Anisotropic threshold stress intensity factor, K IH and crack growth rate in delayed hydride cracking of Zr-2.5Nb pressu
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NTRODUCTION
THE Zr-2.5Nb pressure tubes have been recognized to be very susceptible to delayed hydride cracking (DHC) resulting from hydrogen picked up during operation in a reactor.[1,2] Much attention has been paid to a better understanding of the DHC mechanism in Zr-2.5Nb pressure tubes with a view to improving DHC resistance. Recently, Kim suggested that the DHC resistance of Zr-2.5Nb tubes could be improved through a texture control based on the experimental fact that the threshold-stress intensity factor, KIH , to initiate DHC in Zr-2.5Nb tubes strongly depends on the angle of the {1017} hydride-habit plane and the cracking plane.[3] Since the Zr-2.5Nb has a circumferential texture with a high fraction of (0001) poles oriented in the circumferential direction of the tube,[4] the cracking plane is the radiallongitudinal plane of the Zr-2.5Nb that is lying almost parallel to the {1017} habit plane, as shown in Figure 1. However, though the DHC crack grows along the radial-longitudinal plane of the Zr-2.5Nb pressure tube, the DHC velocity differs YOUNG SUK KIM, Leader of Zr Group, SUNG SOO KIM, Senior Researcher, SANG CHUL KWON and YONG MOO CHEONG, Principal, Researchers, and KYUNG SOO IM, Technical Staff Member, are with the Korea Atomic Energy Institute, Daejon, 305-353, Korea. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11-15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
with the crack-growth direction, either the longitudinal direction or the radial direction of the Zr-2.5Nb pressure tube.[5] In other words, the DHC velocity in the longitudinal direction is faster than that in the radial direction.[5] This suggests faster growth of a DHC crack in the axial direction than in the radial direction. Likewise, the threshold stress intensity factor, KIH , will have an anisotropic behavior with crack growth direction: the initiation of a DHC crack in the longitudinal direction of the Zr-2.5Nb pressure tube would be easier than that in its radial direction. The objectives of this study are to systematically evaluate the DHC velocity and KIH of the Zr-2.5Nb tube in both the radial and longitudinal directions and further elucidate the cause for the anisotropic DHC behavior in the Zr-2.5Nb pressure tube. Compact-tension (CT) specimens and cantilever-beam (CB) specimens taken from the tube were used to determine the DHC velocity and KIH in the longitudinal direction and the radial direction, respectively. Since one of the main driving forces to drive diffusion of hydrogen toward the crack tip is a stress gradient between the crack tip and the bulk area away from the crack tip,[6,7]
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