Fracture initiation at hydrides in zirconium
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INTRODUCTION
IN hydride-forming metals, the presence of hydrides can, under suitable conditions, lead to brittle fracture. Zirconium is a hydride-forming metal that forms the basis of a number of alloys used in CANDU* nuclear reactors.
the same, were all above some threshold level, the results obtained were somewhat similar. Consequently, the present study has been carried out using unalloyed Zr (subsequently referred to as simply Zr). This material has a much lower yield stress than the previously studied alloys.
*CANDU is a trademark of AECL, Ottawa, ON, Canada.
Under certain circumstances, zirconium alloys are susceptible to a process of slow crack propagation called delayed hydride cracking (DHC). Extensive experimental investigations have shown that DHC involves the repeated preferential nucleation, growth, and fracture of hydride platelets at the tip of a pre-existing crack. [~,2,31 A concomitant theoretical description of the DHC process t4,51has provided an explanation for many significant features of the observed cracking behavior, such as the dependence of the crack velocity on temperature and stress intensity factor Kz.tz] Recent improvements in the theory have focused on the effects of direction of approach to the test temperature on DHC. [6,71In addition, various efforts have been made to determine a criterion for the fracture of individual hydride platelets.[8.9] Such work is directed toward developing a fracture mechanism that can be incorporated into the DHC model. It is hoped that this approach will lead to a mechanistic explanation for the K1 dependence of the DHC velocity and, in particular, the abrupt decrease in the velocity at a critical stress intensity value called Kin. In the most recent work, [91 tensile specimens were used to study the initiation of fracture at hydride platelets as a function of stress, stress state, hydride platelet length, and matrix strength. The effect of matrix strength was investigated by testing three high-strength zirconium alloys having yield strengths ranging from 625 to 900 MPa. It appears that because the yield strength values of these materials, although not M.P. PULS is Manager of the Materials and Mechanics Branch, AECL Research, Whiteshell Laboratories, Pinawa, Manitoba ROE 1L0, Canada. Manuscript submitted June 18, 1990. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL
A. General Crack initiation at hydrides at room temperature was studied by deforming in tension smooth or double-notched specimens containing a specified amount of hydrogen (and, hence, hydrides). The shape of the specimens served to create, respectively, a uniaxial or a triaxial state of stress. Hydrogen was added to the specimens gaseously at 673 K. The hydride size (mainly hydride platelet length; in Section I I - B ) was varied by controlling the hydride nucleation and growth processes by choosing a suitable combination of hydrogen content and cooling rate. Initiation of fracture at hydrides was monitored using acoustic emission (AE). Finite element calculations were employed to det
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