The Effect of Stress on Orientation of Hydrides in Zirconium Alloy Pressure Tube Materials

A metallographic and fractographic study of outside-in cracking caused by power ramp tests

“…Since ramp tests usually cause steep temperature gradients along the tube wall, hydrides that precipitated inside the tube before the ramp test would re-dissolve and diffuse to the outside portion, and then would re-precipitate at the outside portion at relatively lower temperatures. At the same time, hoop stress caused by pellet cladding mechanical interaction (PCMI) could help hydrides precipitate along a radial orientation [9][10][11]. It has been reported that radial hydrides relatively crack easily under hoop stress, especially under multi-axial stress conditions [12].…”

Influence of temperature on threshold stress for reorientation of hydrides and residual stress variation across thickness of Zr–2.5Nb alloy pressure tube

“…1. Under unstressed condition, only circumferential hydrides form in Zr-2.5Nb pressure tubes [7,[14][15][16]. However, due to stress reorientation phenomenon, radial hydrides may precipitate and being oriented normal to hoop stress direction [14][15][16] of the pressure tubes, can significantly increase the latter's susceptibility to failure [7,12].…”

“…For dilute Zr-alloy pressure tubes this translates to precipitation of radial hydrides under hoop stress as compared to the precipitation of circumferential hydrides in the unstressed condition. This is usually associated with a critical stress called threshold stress below which no reorientation occurs [7,12,[14][15][16]. Though, threshold stress is reported to increase with material strength, it is reported to decrease with increasing solution annealing temperature [7].…”

“…The phenomenon of reprecipitation of hydrides with a different orientation, when cooled under stress from solution annealing temperature, than in the unstressed condition is known as stress-reorientation of hydrides [7,[14][15][16]. For dilute Zr-alloy pressure tubes this translates to precipitation of radial hydrides under hoop stress as compared to the precipitation of circumferential hydrides in the unstressed condition.…”

“…Leger and Donner [14] based on their observation of the existence of a marked boundary between the radial and circumferential hydrides across the thickness of the pressure tubes reported the variation in threshold stress across the pressure tube thickness and have attributed it to the residual stresses in these materials [17]. The difference between the mean threshold stress value and the local threshold stress across the thickness yielded the residual stress variation across the pressure tube thickness.…”