Secondary cracking in hydrided Zr-2.5 Wt% Nb alloys

Abstract

The occurrence of secondary cracking in cold-worked, hydrided Zr-2.5 wt% Nb pressure tube material has been studied. The results indicate that the secondary cracking can occur during three different modes of propagation of the primary crack, i.e. dynamic fatigue, hydrogen-induced delayed cracking, and ductile fracture. Metallographic studies revealed that the secondary and primary crack planes are approximately perpendicular, and, moreover, the secondary crack path lies either through the brittle zirconium hydride precipitates or the hydride-matrix interface. Evidence is presented to show that the secondary cracks nucleate and propagate within the plastic zone of the primary crack as a result of the transverse (through thickness) component of the triaxial stress at the crack tip. During hydrogen-induced delayed cracking, the secondary cracks either propagated with the primary stable crack, causing pronounced cusping of the crack front, or more typically growth of the secondary cracks terminated at an early stage, and the main crack broke away from the secondary cracks. This difference in behaviour is thought to be related to the degree of hydride continuity and alignment in the matrix.