XRD patterns for alloys Mg-5Sn and Mg-5Sn-3Zn are reported in Fig. 1. XRD analysis result confirms that there is no major impurities or inclusions present in the alloy. Mn addition does not show any significant peaks due to very low percentage of addition. Presence of α-Mg and intermetallics (Mg2Sn) are evident through XRD patterns.
XRD patterns for 0.5 and 2 wt% added Si in Mg-5Sn-3Zn-1Mn alloys is represented in the Fig. 2. Peaks of Diffraction for α-Mg, Mg2Sn and Mg2Si compound are detected in the XRD analysis. According to Mg-Si binary diagram, the Mg2Si phase is formed in the melt during solidification which is confirmed by the results. Moreover, it is noted that with increase of Si content from 0.5 to 2 wt%, the intensity of Mg2Si peaks increases.
Lower and higher magnification optical microscope results of (a) Mg-5Sn, (b) Mg-5Sn-3Zn and (c) Mg-5Sn-3Zn-1Mn alloys are shown in (Fig. 3). In Mg-Sn binary alloys Mg2Sn phase exists in the eutectic form (α-Mg+Mg2Sn) shown in the Fig 3a. It is also observed that Mg-Sn alloy possess dendritic structure consisting of gray α-Mg dendritic grain and black secondary phases. The eutectic Mg2Sn phase is distributed between α-Mg dendritic arms at grain boundaries. In (Fig.3b) the observed microstructure exhibited coarse dendritic morphology. It also shows the Mg2Sn phase in the interdendritic region and some of the particles inside the dendrites.
Fig. 3c shows the Mg-5Sn-3Zn alloy in which the dendritic grain size decreases with the addition of Zn. Moreover the interdendritic region is darker and wider compared with the binary Mg-Sn alloy. It shows a phase in the interdendritic region and some of the independent particles are found inside the dendrites. In fig. 3d it is found that the dendritic size is reduced and darker intermetallic is seen. The decrease in the grain size is clearly visible in the microstructure. Chen et al. studied the microstructure of Mg-5Sn-3Zn alloys and observed results of the cast ingot matrix are influenced by dendritic and irregular sediments. Mg-5Sn-3Zn has a rosette shape which is usually a dendritic structure and their interdentritic spacing is relatively large. In fig. 3e with the addition of 1% Mn shows a refined grain structure with Mg2Sn distributed even and wide on all the grain boundaries. The grain size and amount of dark phase are distributed and the approximate grain size is also gets reduced when compared with Mg-5Sn-3Zn alloy in fig. 3f. This is caused by the addition of Mn which leads to the suppression in the formation of Mg2Sn and MgZn2 phases.
The microstructure of Mg-5Sn-3Zn-1Mn with different Si additions (0.5, 1, 1.5 and 2 wt %) are shown in fig. 4a-d. An interesting observation was made through the microstructure is the morphology and distribution of Mg2Si intermetallic. In (Fig. 4a & b) very few Mg2Si particles are viewed inside the grains and most of them on the grain boundaries. The microstructure of the alloy containing Mg2Si depends on the amount of Si content; alloys with low-Si content represent the eutectic Chinese script Mg2Si. However, Si content greater than 1.5wt% changes the Chinese script Mg2Si to polygon type Mg2Si (Fig. 4c & d) this is clearly visible in higher magnification (fig5d) [24]. The Chinese script shape changes into a polygon and is evenly distributed along the grain boundaries with the increase of Si content. In addition, it was observed that addition of Si did not form another compound with Sn, Zn, or Mn elements.
Figure 5a-d represents the microstructural results of higher magnification to reveal the Chinese script and Polygon shapes of Mg2Si discussed in the context above. Guangyin et al. [25] studied the morphology of the microstructure of Mg-6Zn alloy with the addition of Si.
Microstructure results indicate the presence of interdendritic MgZn and Chinese-script Mg2Si particles in Mg matrix. Karakulak 2018 et al. [17] examined the morphological changes of the Mg-5Sn and Mg-10Sn binary alloys by adding Si. The examined microstructure revealed that addition of silicon to the binary alloy increase the amount of intermetallics at the grain boundary.
Table 1. EDS result for Mg-5Sn-3Zn-1Mn alloy
Element
|
MgK
|
SnL
|
ZnK
|
MnK
|
wt%
|
at%
|
wt%
|
at%
|
wt%
|
at%
|
wt%
|
at%
|
A
|
10.92
|
89.68
|
4.42
|
7.43
|
0.85
|
2.58
|
0.09
|
0.31
|
B
|
5.05
|
73.60
|
7.96
|
23.73
|
-
|
-
|
0.49
|
2.67
|
C
|
16.11
|
98.49
|
0.57
|
0.71
|
0.18
|
0.41
|
0.15
|
0.4
|
Fig. 6 shows the SEM-EDS analysis of Mg-5Sn-3Zn-1Mn alloy for analysing phases and the energy diagram of different region are shown in figure 7. The EDX energy values are tabulated in table 1. SEM-EDX results provide the presence of second phase which is a rod-shaped Mg2Sn on the grain boundary. Area analysis of region A displays the presence of the maximum elements like Mg, Sn, Zn and trace element like Mn. Solid solution mixtures of all the added elements are tabulated in table 1. Region B indicates the point analysis of SEM micrograph which shows that the white region consists of the interdendritic is Mg2Sn phases and similar findings are also observed by Zhang et al. [26]. Area analysis of region C indicates the maximum of Mg i.e., 98at% in the EDS analysis and minimum amount of other elements such as Sn, Zn and Mn. This confirms that this dendritic region is a solid solution of α-Mg matrix.
Fig.8 shows the SEM micrograph of Mg-5Sn-3Zn-1Mn-2Si alloy. In the α-Mg matrix there are two basic types of secondary phases are observed. The SEM-EDS results show the detail of Mg2Sn and Mg2Si secondary phases. Mg2Sn as rod-shaped in the interdendritic region and Mg2Si are like Chinese script and polygonal structure. Upto 1wt% of Si Chinese script Mg2Si structure and above 1.5wt% fine polygonal structure is observed from the EDX analysis.
Table 2 EDS result for Mg-5Sn-3Zn-1Mn-2Si alloy
Element
|
MgK
|
SnL
|
ZnK
|
MnK
|
SiK
|
wt%
|
at%
|
wt%
|
at%
|
wt%
|
at%
|
wt%
|
at%
|
wt%
|
at%
|
A
|
3.42
|
95.27
|
0.4
|
2.3
|
0.23
|
2.43
|
-
|
-
|
-
|
-
|
B
|
1.42
|
65.52
|
3.01
|
28.39
|
-
|
-
|
-
|
-
|
0.15
|
6.08
|
C
|
2.38
|
61.20
|
0.35
|
1.85
|
-
|
-
|
0.01
|
0.12
|
1.66
|
36.83
|
D
|
2.32
|
82.85
|
-
|
-
|
-
|
-
|
-
|
-
|
0.55
|
17.15
|
EDX energy diagram of different region are shown in fig. 9. Region A shows the point analysis, which indicates the presence of Mg, Sn and Zn elements around the grain boundary. The amount of elements present in the region is shown in the Table 2. The point analysis of white region B indicates the presence of Mg2Sn phase. The area EDS analysis of region C primarily indicates the presence of Mg and Si elements and it confirms Mg2Si phase which is tabulated. According to the EDS analysis, the Chinese script structure shows the composition of Mg2Si (region D).
The system has a eutectic temperature of 637°C based on the phase diagram of Mg-Si and the precipitation temperature of Mg2Si is higher than other intermetallics like Mg2Sn and MgZn2. It is almost impossible for these above mentioned particles to act as a heterogeneous site for nucleation on the phase of Mg2Si as prepared in different literatures [26, 27]. This argument is consistent with the results obtained in the paper. It is inferred from the fig. 8 region C has bright particles formed around the gray Mg2Si particles. EDS analysis reports that an irregular reticular arrangement at the boundary of grains having rod-shaped Mg2Sn particles. When comparing Mg-5Sn-3Zn-1Mn, with Si addition, the Chinese script phase of Mg2Si is located within the grain and grain boundaries, and with the addition of Si, its fraction of volume increases [22].
3.1 Hardness
Fig.10 depicts the Vicker’s hardness values for the cast base alloy and different Si added Mg-5Sn-3Zn-1Mn alloys. It is observed from the figure that with increasing Si content, the hardness is increased from 36 to 44 Hv. This enhancement in hardness of the alloy is due to the presence of hard Mg2Si intermetallic phase with increase in the Si addition. Moreover, increasing volume of Mg2Si with Si addition increases hardness. Addition of Si to the binary alloy reduces the size of the alloy grains and increases the hardness. Similar results have been reported by Vignesh. P et al. [20].
Comparing with the base alloys (Mg-5Sn, Mg-5Sn-3Zn and Mg-5Sn-3Zn-1Mn) Si added alloys show increased hardness value. Comparing with hypoeutectic (i.e., 0.5 and 1 wt% Si) alloys, the hypereutectic (i.e., 1.5 and 2 wt% Si) alloys have higher hardness which is due to the presence of the hard brittle polygonal primary Mg2Si crystals in the matrix along with the Chinese script eutectic.
3.2 Tensile Properties
Figure 11 shows the values of the ultimate tensile strength (UTS) and yield strength (YS) of the various Si added alloys. The values of UTS and YS increase by up to 1% Si addition after which the values begin to decrease. However, the UTS and YS values of 2 wt% Si added alloys are greatly reduced. The overall observation from this study shows that 0.5 and 1 wt% of Si provides better tensile properties. The increase in UTS and YS in 0.5 and 1 wt% alloy leads to increase in the eutectic volume. The reduction in the strength properties beyond 1.5 wt% is due to the existence of high volume of coarse Chinese script and brittle polygonal primary Mg2Si phases in the alloy.
The as cast alloys mechanical properties are determined by the intermetallic distributions and its morphology. Chen et al. [22] reported that on matrix Mg2Sn and MgSnY compounds are widely distributed. Mechanical properties are integrated while there are tiny rod-like deposits (MgZn2n and Mg2Sn), the strength and ductility increases and with the presence of huge, asymmetrical MgSnY clusters, elongation is greatly reduced. When the MgZn2 nucleation is stimulated by Mg2Sn, precipitates of fine size (2 - 4 μm) are present at the boundaries of grains and within the grains, the grain boundaries are stabilized in deformation due to fine phases and properties of cast alloy is provided by the precipitates. In addition, dissociation loops form and the dislocation loop moves towards the precipitates. MgZn2 particles stimulated by Mg2Sn are useful in dislocation tangle and improving strength [6, 22]. Similar interpretations are formed in the present study also.
The improvement in tensile strength was mainly due to two aspects: (1) refinement of the microstructure and (2) formation of precipitates like Mg2Sn & Mg2Si. Above 1.5wt% of Si the reduction of mechanical properties are due to the changes of Mg2Si morphology. Along interfaces long crack easily nucleates between Chinese script Mg2Si particle and the Mg matrix. Similarly Guangyin et al. [25] Kumar et al. [28] reported with an increase of 0.25% calcium, the tensile strength decreases. Too much calcium addition enhances the quantity of coarse polygonal Mg2Si particles that has negative outcome on mechanical properties. Fracture surfaces of the tested specimens are shown in the figure 12. In the fractographs it is observed that addition of Si increases brittleness (cleavages) and occurrence of secondary cracks. This implies that the coarse particles initiate sites for fracture. Shallow dimples in the fractograph reveal shear deformation [29].