| Abstract |
| A new model for delayed hydride cracking (DHC) of zirconium alloys is suggested where a driving force for the DHC is not the stress gradient but a supersaturated hydrogen concentration AC over the terminal solid solubility (TSSD) arising from a hysteresis of the terminal solid solubility (TSS) on a heating and a cooling. To demonstrate the feasibility of the newly proposed DHC model, DHC velocity (DHCV) and the threshold stress intensity factor or KIH were determined at different temperatures ranging from 100 to 300℃ on compact tension specimens of Zr-2.5Nb tubes with different yield strengths and hydrogen concentrations of 6 to 100 ppm. As expected, the DHCV and KIH increased and decreased, respectively, with the AC and then leveled off to a constant with the AC in excess of the TSS for precipitation (TSSP) subtracted by the TSS for dissolution (TSSD). Through a supplementary experiment investigating the effect of the degree of supercooling on the DHC velocity which is intentionally introduced on reaching the test temperature by cooling from the peak temperature, the new DHC model is confirmed to be valid. The striation spacing and DHCV increased with the square of the yield strength, and with temperature. By correlating the striation spacings and a plastic zone ahead of the crack tip in the Zr-2.5Nb tubes with the stress gradient we demonstrate that the DHCV is governed by the hydrogen concentration gradient determined by the AC and the plastic zone size which is governed by the square of the yield strength and the Km. Therefore, the activation energy of the DHCV turns out to be a sum of the activation energy of the hydrogen diffusion and that of the striation spacing. (Received January 19, 2004) |
|
|
| Key Words |
| Delayed hydride cracking, Zirconium, Yield strength, Striation spacing, Hydrogen concentration |
|
|
 |
|
