The influence of hydride size and matrix strength on fracture initiation at hydrides in zirconium alloys
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INTRODUCTION
PRESSUREtubes of cold-worked Zr-2.5
wt pct Nb alloy are the primary containment for the coolant of CANDUTM nuclear reactors. In zirconium and its alloys, excess dissolved hydrogen precipitates as hydrides, the presence of which could embrittle the material. Two embrittlement processes due to hydrides are possible. One process is the result of a mechanism of crack initiation and slow propagation called delayed hydride cracking (DHC). rE21 This mechanism can operate at fairly low hydrogen concentrations, provided the temperature is also low enough so that hydrides have precipitated in the material. The second embrittlement process is a reduction in the fracture toughness of the alloy due to the presence of a high concentration of hydride platelets that have their inplane dimensions in the crack growth direction (radial hydrides). I3j This can reduce the critical crack length to levels such that a leak-beforebreak criterion may no longer be applicable. The embrittlement process due to a high concentration of bulk hydrides is strongly temperature dependent, t3] Below a transition temperature, the crack propagates almost entirely through radial hydrides, the fracture surface is flat and brittle, and the corresponding critical crack length is small. A temperature increase of just five or ten degrees will take the material through the transition, and the hydrides now have no influence at all on the fracture mechanism. The fracture mechanism above this temperature becomes one of ductile tearing with a high critical crack length. To understand this ductile-to-brittle transformation, a criterion for the initiation of fracture at hydride platelets is needed. It is also needed to provide a theoretical underM.P. PULS is Research Scientist, Materials and Mechanics Branch, Atomic Energy of Canada, Ltd., Whiteshell Nuclear Research Establishment, Pinawa, MB, Canada, ROE 1L0. Manuscript submitted May 7, 1987.
METALLURGICAL TRANSACTIONS A
standing for the initiation of DHC on a smooth pressure tube surface and for predicting the critical threshold stress intensity factor, Kin, below which DHC stops. This paper describes the results of the first phase of an experimental and theoretical program which is designed to examine the mechanism of the crack initiation event at hydrides. It is hoped that by studying the fracture of hydrides in situations where the stress and strain state can be easily determined and controlled, it will be possible to obtain quantitative information on crack initiation at single hydride platelets. This information should be useful in establishing fracture criteria under the more complex conditions at high hydride concentrations or in front of a crack tip that is propagating by the DHC mechanism. The emphasis in this study is to determine the effects of matrix strength (as exhibited by different alloys) on the conditions for crack initiation at room temperature in hydrides. The effect of matrix strength is studied as a function of stress state, hydride size, hydrogen content, and matrix cons
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