Fracture Initiation Due to Hydrides in Zircaloy-2

Abstract

In hydride-forming metals, the presence of hydrides can sometimes lead to brittle fracture. Zirconium is a hydride-forming metal that forms the basis of a number of alloys used in CANDU^TM nuclear reactors. Under certain circumstances, zirconium alloys are susceptible to a process of slow crack propagation called delayed hydride cracking (DHC). Extensive experimental investigations have shown that DHC involves the repeated preferential nucleation, growth and fracture of hydride platelets at the tip of a pre-existing crack (1,2). A concomitant theoretical description of the DHC process (3,4) has provided an explanation for many significant features of the observed cracking behaviour, such as the dependence of the crack velocity on temperature and Mode I stress intensity factor K_I. An important deficiency of the theoretical model, however, is that it cannot predict the critical stress intensity factor, K_IH below which DHC crack growth would stop. Experiments have shown that DHC crack velocity is approximately independent of K_I up to values approaching those of the fracture toughness of the bulk material, whilst at low K_I-values, a sharp drop-off occurs in crack velocity, suggesting a critical K_IH for DHC.