Influence of Impact on the Mechanical Behaviour of a Cast and a Forged Gamma Based TiAl alloy

Kurzfassung

One of the main problems representing a barrier for TiAl gas turbine applications is its low ductility, especially in reference to impact damage of turbine blades or vanes. For that reason the influence of high-speed impact damage on the mechanical behaviour of a cast TNBV3B TiAl alloy and a forged alloy with a similar composition but improved ductility has been investigated. Impact tests are performed making use of a ballistic gun. The caused damage has been evaluated as a function of the impact speed, the particle weight and, thereby, the impact energy level in relation to the specimen thickness. The location of the impact on the specimen plays an important role. Impact near the specimen edge appears to produce a severer damage as the depth of the damage zone close to the edge is larger and damage of a given size is more critical if it is located at the edge. This is shown with the aid of residual strength and fatigue experiments. The residual strength could be described on the basis of a linear elastic fracture mechanics analysis and a fracture toughness of 15 to 17MPa(m)1/2. Fatigue experiments were performed to determine the threshold for crack growth as a function of the impact damage size.