Search

Abstract: This work was prompted by the publication of Puls''s recent papers claiming that the Dutton–Puls model is valid enough to explain the stress and temperature dependences of the crack growth rate (CGR) in zirconium alloys. The first version of the Dutton–Puls model shows that the CGR has positive dependences on the concentration difference ΔC, hydrogen diffusivity D H, and the yield strength, and a negative dependence on the applied stress intensity factor K I, which is one of its critical defects. Thus, the Dutton–Puls model claiming that the temperature dependence of CGR is determined by D H C H turns out to be incorrect. Given that ΔC is independent of the stress, it is evident that the driving force for DHC is ΔC, not the stress gradient, corroborating the validity of Kim''s model. Furthermore, the predicted activation energy for CGR in a cold-worked Zr–2.5Nb tube disagrees with the measured one for the Zr–2.5Nb tube, showing that the Dutton–Puls model is too defective to explain the temperature dependence of CGR. It is demonstrated that the revised Dutton–Puls model also cannot explain the yield stress dependence of CGR. [Copyright &y& Elsevier]

Abstract: This paper evaluates the important elements of delayed hydride cracking (DHC) for transgranular stress corrosion cracking (TGSCC) of Mg alloys. A DHC model was formulated with the following components: (i) transient H diffusion towards the crack tip driven by stress and H concentration gradients; (ii) hydride precipitation when the H solvus is exceeded; and (iii) crack propagation through the extent of the hydride when it reaches a critical size of ∼0.8μm. The stress corrosion crack velocity, V c, was calculated from the time for the hydride to reach the critical size. The model was implemented using a finite element script developed in MATLAB. The input parameters were chosen, based on the information available, to determine the highest possible value for V c. Values for V c of ∼10−7 m/s were predicted by this DHC model. These predictions are consistent with measured values for V c for Mg alloys in distilled water but cannot explain values for V c of ∼10−4 m/s measured in other aqueous environments. Insights for understanding Mg TGSCC are drawn. A key outcome is that the assumed initial condition for the DHC models is unlikely to be correct. During steady state stress corrosion crack propagation of Mg in aqueous solutions, a high dynamic hydrogen concentration would be expected to build up immediately behind the crack tip. Stress corrosion crack velocities ∼10−4 m/s, typical for Mg alloys in aqueous solutions, might be predicted using a DHC model for Mg based on the time to reach a critical hydride size in steady state, with a significant residual hydrogen concentration from the previous crack advance step. [Copyright &y& Elsevier]

Hydrides of intermetallic compounds (IMC) belong to the class of functional materials whose possible applications cover a wide temperature range, from a few Kelvin up to 500 K. Each application is correlated to a well-defined property, i.e. chemical reactivity for purification, temperature/pressure relationship for sensors, hydrogen storage capacity converted into energy for hydride chemical engines, or converted into electrical current for hydride batteries. Up to now, only the hydride batteries have reached the consumer level while for hydrides obtained by direct reaction between hydrogen gas and the parent solid phase, applications have not yet been successful suggesting that many problems remain to be solved. In this paper, we will be concerned with the factors influencing the absorption properties according to the type of application. It will also be shown that the dynamic behavior of these highly defective materials is difficult to control due to the hysteresis accompanying the formation–decomposition of the hydrides compounds. Hysteresis reflects irreversibility and contributes to the degradation of the absorption properties over the long-term behavior. [Copyright &y& Elsevier]