On some scale-related aspects influencing the fracture behavior at room temperature of preoxidized MA956 alloy

The effect of the scale of preoxidized MA956 alloy on the brittle fracture behavior at room temperature was examined in order to discern the fracture mechanism. With the aid of the Charpy impact tests, it is inferred, that the constraint exerted by the scale is more severe than that of the notch itself. The hardness tests in cross sections of preoxidized specimens reveal that the oxide scale prevents the plastic deformation of the substrate near the scale/metal interface. This constraint increases the plastic flow strength and causes that the cleavage stress can be reached more easily. Fracture mechanic analysis of a circumferential through scale crack in a tensile specimen reveal that the crack is not the cause of the brittle fracture of the substrate. Moreover, a relaxation of the stress state ahead crack tip is predicted because the crack tip blunting induced by the scale failure at the crack tip region. Therefore, it is concluded that the plastic constraint of the flaw-free scale on the metal accounts for the brittle fracture observed in the preoxidized specimens.