Evolution of ternary Al-Cu-Fe. Data were continuously collected as the temperature increased from 350°C to 580°C. It was during this stage, specifically at 415°C, that Fe began to mix with the Al-Cu compositions. As the temperature exceeded 415°C, the XRD data showed that the peak of θ-phase in [112], [220] and [211] orientations were smaller and changed to a η-phase with [203, 403] orientation. Between 350°C to 435°C the peak position for Fe shifted to the right, but at 450°C it shifted left (Figure 2.a). The lattice parameter of Fe decreased from 2.8883Å at 350°C to 2.8657Å at 435°C, and then increased again to 2.878Å at 450°C. This decrease and increase could be attributed to the elimination of defects, structural relaxation or thermodynamic stability. At 470°C, Fe had completely mixed the Al-Cu compositions, as indicated by the appearance of the cubic β-phase (Al(CuFe)) with [110] orientation (Figure 2.a).
This phase evolution was also seen in the in-situ TEM images. From 350°C to 470°C, the void areas suddenly increased (Figure 2.b, c and d). And, at the 470°C, the diffraction pattern for β-phase was seen (Figure 2.d). As mentioned above, Fe cannot react with Cu at this temperature. Therefore, the interaction could be between Al and Fe or Al-Cu (η and θ-phases) and Fe. The interaction between Al and Fe has been shown to take place as it cools from a higher temperature 46. The addition of a few percent Fe to an Al-Cu alloy has also been shown to have a minimal effect, and with increasing the Fe wt% into Al-Cu, Al+ cubic β-phase (Al (CuFe)) will form 47. From Figure 2.b, the Fe started to mix at 415°C at the Cu-Fe interface. The Fe mixing increased as the sample was kept at 415°C for 2 minutes (Figure 2.c). When the temperature exceeded 465°C, the β-Al (CuFe) crystal appeared and the whole sample was covered with the β-phase after 2 minutes (Figure 2.d).
The phases and crystallization continued to evolve as the temperature was further increased to 580°C (Figure 2.a). When the temperature reached 500°C, the β-phase peak was sharper and there was no change in the β-phase peak position (Figure 2.e) However when the temperature increased to 540°C, the peak of the β-phase was smaller and other new peaks were observed (Figure 2.a). After the sample reached 580°C (Figure 2.f) and was kept at this temperature for 20 minutes (Figure 2.g), the β phase completely changed into a ω-phase (ω-AlCuFe), which remained throughout the following stage. Also, when the sample was kept at 580°C for 20 minutes, the void area increased and the area of ω-phase grew continuously (Figure 2.g).
These results indicate that when the temperature increases, the ternary phase is more stable than the binary phase. Saadi et al. reported the enthalpy of formation for θ-(Al2Cu), β-(Al50Cu38Fe12), ω-phase (Al7Cu2Fe) obtained indirectly by measuring the heats of dissolution of pure metals to be -15.95KJ and -24.23KJ and -24.09KJ, respectively 48, 49. However, others have reported that the composition of the β-phase is more likely to be Al50Cu40Fe10 or Al50Cu42Fe8, which would mean that the enthalpy of formation could be lower than -24.23KJ, around -23.83 KJ 50, 51. Overall, from our observations and from the enthalpy of formations, we can conclude that the ω-phase is more stable during the crystallization at high temperature and the system can be in a better state of equilibrium.