Deformation twinning and texture variation in Zr-2.5 pct Nb alloy during rolling
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I. INTRODUCTION
A pressure tube made of cold-worked Zr-2.5 pct Nb alloy is used for the pressure vessel of the pressurized heavy water reactor. The mechanical properties of the pressure tube are degraded due to embrittlement by neutron irradiation during operation. It appears that the strength is increased and the fracture toughness and the critical crack length (CCL) are decreased.[1,2] It is understood that an unstable fracture occurs if the length of the through wall crack is over the CCL. The pressure tube absorbs the hydrogen formed by the corrosion process during operation, and shows a susceptibility to delayed hydride cracking (DHC), during which the crack is propagated by a repetition of the precipitation and a fracture of the hydride at the notch or crack tip. Considerable hydrogen content over the terminal solid solubility is required at the temperature of concern. The hydrogen moves to the crack tip and the hydride is precipitated at the notch or crack tip. Then, the hydride is fractured and the crack grows if the condition is satisfied. A through wall crack can be formed and an unstable fracture may occur by a DHC.[3] The characteristics of a DHC are evaluated by two different parameters, namely, the threshold stress intensity factor (KIH) and the DHC velocity (DHCV).[4,5] It is known that the susceptibility for a DHC appears in the radial-axial plane of a pressure tube. Both the KIH and DHCV in the radial direction (DHCVradial) are important at the stage of forming a through wall crack because the crack must grow in the radial direction primarily, while the DHCV in the axial direction (DHCVlongitudinal) becomes important after the through wall crack is formed. However, it has been confirmed that the DHCVlongitudinal (T-L) is about 2 times greater than the DHCVradial (T-R), even though the crack planes are the same SUNGSOO KIM, Principal Researcher, is with the Korea Atomic Energy Research Institute, Taejon 305-503, Korea. Contact e-mail: [email protected] This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
and only the cracking direction varies, where T-L and T-R mean that the cracking plane is normal to a transverse direction and the crack growth (cracking) directions are longitudinal (L) and radial (R), respectively, as shown in Figure 1. Meanwhile, it is thought that the DHC crack grows in a perfectly brittle manner, because it is understood that the DHC crack growth is achieved simply by a repetition of the precipitation and a fracture of the hydride. However, it is evident that a DHC is accompanied by deformation twinning.[6] This suggests that the deformation behavior is closely related to the DHC behavior. Considering the DHC propagation process in detail, the crack tip becomes blunted after loading; time is required to satisfy this proce
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