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XD TiAl alloys (Ti-45 and 47Al-2Nb-2Mn ϩ 0.8 vol pct TiB 2 ) (at. pct) were oil quenched to produce fine-grained fully lamellar (FGFL) structures, and aging treatments at different temperatures for different durations were carried out to stabilize the FGFL structures. Microstructural examinations show that the aging treatments cause phase transformation of ␣ 2 to ␥, resulting in stabilization of the lamellar structure, as indicated by a significant decrease in ␣ 2 volume fraction. However, several degradation processes are also introduced. After aging, within lamellar colonies, the ␣ 2 lamellae become finer due to dissolution, whereas most of the ␥ lamellae coarsen. The dissolution of ␣ 2 involves longitudinal dissolution and lateral dissolution. In addition, at lamellar colony boundaries, lamellar termination migration, nucleation and growth of ␥ grains, and discontinuous coarsening occur. With the exception of longitudinal dissolution, all the other transformation modes are considered as degradation processes as they result in a reduction in ␣ 2 /␥ interfaces. Different phase transformation modes are present to varying degrees in the aged FGFL structures, depending on aging conditions and Al content. A multiple step aging reduces the drive force for phase transformation at high temperature by promoting phase transformation via longitudinal dissolution at low temperatures. As a result, this aging procedure effectively stabilizes the lamellar structure and suppresses other degradation processes. Therefore, the multiple step aging is suggested to be an optimal aging condition for stabilizing FGFL XD TiAl alloys.

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Surface defects such as streaks are often present on anodized extrusions of 6xxx series alloys. Microstructural examination and extrusion trials were conducted to investigate the microstructural effect on the surface appearance and the formation of streak defects. It was found that the formation of an inhomogeneous distribution of grain boundary grooves is a critical factor for the formation of die streaks on anodized aluminum extrusions. This is influenced by various microstructural features. Fe-rich intermetallic particles reduced the difference in severity of grain boundary grooves on the same extrusion surface through their effect on other microstructure features, reducing the tendency for streaking. Also, grain size was found not to be a direct cause for the formation of grain boundary grooves, but it does influence the distribution and amount of grooves. Furthermore, variation of grain orientations caused the formation of grain etching steps, which influenced the morphology of grain boundary grooves, but these effects were not significant in the formation of the streak defects. However, Mg 2 Si precipitates influenced the difference in dissolution rates between grain interiors and grain boundaries during etching, dramatically influencing the formation and severity of grain boundary grooves. Therefore, it is important to reduce any inhomogeneous distribution of precipitates on the surface of extrusions to prevent die streaking and improve the surface appearance of the anodized product.

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