Microstructure of the modified AZ61 alloy after subsequent compressive loading50

Based on the above results, the 4P45 modified alloy could possess the microstructure of fully recrystallized and fine grains, and a second subsequent processing of compression loading along the normal direction was applied on this 4P45 modified alloy. After a further compressive strain of ~3%, the microstructure of the FSP modified AZ61 alloy contained a high amount of twins inside the grains, as shown in Fig. 3-8. It could be known that the

deformation twining would be a dominant deformation mechanism for the modified AZ61 alloy under compression. Further discussions on the induced twinning during subsequent compression will be presented in Sec. 4.2.1.

3.3 The macrostructure and microstructure of Mg based composites made by FSP

3.3.1 The appearance of the stirred zone of composites

Figure 3-9 shows the top and cross-sectional views of the Mg based composites made by FSP. No sign of the grooves filled with silica powders can be observed in the nugget zone.

This result implied that the silica powders placed in the grooves have been stirred into the Mg alloys matrix. In order to fabricate bulk composites, it is necessary to achieve sufficient powder dispersion in the middle and bottom region away from the surface. Severe clustering silica powder was not present in OM micrographs after 2P FSP, as shown in Fig. 3-9(c).

Therefore, this method of fabricating Mg based composites could uniformly and effectively disperse the initial lump of silica powders into the Mg matrix. The microstructure observations by SEM and TEM for Mg based composites with the different passes and grooves are presented in the next section.

3.3.2 SEM characterizations of Mg matrix composites made by FSP

The dispersion of the nano-sized SiO₂ particles appeared to be reasonably uniform under OM. However, the OM observation is very limited due to its depth of field and resolution.

SEM observations are carried out to examine the dispersion at magnifications greater than 1000 times, and the observed results are analyzed by the Optimas image analysis software.

Severe clustering of silica particles would occur after 1P FSP on the outside rim of the onion ring, especially in the top region near surface. Nevertheless, this clustering phenomenon would be improved after 2P, as shown in Fig. 3-10. When the number of passes is greater than 3 (3P), the clustered silica powders on the outside rim of the onion ring became hardly seen. Note that severe clustering of silica particles like Fig. 3-10, seen in the outside rim, has never been observed in the central region of the onion ring, even after 1P FSP.

After 1P, the particle dispersion within the central cross-sectional area (i. e. T plane) was macroscopically uniform, as shown in Fig. 3-11(a) for the one deep groove (1D1P) and in Fig.

3-12(a) for the two deep grooves (2D1P). However, the observed particle size is frequently 0.3-1 μm, much larger than the individual SiO2 size (~20 nm). The situation after 2P FSP, with opposite FSP direction for the second pass, appears to be further improved, as shown in Fig. 3-11(b) for the one deep groove (1D2P) and in Fig. 3-12(b) for the two deep grooves (2D2P). If the number of passes was pushed into three passes or even to four passes, the phenomenon of clustered silica could be improved effectively.

The statistical measurements for the volume fraction of silica particles in matrix give the values around 5 vol% and 10 vol% for one groove and two grooves, respectively. These statistical results were analyzed on micrographs taken at 1000, 3000, and 7000 times of magnification. Figure 3-13 shows another statistical result for the evolution of the size of clustered silica under different FSP passes. The size of clustered silica could be smaller and smaller with increasing number of passes, but some larger particles around 2-5 μm in diameter can still be seen occasionally. These larger particles were identified to contain the manganese and aluminum elements by SEM-EDS, as shown in Fig. 3-14, most likely the Al₄Mn dispersoids formed in the AZ61 billet during semi-continuous casting. Table 3-2 lists the summary of the average SiO2 cluster size in the 1D (with Vf~5%) and 2D (with Vf~10%)

FSP specimens.

At high magnifications, these clustered silica are sited on the grain boundaries or triple junctions, but some smaller clustered silica particles are embedded inside the grains, as shown in Fig. 3-15. This implies that FSP in the condition of 800 rpm and 45 mm/min could provide enough energy to overcome the obstruction of smaller clustered silica particles, so that grain boundaries could migrate across these silica particles.

3.3.3 TEM phase identification

Figures 3-16 and 3-17 present the typical TEM micrographs of the Mg matrix composites made by FSP. It has a tendency for the grain size to decrease gradually with increasing FSP passes, different from the trend observed form the FSP modified alloys. For example, the average grain size of the 1D1P specimen is around 2-3 μm, but the average grain size of the 1D4P specimen could progressively refine to 1-2 μm. The grain size of the 2D1P specimen could directly refine to 1-2 μm, although the FSP pass number is only one pass. The average grain size of the 2D4P specimen is as low as 0.8 μm. This result means that silica particles could effectively restrain the rapid growth of grains during FSP, as compared with the un-reinforced AZ61 alloy. Table 3-2 lists the summary of the average AZ61 matrix grain size in the 1D (with Vf~5%) and 2D (with Vf~10%) FSP specimens.

Within the grain interior, tangled dislocations and SiO2 particles measuring around 20 nm can be seen in Fig. 3-18. The nano-sized silica particles added into the Mg alloy matrix were often observed to appear as clusters measuring 100-200 nm in size, as shown in Fig.

3-19(a). This result is also consistent with the statistical measurement for SEM micrographs, i.e., the cluster of 100-300 nm size is most frequently seen. Some of the original silica