Physical metallurgy of single crystal gamma titanium aluminide alloys:: orientation control and thermal stability of lamellar microstructure

This paper summarizes our recent work on the development of cast single crystal gamma TiAl alloys (PST) focusing on both lamellar orientation control of PST crystal in process and the thermal stability of the lamellar microstructure in use at elevated temperatures. PST crystals can easily be produced by unidirectional solidification regardless of the kinds of primary solidification phases of bcc β-Ti or hcp α-Ti solid solutions. The lamellar orientation control can be achieved by controlling the orientation of α single crystal existing just underneath the liquid/solid interface. Seeding with Ti-rich PST crystal does not work because of random nucleation of new grains due to the reverse phase transformation of γ to α during heating. However, an Al-rich γ single crystal with no solid/solid phase transformation up to the melting point is a promising seed when the average composition of the seed and alloys to grow is in the region of α solidification. During exposure at elevated temperatures interface reaction of energetically unstable variant interface takes place, resulting in the coarsening of γ plates and eventually leading to the collapse of the lamellar microstructure unless α2 plates exist. Thus, thermodynamically stable α2 plates play an important role in pinning the coarsening of γ plates across the lamellae, which is responsible for the high thermal stability of the lamellar microstructure.