Lamellar orientation control of Ti-48Al PST crystal by unidirectional solidification

Control of lamellar orientation in unidirectionally solidified Ti-48Al PST crystals has been attempted by an optical floating zone method using various seed crystals of Ti-(48-57)Al alloys. Regardless of the seed crystals, the Ti-48Al PST crystals can be grown without any difficulty. However, the lamellar orientation can never reproducibly be controlled by using any seeds but single crystal of Ti-57Al. In case of the 48Al PST seed, polycrystallization due to γ→α phase transformation takes place in heating to melt. The seeds with composition range of a peritectic reaction (L+α→γ) to occur in melting exhibit a cellular liquid/solid interface, resulting in new grains there. In these cases, the grown crystal follows an orientation of one of the newly formed grains. The single crystal seed of Ti-57Al exhibits a flat liquid/solid interface in melting (γ→γ+L) even after making contact with 48Al alloy to grow, and the grown crystal follows the seed orientation. It is concluded that composition travel to skip the peritectic reaction (L+α→γ) using γ single phase seed makes the control of lamellar orientation of PST crystals possible.