized grains (Figure 5(a)). As shown in Figures 4(b) and 5(b), the average grain size of as-extruded AZ80–2Sn alloy with addition of 0.5 mass % Bi decreases from 25 μm to 12 μm and fine granular Mg₁₇Al₁₂ and Mg₃Bi₂ phases distribute dispersedly in the α-Mg matrix. The effect of 0.5 mass % Bi addition on grain refinement of AZ80–2Sn alloy is obvious. On one hand, Mg₃Bi₂ phase formed by addition of Bi has good thermal stability, thus can be broke and distribute along grain boundary during extrusion process. The Mg₃Bi₂ phases along grain boundary are hard to be softened at elevated temperature and can prevent merge of grains and growth. On the other hand, serious distortion around Mg₃Bi₂ phases can occur during extrusion process, which results in the formation of distorted regions with high dislocation density. The distortion regions can promote the recrystallized grains to nucleate, thus increasing the nucleation rate and refining grains. However, when the content of Bi exceeds 0.5 mass %, the average grain sizes of as-extruded AZ80–2Sn-1 mass %Bi and AZ80–2Sn-1.5 mass %Bi alloys both increase to about 15 μm (Figure 4(c) and 4(d)). In addition, the Mg₁₇Al₁₂ and Mg₃Bi₂ phases begin to concentrate and grow near grain boundary with addition of 1 mass % Bi (Figure 5(c)), and continue to grow further with addition 1.5 mass % Bi (Figure 5(d)). With an increase in the content of Bi (>0.5 mass %), Mg₃Bi₂ phases can agglomerate and the dispersion rate decreases.

Figure 4:

Optical micrographs of the as-extruded AZ80–2Sn alloys with different Bi additions: (a) 0 mass %Bi; (b) 0.5 mass %Bi; (c) 1 mass %Bi; (d) 1.5 mass %Bi.

Figure 5:

SEM back scattered electron images of the as-extruded AZ80–2Sn alloys with different Bi additions: (a) 0 mass %Bi; (b) 0.5 mass %Bi; (c) 1 mass %Bi; (d) 1.5 mass %Bi.

The tensile test results of the as-extruded AZ80–2Sn alloys with different Bi additions are shown in Figure 6. It can be seen that the tensile strength (UTS), yield strength (YS) and elongation of the as-extruded AZ80–2Sn alloys with the addition of Bi are improved. The UTS, YS and elongation of the as-extruded AZ80–2Sn alloy are 349.1 MPa, 229.8 MPa and 9.6 %, respectively. The as-extruded AZ80–2Sn-0.5 mass %Bi alloy has the best combination of UTS, YS and elongation, which are 379.6 MPa, 247.1 MPa and 14.8 %, respectively. Under as-extruded conditions, the UTS, YS and elongation of AZ80–2Sn-0.5 %Bi alloy compared with AZ80–2Sn alloy are improved by 8.7 %, 7.5 % and 54.2 %, respectively, which may be the effect of fine-grain strengthening and dispersion strengthening. According to Hall-Petch equation, σ=σ₀ + kd^−1/2 [17], the yield strength is in inverse proportion to the square root of grain diameter, so the yield strength of alloys will increase with the refinement of grain. Therefore, the yield strength of as-extruded AZ80–2Sn-0.5 mass % alloy increases with the decrease of grain size. Meanwhile, the fine granular and uniformly distributed Mg₁₇Al₁₂ and Mg₃Bi₂ phases can act as the obstruction to dislocation slipping, which can also improve the yield strength. When the content of Bi exceeds 0.5 %, the mechanical properties of experimental alloys decrease with further addition of Bi, this is because that the Mg₃Bi₂ and Mg₁₇Al₁₂ phases become coarsened, which can weaken the effect of dispersion strengthening. Moreover, Mg₃Bi₂ phase has good thermal stability, so it seems possible that Bi addition can strengthen AZ80–2Sn alloy at elevated temperature. Further investigations are under way.

Figure 6:

Tensile properties of the as-extruded AZ80–2Sn alloys with different Bi additions tested at room temperature.