Fracture Resistance of a Zirconium Alloy with Reoriented Hydrides
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Hydride Re-Orientation
Hydride Re-Orientation
Unirrad. Sheet [11]
Compiled by Chung [6] Unirrad. Rod [7]
Unirrad. Rod [8] Unnirad. & irrad. Rods [9] Irrad. Rod [10]
Unirrad. Rod [8] Unirrad. & Irrad. Rods [9] Irrad. Rod [10]
Compiled by Chung [6]
250 Hydride Re-orientation
Zircaloy-4
HRT Stress, MPa
200
150 Chung [6]
100 ISG-11 Rev. 3 [5]
50
No Hydride Re-orientation
0 200
250
Ferry and Poinssot [7]
300
350
400
450
500
HRT Temperature,°C
(a) Zircaloy-2 No Hydride Re-Orientation
200
HRT Stress, MPa
It is well known that radial hydrides reduce the fracture resistance of Zircaloy cladding tubes at ambient temperatures. Several studies[6–14] have been conducted recently to investigate the conditions that lead to HRT and the corresponding impacts on the integrity of irradiated and unirradiated Zircaloy cladding rods. The effects of reoriented hydrides on the fracture resistance of Zircaloy-2 or Zircaloy-4 at intermediate temperatures [473 K (200 C)] have not been established. Furthermore, there are also uncertainties on the critical stress required for instigating HRT in Zircaloy-2 and Zircaloy-4. An extensive review[15] of literature data[6–14] indicates that the critical stress for HRT in Zircaloy-4 is about 90 MPa for temperatures at or below 673 K (400 C) for both irradiated and unirradiated rods, as shown in Figure 1(a). In contrast, the critical stress for HRT appears to be significantly lower than 90 MPa for Zircaloy-2, as shown in Figure 1(b). A comparison of the HRT maps for Zircaloy-2 and Zircaloy-4 in Figure 1 reveals that the critical stress of 90 MPa appears to be valid for Zircaloy-4 at the temperature of 573 K and 623 K (300 C and 350 C), while this critical stress value is too high for Zircaloy-2 at 573 K and 623 K (300 C and 350 C) by almost 20 MPa. At the present time, it is uncertain why the HRT stress for Zircaloy-2 is lower than the 90 MPa limit observed in Zircaloy-4. Thus, there is a need to better define the critical stress for HRT in Zircaloy-2, particularly at 573 K and 623 K (300 C and 350 C). The objective of this paper is to report the results of an investigation focused on quantifying the critical stress for HRT and the effects of reoriented hydrides on the fracture resistance of Zircaloy-2 at 473 K (200 C). One of the main goals of this study was to determine the reason(s) behind the low critical stress value for HRT in Zircaloy-2,[10,13,14] as shown in Figure 1(b). Both HRT and fracture toughness testing of hydrided Zircaloy-2 materials have been performed in a scanning electron microscope equipped with a high-temperature loading stage so that the HRT and embrittlement processes can be characterized by direct observations. The results of this investigation indicate that HRT indeed occurs ahead of a loaded crack tip, but the resulting hydrides, which are generally on the order of 1 to 10 lm in lengths, form voids after they are fractured. Since the fractured hydrides are separated by a ductile Zr-matrix at 473 K (200 C), the crack growth process occurs in a s
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