The stress orientation of hydride in zirconium alloys

Stress-reorientation of hydrides and hydride embrittlement of Zr–2.5 wt% Nb pressure tube alloy

“…The microstructure of the CWSR Zr-2.5 wt% Nb alloy pressure tube material is such that under unstressed condition, only circumferential hydrides form [6,17]. For these components, mode of stress-reorientation of hydrides is the precipitation of radial hydrides, while cooling under stress from a solution-annealing temperature [6,[9][10][11][12][13][14][15][16][17][18][19][20][21]. This is associated with a critical stress called threshold stress for reorientation below which no reorientation occurs [6,17,18].…”

“…It is suggested that the threshold stress for reorientation of hydrides in CWSR Zr-2.5 wt% Nb pressure tube alloy reduces to 0 at a temperature of 704 K [18]. A high concentration of basal poles parallel to the applied stress is necessary for reorientation of hydrides [6,20,21]. However, the degree of reorientation is reported to be insensitive to time under stress; and also small variation in texture, grain-size and dislocation density are found to have apparently no consistent effects [6].…”

Hydride reorientation in Zircaloy-4 examined by in situ synchrotron X-ray diffraction

“…Their first stage of nucleation into the α-Zr lattice (in this case when cooled from a temperature above the terminal solid solubility for the concentration of hydrides) is considered crucial when determining their orientation with Ells [30] concluding that the preferential orientation of hydride packets is most efficient during nucleation rather than through growth of a stable nuclei. The application of a tensile stress during hydride re-precipitation from solution may change the preferential sites for hydride formation by reducing the energy barrier for nucleation.…”

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].…”

“…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.…”