Effects of the colony size on the fracture toughness of TiAl−Mo−B alloys

The effect of the colony size on the fracture toughness of Ti−46.5Al−1.5Mo−xB (x=0.1, 0.6, 1.0) alloys was investigated. The colony size was varied by a heat treatment in the alpha-single phase region and by directional solidification (DS). The colony size decreased as the boron content increased. Fracture toughness was measured at room temperature by a three-point bend test. The heat-treated Ti−46.5Al−1.5Mo−0.1B alloy, which had a colony size of ∼350 μm, showed the best fracture toughness, and the fracture toughness decreased rapidly as the boron content increased. The fracture toughness of the DS ingots was similar with different compositions and was lower than that for the heart-treated Ti−46.5Al−1.5Mo−0.1B alloy. This study confirms that the colony size plays a major role in determining the fracture toughness of TiAl alloys with a lamellar microstructure.