J. Zhang, X. Li, H. Oskarsson

TiAl alloys posses a promising strength to make the rotating parts that work in hot environments. It is believed that the weight-saving by substituting TiAl alloys for Ni-based superalloys would help to increase the turbine efficiency. However, the mechanical properties, especially, the tensile ductility and fracture resistance, are still not yet competitive with conventional superalloys.
The mechanical anisotropy of the cast microstructure of TiAl alloys has been well demonstrated by producing the polysynthetically twined (so called PST) crystals. This study has found that a textured microstructure can also be produced in the platelike TiAl samples during casting, in which the lamellar orientation is parallel to the plate surfaces. This kind of textured microstructures are transformed in the cast columnar crystals with Ti/Al ratio at 1.03 – 1.07. Similar to the PST crystals, the textured TiAl alloy obtained exhibits much improved tensile properties, fracture resistance, and creep rupture life along the preferential orientation, compared with those containing random lamellar colonies. Besides, it has also been found that the strength and ductility could be retained after the tensile specimens being exposed at 700°C for 100 hours. Therefore, it is a strong potential to improve the mechanical properties of cast TiAl alloys specific to the application for industrial turbine blades. In order to optimize the mechanical properties of this material, the relationship between the strength level and the microstructure characteristics such as the consistency of lamellar orientation and the lamellar spacing will be discussed as well.

Microstructure Development

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