High temperature mechanical behaviour of alloy 617. Pt. 2

The high temperature mechanical behaviour of a single heat of Alloy 617 (DIN 2.4663, Nicrofer 55 20 Co) was studied at five laboratories using tensile, creep and fatigue tests. The first part of the report summarizing this work was primarily devoted to a discussion of the deformation mechanisms. The lifetime and damage mechanisms will be dealt with in this second part. In the temperature range investigated (600 to 1000deg C), the lifetime at stresses well below the UTS is in most cases determined by one of two damage mechanisms. For monotonic loading and for asymmetric cycling with a slow tension part the crack starts from the surface and proceeds along the grain boundaries. In this case preliminary damage of the boundaries in front of the crack tip plays a role, either by oxidation of the grain boundary carbides or by cavity formation. The second mechanism is observed under symmetric cyclic loading and results in transgranular crack propagation. In this case, under specific creep-fatigue testing conditions an inversion of the frequency effect is observed: The cyclic lifetime Nf increases with increasing cycle time. (orig.)