A model of slow crack propagation based on the delayed hydride cracking (DHC) mechanism in hydride-forming alloys has been critically examined and evaluated to take account of recent experimental and theoretical advances in the understanding of hydride fracture and terminal solid solubility (TSS). The model predicts that the DHC velocity is a sensitive function of the hydrogen concentration induced in the bulk of the material as a result of the direction of approach to test temperature. For test temperatures approached from below, factors such as the hydridematrix accommodation energies, the stress state at the crack tip, and the value of the yield stress have a strong effect on the DHC arrest temperature in the technologically interesting temperature range of 400 to 600 K. A fracture criterion is explored based on the need to achieve a critical hydride length in the plastic zone at the crack tip. A necessary condition for DHC is that the crack tip hydride must grow to this critical length. An approximate estimate is made for the steady-state growth limit of the crack tip hydride as a function of the direction of approach to temperature and the crack tip stress state. For temperatures approached from below, growth of the crack tip hydride is limited to just outside the plastic zone boundary at low temperature, gradually receding toward and inside the plastic zone boundary with increasing temperature. At lowK
 I
 values, this limits the crack tip hydride lengths to below their critical values for fracture. This could be one condition forK
 IH
 . For test temperatures approaches from above, the growth limit is significantly increased, and the sensitivities to the above parameters become less evident.

Effects of crack tip stress states and hydride-matrix interaction stresses on delayed hydride cracking

Review of the thermodynamic basis for models of delayed hydride cracking rate in zirconium alloys

Delayed hydride cracking in Zr–2.5Nb pressure tube material☆

The delayed hydride cracking (DHC) behavior for ZIRCALOY-2 tubing was characterized at temperatures ranging from 93 °C to 288 °C. Testing was performed on the three types of pressure tubes that were used in the construction of the Hanford N Reactor, Richland, WA. A two-stage cracking response was observed for all tube types, with a very high K sensitivity in the threshold (stage I) regime followed by a region that was insensitive or moderately sensitive to the applied K level (stage II). The stage II crack velocity was a thermally activated process with an apparent activation energy of 68 kJ/mol. This energy level was consistent with the theoretical activation energy associated with hydrogen diffusion into the triaxial stress field ahead of a crack. Above a critical temperature of 180 °C, cracking occurred only when specimens were subjected to an overtemperature cycle. Below this critical temperature, DHC occurred regardless of whether specimens were heated or cooled to the test temperature. Differences in crack velocities and threshold responses for the various tube types were attributed primarily to variations in crystallo-graphic texture. Postirradiation testing revealed that irradiation to a fluence of 7.7 × 10(21) n/cm2 caused a 50-fold increase in crack growth rates. Metallographic and fractographic examinations were performed to understand operative DHC mechanisms.

Delayed hydride cracking behavior for ZIRCALOY-2 tubing

The effect of hydrogen concentration on the delayed hydride cracking velocity (DHCV) and the threshold stress intensity factor, KIH of a Zr-2.5Nb tube were examined at test temperatures ranging from 100 to 280°C by subjecting compact tension specimens with a hydrogen concentration of 12 to 100 ppm H to an overtemperature cycle. The DHCV and KIH increased and decreased, respectively, with an increase in the supersaturated hydrogen concentration over the terminal solid solubility for dissolution (TSSD) or ΔC. They then leveled off to constant values at ΔC in excess of the ΔCmax corresponding to a difference of the terminal solid solubility of the hydrogen on cool-down and on heat-up. Further, intentional introduction of an undercooling by 0 to 40°C at the test temperature decreased the DHCV of the Zr-2.5Nb tube, indicating that ΔC between the bulk region and the crack tip governs the DHCV. A new DHC model is proposed where the driving force for DHC is the difference in the hydrogen concentration between the bulk region and the crack tip by preferentially nucleating the hydrides only at the crack tip under an applied tensile stress, due to a hysteresis in the TSS of hydrogen on heat-up and on cool-down. A supplementary experiment was conducted to validate the feasibility of the proposed DHC model.

Driving force for delayed Hydride cracking of zirconium alloys

The effect of heat treatments on delayed hydride cracking (DHC) in Zr-2.5 wt pct Nb has been studied. Crack velocities were measured in hydrided specimens, which were cooled from solution-treatment temperatures at different rates by water-quenching, oil-quenching, liquid-nitrogen quenching, and furnace cooling. The resulting hydride size, morphology, and distributions were examined by optical metallography. It was found that fast cooling rates, which produce very finely dispersed hydrides, result in higher crack growth rates and a stronger dependence of the crack velocity on the applied-stress intensity factor. Also, the incubation period before cracking commences was found to be relatively short for specimens with fine hydrides, whereas specimens with coarse hydrides required considerably longer incubation periods. These results can be explained by rapid growth of preexisting hydrides within the crack-tip plastic zone. In addition, different solution temperatures were used to investigate the effect of the continuity of the grain-boundary phase(β-phase) on the crack velocity. Transmission electron microscopy was used to examine the structure of this grain-boundary phase. It was found that for heat treatments, which destroyed theβ-phase continuity, the crack velocity was significantly reduced, as would be expected from the theory of enhanced diffusion through grain boundaries.

K. F. Amouzouvi

Papers

Effect of heat treatment on delayed hydride cracking in Zr-2.5 Wt Pct Nb