Chapter 3 Experimental results
3.7 Mechanical tests
Mechanical testing is focused on the tensile response of the modified alloys and Mg based composites, and the test temperatures are selected to be at the room temperature and the elevated temperatures of 250oC, 300oC, 350oC and 400oC. These mechanical properties are presented below.
3.7.1 Mechanical properties at room temperature
3.7.1.1 Mechanical properties of the modified alloys made by FSP
Figure 3-32 shows the engineering stress and strain curves of the tensile tests for the modified AZ61 alloy. These tensile samples are machined parallel to the welding direction, called the “WD” specimens. Table 3-8 summaries the yield stress (YS), ultimate tensile strength (UTS) and elongation for these FSP modified AZ61 alloys. The YS of the as-received AZ61 billet was 140 MPa. It must be noted that the YS was not apparently improved for the AZ61 Mg alloy after FSP, namely, from 140 MPa to 147 MPa only.
However, the grain size has been refined from 75 μm to 7 μm. If the pass number of FSP was pushed up to 4 passes, the YS of the 4P45 sample was similar to that of the 1P45 modified alloy, only 140 MPa. But the UTS of the modified AZ61 alloy could be raised from 190 MPa to 242 and 327 MPa for the 1P45 and 4P45 modified alloys, respectively. If the Hall-Petch relation is followed, apparent improvement on the YS should be observed. The reasons for this situation will be discussed in Sec. 4.1.3.1, and a feasible solution to improve such a lower YS for the FSP modified Mg alloys is proposed.
In order to measure and compare the difference of the mechanical properties parallel to the welding direction (WD) and perpendicular to the welding direction (TD) of the modified alloy, the mini tensile specimen, as shown in Fig. 2-11, was machined along these two
directions. Figure 3-33 shows the engineering stress and strain curves of the tensile tests for the WD and TD specimens of the 1P45 and 4P45 modified alloys. Table 3-9 summaries the tensile properties for the 1P45 and 4P45 samples along WD and TD. Broadly speaking, the YS data for the WD or TD specimens were both around 130-140 MPa, and the difference of YS was very small, no matter of the 1P45 or 4P45 modified alloys. It implies that the FSP modified Mg alloys do not possess the strong anisotropy for the tensile properties at room temperature.
Figure 3-34 shows the comparison of engineering stress and strain curves of the tensile tests for the modified AZ61 alloy with and without subsequent compression. The 4P45 modified alloy with subsequent compression, namely, the 4P45-cp modified alloy, could raise the YS from the 140 MPa to 178 MPa. Except for the increased YS, the strain-hardening rate after yielding deformation of the 4P45-cp modified alloy was higher than that of the modified alloy without compression. In addition, the UTS of the 4P45-cp modified alloy was also raised from 327 MPa to 362 MPa. It will be clear from the above results that the FSP modified Mg alloys with a subsequent second compression could remarkably improve the disadvantage of lower yielding stress. The reason will be discussed in Sec. 4.2.3.
3.7.1.2 Mechanical properties of Mg based composites fabricated by FSP
As for the Mg matrix composites, Fig. 3-35 shows the engineering stress and strain curves of the tensile tests for Mg based composites in comparison with the 4P45 modified alloy. Table 3-8 summaries the YS, UTS, and elongation for the modified alloys and Mg based composites. The YS of the FSP composites could be improved to 214 MPa for the 1D4P specimens and to 225 MPa for the 2D4P specimens, compared with only 140 MPa in the 4P45 modified AZ61 alloy without any silica reinforcement. But the UTS of the
composite specimen was lower than that of 4P45 modified alloy. It could be seen that the YS and UTS would not continuously increase their values for FSP passes greater than 3P; the 4P specimens exhibit little increment as compared with the 3P or 2P specimens. However, the YS data on all Mg based composites, no matter of the 1D, 2D, 2P or 4P samples, were all higher than that of the modified alloy.
3.7.2 Tensile behavior of the FSP Mg alloy at elevated temperatures
Systematic elevated temperature tensile tests were performed on the FSP modified Mg alloy subject to 1P and 4P, as well as on the 1D4P, and 2D4P Mg based composites. Both the WD and TD specimens of the FSP modified alloy samples were included for the elevated temperature testing. As for the FSP Mg based composites, the high strain rate superplasticity behavior is of concern of the elevated temperature tensile testing. In order to simplify the experiments, the tensile direction was limited to transverse direction (TD) and the tensile specimen size was a mini dog-bonded tensile sample, as shown in Fig. 2-11.
3.7.2.1 Tensile behavior of the WD specimens of the modified alloy at elevated temperatures
Figures 3-36 and 3-37 show the true stress and strain curves for the WD specimens of the 1P45 and 4P45 modified alloys, respectively.
Table 3-10 summaries the results of the ductility at elevated temperatures for the FSP modified AZ61 alloy. It appears difficult to find the difference of tensile behavior from the numerous experimental data in Table 3-10; therefore, the variation of the ductility at 300oC and 400oC as a function of loading strain rate is plotted in Fig. 3-38. It is clear that the
specimens of the 1P90 modified alloy possessed the better ductility at 300oC than that of the 1P45 modified alloy, and the ductility at a higher elevated temperature, 400oC, was superior to that at 300oC. But these ductility results of the WD specimens of the 1P modified alloy did not reach the minimum requirement of superplasticity, namely, a ductility greater than 200%.
The ductility of the WD specimens of the 4P modified alloy could have a 230% elongation at 300oC and 1x10-4 s-1.
3.7.2.2 Topography of deformed specimens
In addition, the 1P specimens after deformation at elevated temperatures revealed the onion-split characteristic in appearance, as shown in Fig. 3-39(a)-(c); however, the 4P specimens did not, as shown in Fig. 3-39(d). The onion-split for the 1P modified alloy seemed to influence the ductility at elevated temperatures. The reason in causing this problem and the solution to improve this disadvantage will be discussed in the next chapter. Figure 3-40(a) shows the characteristic of grain boundary sliding on the deformed surface. The emerging grains at the onion-split position appear the striation marks on the surface of grains.
Striations were the evidence of exposed grain boundaries after grain boundary sliding [146].
3.7.2.3 Tensile behavior of the TD specimens of the modified alloy at elevated temperatures
Figures 3-41 and 3-42 show the true stress and strain curves for the 1P45 and 4P45 specimens, respectively.
Figure 3-43 shows the variation of elongation at elevated temperatures of the 1P45 and 4P45 TD alloy specimens as a function of loading strain rate. It is clear that the modified
alloy did not possess the better ductility (only 20-50%) at temperatures higher than 350oC. In contrast, the elongation at 300oC and 1x10-4 s-1 was the maximum (~230-250%), no matter of the 1P45 or 4P45 sample, but the elongation became around 100% as the temperature was lowered down to 250oC. The TD specimens after deformation did not appear the onion-split like the WD specimens. In addition, it is worth noting that the 1P45 modified alloy showed an elongation difference for the WD and TD specimens, especially at strain rates lower than 1x10-3 s-1, as shown in Fig. 3-44. However, the 4P45 modified alloy did not have this difference, and the ductility ability was very similar at all strain rates. The reason for the elongation difference for the WD and TD specimens of the 1P45 modified alloy and the reason for the suitable deformation temperature at 300oC, will be discussed in Sec. 4.1.3.2.
3.7.3 Tensile behavior at elevated temperatures for the FSP Mg based composites
As mentioned in Sec. 3.2, it is known that Mg based composites could have a better microstructure in terms of grain size and the reinforcement dispersion, and show the better mechanical strength after 4P FSP. In addition, the multi-passes FSP, such as 4 passes, would effectively improve the onion-split disadvantage to decrease the anisotropy of the ductility for the WD and TD specimens. Therefore, the tensile tests at elevated temperatures for the FSP Mg based composites were focused on the TD and 4P FSP specimens, the specimen size shown in Fig. 2-11. Systematic tensile tests were performed on the one-groove (1D) and two-grooves (2D) Mg based composite samples.
3.7.3.1 Tensile behavior at elevated temperatures for the 1D Mg based composites
Figure 3-45 shows the true stress and strain curves for the 1D4P Mg based composites samples.
Figure 3-46(a) shows the variation of tensile elongation as a function of strain rate at 250-400oC for the FSP 1D4P Mg based composite samples. Table 3-11 summaries the tensile results. It can be seen that the optimum strain rate for superplasticity performance increased with increasing loading temperature. For example, the optimum strain rate at 250oC for the 1D4P samples with an elongation of 208% is the range of 6x10-4 s-1; and the optimum strain rate at 300, 350oC, and 400oC with an elongation of 281%, 238%, and 136% could be raised to 1x10-3, 1x10-2,and 1x10-1 s-1, respectively. The superplasticity performances, including higher elongation, lower temperature, and higher strain rate, of the 1D4P Mg based composites samples were better than those of the FSP modified Mg based alloy processed at the same parameters of 800 rpm, 45 mm/min and 4 passes. Figure 3-47 shows the 1D4P specimens after deformation. The specimens appear a severe level of local necking at 300oC and 350oC, and strain rate of 6x10-4 s-1, but the specimens show free-necking gage section characteristic of superplastic flow at 300oC and 350oC, as the strain rate is pushed up to 1x10-3 s-1. The specimens at 400oC appear the local necking phenomenon again at a strain rate of 1x10-3 s-1, in contrast to that at 300oC and 350oC.
3.7.3.2 Tensile behavior at elevated temperatures for the 2D Mg based composites
Figure 3-48 shows the true stress and strain curves for the 2D4P Mg based composite samples. In average, the flow stress levels of the 2D4P specimen are typically lower than those of the 1D4P and the FSP modified alloy specimens at the same loading rate and temperature, suggesting the smoother operation of GBS.
Figure 3-46(b) shows the variation of tensile elongation as a function of strain rate at 250-400oC for the FSP 2D4P Mg based samples. Table 3-11 summaries the tensile results.
The 2D4P Mg based composites samples also had the same trend of the optimum strain rate for superplasticity performance which increases with increasing loading temperature.
However, the optimum strain rate of the 2D4P Mg based composite samples was superior to the 1D4P Mg based composite samples or the FSP modified Mg based alloy. For example, the optimum strain rate at 300oC for the 2D4P samples with an elongation of 470% is in the range of 1x10-2 s-1; and the optimum strain rate at 350 and 400oC with an elongation of 410%
and 454% could be raised to 1x10-1 and 3x10-1 s-1, respectively, all exceeding the criteria for HSRSP. The optimum strain rate and elongation performance of the 2D4P samples were the best as compared with the 1D4P Mg based sample or the modified alloy. Figure 3-49 shows the appearance of the 2D4P specimens after deformation. The specimens appear a severe level of local necking at 1x10-3 s-1 and 350oC or 400oC, but the specimens show the neck-free gauge section characteristic of superplastic flow as the strain rate exceeding 1x10-2 s-1 and 1x10-1 s-1 at 350oC and 400oC, respectively. It implied that the cavity effect would dominate the failure mechanism at the lower strain rates for the composite samples.
3.7.3.3 Topography of deformed specimens
Figure 3-50 shows the surface topography of the 2D4P specimens deformed at 350oC and 1x10-1 s-1 to the different strain levels. It reveals the evidence of grain boundary sliding.
The grain size can be seen to be maintained in the range of 1-2 μm. The inserted nano particles have played an effective role in restraining grain growth not only during FSP but also during the subsequent static annealing and superplastic deformation at elevated temperatures.