Crack Growth Rate of Zirconium Alloys with Temperature

One of the unresolved issues related to delayed hydride cracking (DHC) of zirconium alloys is arrest of a DHC crack above 300 .deg. C despite the test temperatures being approached by cooling. Two hypotheses were suggested according to the old DHC models, which argued that crack arrest occurs either when a difference in hydrogen concentration, ΔC between the bulk and a crack tip is reduced to zero or when the bulk hydrogen concentration is equal to the terminal solid solubility for hydride dissolution (TSSD). Given that ΔC or the magnitude of the bulk hydrogen concentration in solution is a factor to affect nucleation of hydrides in the matrix, those hypotheses argued that crack arrest at higher temperatures is associated with nucleation of hydrides. The fact is that DHC is arrested despite sufficiently large hydrides being precipitated at a crack tip. Therefore, it is clear that arrest of a DHC crack has nothing to do with nucleation of the hydrides, demonstrating that the old DHC models are defective. In contrast with the old DHC models assuming that DHC is a hydrogen diffusion-driven process, DHC consists of three consecutive processes: nucleation, growth and cracking of hydrides. Hence, according to Kim's DHC model, the crack growth rate by DHC (CGR) is governed by the slowest process among the three consecutive processes. When the hydrogen concentration charged to zirconium alloys are large enough to exceed the critical supersaturation of hydrogen, ΔCmax above which the CGR becomes constant, the hydride nucleation rate becomes too fast to govern the CGR. Thus, it is either hydride growth or hydride cracking that determines the CGR of zirconium alloys with sufficient hydrogen. The aim of this work is to understand which of the two factors affect the CGR of zirconium alloys, leading to DHC arrest. To this end, the CGR and striation spacing corresponding to a critical hydride length were determined for a cold-worked Zr-2.5Nb tube with sufficient hydrogen after DHC tests at different temperatures ranging from 144 .deg. C to above 300 .deg. C