Fig. 2 shows the particle size analysis result, including the differential and cumulative particle size distributions. According to the figure, more than 86% of the particles have a particle size of fewer than 90 μm. Fig. 3 shows the optical microstructure of the Mg-Zn with various Zn contents produced under different temperatures, applied pressures, and times. SPS parameters were set the same as values in Table 2, and the image numbers are consistent with the sample numbers already listed in Table 2. In Fig. 3, images of 1,2,3½4,5,6½7,8,9 indicate Mg-1Zn½Mg-3Zn½Mg-5Zn, respectively. The images clearly show Mg grains, in which the black holes are porosities. As seen, the increase in Zn percent has increased the grain size. In the solidification of Mg alloys, Mg-rich regions forming at the solid/liquid interface inhabit the grain growth, and thus the grain size becomes smaller. Indeed, the alloy element helps to increase the nucleation sites, and its increase causes more decrease in grain size. The excessive amount of the alloying element leads to the creation of dendritic separation defects along the grain boundaries, which weakens the strength and elongation of the alloy. However, in powder metallurgy alloying, since there is no nucleation and the sintering temperature is close to the melting temperature of the elements, high temperature causes an increase in grain size.
The effect of all parameters on the grain size can be examined by calculating the average diameter of the grain using the MIP software and predicting other states using the Taguchi method. Taguchi method prediction and responses for different signal-to-noise (S/N) ratios and grain sizes are provided in Fig. 4 and Table 3. As seen, the Zn percentage is the most critical parameter which affects the grain size. The hardness and density measurement results as a function of SPS parameters based on the Taguchi method are given in Table 4. For Sample 1 (Mg-1Zn), the SPS temperature/time/pressure of 340 ℃/5 min/30 MPa resulted in the highest hardness value of 92.5 HV.
Table 3. Response table of S/N ratios for grain size.
Level
|
Zn content
(wt. %)
|
Time
(min)
|
Temperature
(ºC)
|
Pressure
(MPa)
|
1
|
37.16
|
-36.80
|
-37.61
|
-37.21
|
2
|
-36.56
|
-37.58
|
-36.70
|
-38.07
|
3
|
-38.38
|
-37.71
|
-37.78
|
-36.81
|
Delta
|
1.82
|
0.91
|
1.08
|
1.27
|
Rank
|
1
|
4
|
3
|
2
|
In porous samples, the higher the pressure applied to the press, the higher the density and hardness of the sample. However, the density does not exceed a specific limit by applying more pressure. Applying pressure on dense samples leads to the interconnection of grains and an increase in grain size, decreasing the hardness and strength of the material (according to the Hall-Petch relationship). Increasing the pressure also helps the recovery of samples in the early sinter stage and increases the plastic deformation ability and density. Table 4 shows that 30 MPa pressure is the optimized amount to reach the highest hardness. On the other hand, applying pressure causes micro-cracks in the sample, which also reduces the hardness and density of the sample. In MgZn alloys, volume shrinkage occurs during sintering at high temperatures due to the difference in Mg and Zn diffusion rates. By applying pressure, the volume shrinkage does not decrease but turns into micro-pores. For this reason, consistent with Fig. 5b, increasing the pressure causes a decrease in the hardness of the Mg-Zn alloy. Applying higher pressure will lead to the loss of micro-pores and more density of the material and, as a result, increase the hardness of the samples.
It is typically expected that sufficient bonding between powder particles is not formed during the short-time sintering. During the long-time sintering, two mechanisms conflict: i) grain growth and reduction of hardness and strength, and ii) the possibility of more diffusion of Zn in the surface layer and forming more bonds and thus increasing hardness and strength. An excessive increase in sintering time can lead to more grain growth and a reduction in the hardness of powder particles.
Table 4. Results of hardness and density measurements as a function of SPS parameters.
No.
|
Zn amount
(wt. %)
|
Time
(min)
|
Pressure
(MPa)
|
Temperature
(ºC)
|
Hardness
(HV)
|
Density
(g/cm3)
|
1
|
1
|
5
|
30
|
340
|
92.50
|
0.902
|
2
|
1
|
7
|
45
|
365
|
62.35
|
0.873
|
3
|
1
|
10
|
60
|
390
|
53.26
|
1.033
|
4
|
3
|
5
|
60
|
365
|
54.35
|
1.040
|
5
|
3
|
7
|
30
|
390
|
67.23
|
1.000
|
6
|
3
|
10
|
45
|
340
|
57.85
|
1.069
|
7
|
5
|
5
|
45
|
390
|
59.50
|
1.120
|
8
|
5
|
7
|
60
|
340
|
66.30
|
0.974
|
9
|
5
|
10
|
30
|
365
|
71.68
|
0.924
|
As the sintering temperature increases, the pores between the particles grow and the grain size increases, reducing hardness. Also, increasing the sintering temperature leads to an increase in diffusion kinetics, flexibility, and thus the compressibility of the material, resulting in a lower hardness reduction rate. A simultaneous rise in temperature and pressure can cause faster diffusion and bonding of powder particles [29].
According to the Mg-Zn binary phase diagram, the alloys of this research lie in the single-phase region, where increasing the amount of Zn causes an increase in density and a decrease in hardness [30]. Also, the reduction in hardness may be due to forming a secondary phase that is less hard than Mg. According to the Mg-Zn binary phase diagram, the Mg7Zn3 phase is created at 325 ℃ by eutectic reaction and decomposes into α-Mg and MgZn intermetallic compound during cooling. In addition, Zn can be wholly dissolved in α-Mg due to its relatively high solubility of up to 1.6 wt. % at room temperature [31]. Therefore, when the amount of Zn is less than the solubility limit, it tends to dissolve in the α-Mg matrix. Accordingly, Mg-1Zn is a single-phase alloy where Zn is dissolved in the α-Mg matrix [11]. On the other hand, by increasing Zn content, the grain size decreases and the hardness of the samples increases. Also, increasing the amount of Zn from 3 to 5 wt. % leads to the formation of a hard intermetallic compound of MgZn, increasing hardness.
Meanwhile, Taguchi response tables of S/N ratios for density (Table 5) and hardness (Table 6) are reported, consistent with Fig. 5(a,b), respectively. As shown in Table 5, temperature is the most effective SPS parameter in density, followed by the percentage of Zn content. According to Table 6, pressure has the most significant effect on hardness.
Table 5. Responses for S/N ratios for density of the samples.
Level
|
Zn amount
(wt. %)
|
Time
(min)
|
Temperature
(ºC)
|
Pressure
(MPa)
|
1
2
3
Delta
Rank
|
0.02299
-0.59786
0.30674
0.90460
2
|
0.14305
-0.46951
0.05833
0.61256
4
|
-0.18171
-0.50854
0.42212
0.93066
1
|
-0.52748
0.12807
0.13128
0.65876
3
|
Table 6. Responses for S/N ratios for the hardness of the samples.
Level
|
Zn amount
(wt. %)
|
Time
(min)
|
Temperature
(ºC)
|
Pressure
(MPa)
|
1
2
3
Delta
Rank
|
36.58
35.50
36.34
1.08
3
|
36.51
36.29
35.63
0.88
4
|
37.00
35.90
35.52
1.48
2
|
37.66
35.54
35.22
2.44
1
|
For further investigation of the Zn role in the Mg matrix, samples with different Zn percentages were studied under SEM, as shown in Fig. 6. If the amount of Zn is less than the solubility limit, it tends to be solved in the α-Mg matrix. Accordingly, as seen in Fig. 6(b), Mg-1Zn is a single-phase alloy with Zn as soluble. The Zn content is dissolved in the α-Mg matrix, while the Zn particles are homogeneously dispersed in the Mg matrix. More increase in the Zn content from 1 to 5 wt. % results in the creation and growth of intergranular boundaries. Also, the secondary phase formation is visible in the alloy with higher Zn content, especially in the Mg-5Zn alloy (white points in Fig. 6(f)).