In α+β titanium alloys such as Ti-6Al-4V, cooling from above the β transus gives a lamellar structure of colonies of plate-like α, while working below the β transus and annealing gives a structure containing equiaxed α. Which statement about the difference in properties between these two structures is correct?

Answer: B. The equiaxed structure has greater ductility and resistance to fatigue crack initiation, and the lamellar structure has greater fracture toughness and resistance to fatigue crack growth

Equiaxed structures (worked below the β transus and recrystallized, or bimodal structures mixing equiaxed α with lamellar regions) have short slip lengths and distribute deformation evenly, so they have greater ductility and resist the initiation of fatigue cracks. In the lamellar structure cooled from above the β transus, on the other hand, cracks follow a tortuous path along the colonies of plate-like α, so the crack path is longer, the fracture toughness is higher and long cracks grow more slowly. The lamellar structure is also better in creep strength at high temperature. The option that swaps the two sets of properties is wrong, and because the shape of the microstructure strongly affects mechanical properties, the option saying they are almost the same is also wrong. Microstructures are tailored to the properties each part needs through the working temperature and cooling rate.