5.1 Microstructure characterization
5.1.4 Aged alloys
Precipitation is affected by both the temperature and solute content. A high solute content alloy can accelerate the precipitation process compared to a low solute content one. Temperatures, on the other hand, affect not only the precipitation kinetics but also the precipitation sequence.
2 mm
2 mm
2 mm
2 mm
a)
d) c)
b)
47 Since the precipitation sequence plays a more important role in precipitation strengthening, the influence of temperature on the precipitation sequence is of more interest in the current study.
Therefore, alloys with the highest solute content of each series are selected to represent the precipitation behaviour under different ageing temperatures.
5.1.4.1 Mg-Al alloys
From Fig. 5-17 to Fig. 5-19, the microstructures of Mg-8Al alloys aged at 175 °C, 200 °C and 225 °C are shown. The precipitates in the peak-aged and over-aged condition (Fig. 5-32) are determined using X-Ray diffraction (Fig. 5-20). The precipitates are confirmed to be Mg17Al12
phase in both conditions.
At the early stage of ageing at 175 °C, discontinuous precipitation first occurs at the grain boundaries and no continuous precipitation is observed, as the sample was aged for 4 hours in Fig. 5-17. The volume of discontinuous precipitation increases after the aged time extends to 32 hours and continuous precipitation occurs inside the grain. When the sample reaches peak-aged for 96 hours, the precipitation occurs everywhere. Both discontinuous and continuous precipitation exist in the alloy aged at 175 °C.
Fig. 5-17 Microstructures of Mg-8Al alloy aged at 175 °C.
Fig. 5-18 and Fig. 5-19 are microstructures of Mg-8Al alloys aged at 200 °C and 225 °C, respectively. The main characteristics of the microstructure are similar to that aged at 175 °C,
50μm 50μm
20μm 4 hrs
96 hrs
32 hrs
48
but the precipitation process is accelerated. In the sample aged at 175 °C for 32 hours, discontinuous precipitation dominates and a small amount of continuous precipitation occurs inside the grains. In the sample aged at 200 °C for 16 hours, except the discontinuous precipitation, a large number of continuous precipitation occurs inside the grain. In the sample aged at 225 °C for 8 hours, precipitation occurs through the entire sample.
Fig. 5-18 Microstructures of Mg-8Al alloy aged at 200 °C.
Fig. 5-19 Microstructures of Mg-8Al alloy aged at 225 °C.
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20μm 4 hrs
96 hrs
16 hrs
4 hrs
32 hrs
8 hrs
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20μm
49 Fig. 5-20 X-Ray diffraction of Mg-8Al alloys aged at different conditions.
5.1.4.2 Mg-Gd alloys
Fig. 5-21 and Fig. 5-22 show the microstructures of Mg-2.5Gd alloys aged at different conditions. Since God has a high absorption coefficient of X-ray generated by Cu target, synchrotron radiation diffraction (Fig. 5-23) is used to analyse the peak-aged and over-aged condition (Fig. 5-33). The results show that the phases in the peak-aged condition are the 𝛽′ phase, 𝛼-Mg and preformed GdH2 phase during solution treatment. In the over-aged condition, the peaks of Mg5Gd are observed.
Fig. 5-21 Microstructures of Mg-2.5Gd alloys aged at 200 °C and 225 °C.
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50
As in Fig. 5-21, a high number density of precipitates are observed and uniformly distributed in the magnesium matrix in the alloy aged at 200 °C for 144 hours. Although in the SEM micrographs, no obvious phase is observed for alloy aged at 225 °C for 8 hours. The synchrotron patterns show the existence of the 𝛽′ phase. The precipitates are not observed because of the limited resolution of the SEM. In the alloy aged at 200 °C for 144 hours, a solute-depleted area is observed near the grain boundary. This solute-depleted area is formed due to the consumption of the solute atoms by the grain boundary precipitates. The microstructures in the over-aged condition show only a slight difference to that in the peak-aged condition. In the sample peak-aged at 225 °C for 240 hours, a coarser and brighter phase is observed. A solute-depleted area is also formed around the newly formed precipitates, suggest that the newly formed phase has a high Gd content.
Fig. 5-22 shows the microstructure evolution of Mg-2.5Gd alloys aged at 250 °C. After aged for 1 hour, the precipitation first occurs at the grain boundary. After aged for 2 hours, except for the grain boundary precipitates, a large number of the 𝛽′ phases are observed inside the grain. When the sample is over-aged to 16 hours, the dominant precipitates are still the 𝛽′ phase, but the 𝛽′ phase becomes coarser. After extending the aged time to 240 hours, the 𝛽′ phase becomes coarser and another type of phase, which is similar in the sample aged at 225 °C for 240 hours, is observed. According to the synchrotron diffraction pattern, this phase is the equilibrium 𝛽 phase. The solute-depleted area around the 𝛽 phase is very clear.
Fig. 5-22 Microstructures of Mg-2.5Gd alloys aged at 250 °C.
1 μm 200 nm
1 μm 1 hrs
16 hrs
2 hrs
240 hrs
500 nm
51 Fig. 5-23 Synchrotron diffraction of Mg-2.5Gd alloys aged at different conditions.
5.1.4.3 Mg-Sn alloys
From Fig. 5-24 to Fig. 5-26 are the microstructures of Mg-2.5Sn alloys aged at 175 °C, 200 °C and 225 °C, respectively. The precipitates in the peak-aged and over-aged condition (Fig. 5-34) are determined using X-Ray diffraction (Fig. 5-27). The precipitates are confirmed to be Mg2Sn phase in both conditions.
Fig. 5-24 Microstructures of Mg-2.5Sn alloy aged at 200 °C.
1 2 3 4 5 6
52
Fig. 5-25 Microstructures of Mg-2.5Sn alloy aged at 225 °C.
Fig. 5-26 Microstructures of Mg-2.5Sn alloy aged at 250 °C.
Continuous precipitation is the primary type in Mg-Sn alloys. As in Fig. 5-24, grain boundary precipitation and continuous precipitation occur in the sample aged at 200 °C for 16 hours.
When the ageing time is prolonged, the volume of continuous precipitation inside the grain increases and discontinuous precipitation is not observed. The continuous precipitation has a visible orientation relationship with the matrix grains. The precipitation processes at 225 °C and 250 °C are similar to those at 200 °C, but with accelerated precipitation kinetics. After being aged for 16 hours, the sample aged at 250 °C has a much higher density of precipitates than those aged at 200 °C.
10μm 10μm
10μm 16 hrs
72 hrs
32 hrs
10μm 10μm
10μm 8 hrs
120 hrs
16 hrs
53 Fig. 5-27 X-Ray diffraction of Mg-2.5Sn alloys aged at different conditions.
5.1.4.4 Mg-Zn alloys
From Fig. 5-28 to Fig. 5-30 are the microstructures of Mg-2.5Zn alloys aged at different conditions. The precipitates in the peak-aged and over-aged condition (Fig. 5-35) are determined using X-Ray diffraction (Fig. 5-31).
Fig. 5-28 Microstructures of Mg-2.5Zn alloy aged at 175 °C.
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54
Fig. 5-29 Microstructures of Mg-2.5Zn alloy aged at 200 °C.
Fig. 5-30 Microstructures of Mg-2.5Zn alloy aged at 225 °C.
The precipitates are confirmed to be MgZn2 in the over-aged condition. In the peak-aged condition, broad peaks appear around the same position as the MgZn2 phase. However, the Mg4Zn7 phase also has peaks between 40 ° to 42 °, so the precipitates in the peak-aged condition may be the Mg4Zn7 phase. Fig. 5-30 shows the typical microstructure evolution of Mg-2.5Zn during ageing treatment. When peak-aged, large numbers of fine and elongated precipitates are found inside the grain, some block-shaped precipitates also exist. If the ageing time is prolonged to reach an over-aged condition, the elongated precipitates become coarse
2 μm 2 μm
2 μm
1 hrs 4 hrs
120 hrs
1 μm 2μm
2 μm
1 hrs 16 hrs
240 hrs
55 and the number of the block-shaped precipitates increases. When the ageing time is further prolonged to 240 hours, the coarsening of the elongatedprecipitates and increased number of the block-shaped precipitates becomes more obvious. The microstructure characterization of Mg-2.5Zn aged at 175 °C and 200 °C is similar to that aged at 225 °C with the fact that the precipitation process takes a longer time due to the lower ageing temperatures.
Fig. 5-31 X-Ray diffraction of Mg-2.5Zn alloys aged at different conditions