The most noticeable outcome of Figure
7 is the increase in the fraction of all the particles during homogenization at low temperatures, which acts in contrary to the aim of the homogenization process.[
3] This increase in the fraction of particles during the first 2 hours of homogenization at 390 °C and 430 °C, as shown in Figure
7, is due to the formation of a significant number of new particles, namely, MgZn
2 (
η) and Mg
2Si (
β). As stated previously, this finding indicates that part of the elements precipitate out by forming precipitates during the first 2 hours of homogenization.
To account for the dissolution of the GB phases, a study on the solubility limits of the elements composing the main GB phase, namely, Fe, Mn, and Si, is essential. The data on the solubility limits of the elements in Al-Mn-Fe-Si are scarce in the literature.[
32] In this case, the solubility of the elements in Al in the four-component Al-Mn-Fe-Si regions adjacent to the ternary systems may be estimated, based on three-component regions of the Al-Fe-Mn, Al-Fe-Si, and Al-Mn-Si ternary systems.[
32] Table
IV shows the solid solubility limits of iron, manganese, and silicon in the four-component Al-Fe-Mn-Si system at different temperatures.[
32] It is clear that, at low temperatures,
i.e., lower than 470 °C, the solubility limits of the elements forming the GB components are small in the
α-Al matrix. Therefore, considerable dissolution of the GB phases is not expected at these temperatures. This is in agreement with the results of the quantitative image analyses of the OM images, presented in Figure
7. At higher temperatures, however, the solubility limits increase, as shown in Table
IV, which indicates that most of the alloying elements along the GB phases are dissolved in the
α-Al matrix. It is also clear that, even at such high temperatures (
e.g., 550 °C), the solubility limits of the elements in the Al matrix are less than the weight percentages of those elements in the composition of the alloy. Therefore, complete dissolution of all the GB phases is not expected even after holding for a long time. This explains the observation in Figure
3(f) that there are still some GB phases remaining in the structure after homogenization at 550 °C. The EDX analyses confirmed the existence of these phases even after homogenization at 550 °C for 48 hours.
Table IV
Solid Solubility Limits of Iron, Manganese, and Silicon in the Four-Component Al-Fe-Mn-Si System at Different Temperatures[
32]
390 | 0.002 | 0.026 | 0.03 |
430 | 0.004 | 0.05 | 0.06 |
470 | 0.009 | 0.15 | 0.08 |
510 | 0.016 | 0.25 | 0.11 |
550 | 0.044 | 0.44 | 0.2 |