Criteria for Fracture Initiation at Hydrides in Zirconium-2.5 Pct Niobium Alloy
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great deal of work has been done to measure the fracture resistance of the material, much of which is summarized in a recent article by Langford and Mooder. l However, most of this work deals with direct measurements of fracture criteria, with very little attention paid to the micromechanisms of fracture. Without an understanding of the micromechanical details of the fracture process, a framework to predict, with confidence, the effect of microstructural and mechanical property changes on the fracture criteria cannot be established. 1.1 The Alloy and Crack Initiation The pressure tubes are fabricated at 880 ~ by extrusion of ingots of Zr-2.5 pct Nb. They are then cold-drawn twice to a total strain of approximately 20 pct, resulting in a microstructure of elongated grains of a-zirconium containing about 1 wt pct niobium in solution, and a grain boundary phase of B-zirconium containing about 20 wt pct niobium. The a-grain size is typically 8/~m in the extrusion direction, and about 0.5 /~m in the other two principal directions. Previous fracture studies 2,3 have shown that, like many other ductile alloys, crack initiation in Zr-2.5 pct Nb follows the nucleation and growth of voids in the plastic zone of the specimen. In other ductile alloys, crack initiation can occur by either the growth and coalescence of primary voids or the development of intense shear bands between primary void sites. 4,5 These shear bands ultimately fail by the nucleation of void sheets, a dense array of small, closely spaced voids. The nucleation of voids is almost always associated with either the fracture of second-phase particles or the L. A. SIMPSON, formerly on leave with the Materials Research Laboratory, Brown University, Providence, RI 02912, is with the Materials Science Branch, Whiteshell Nuclear Research Establishment, Pinawa, Manitoba ROE 1L0 Canada. Manuscript submitted February 23, 1981. METALLURGICAL TRANSACTIONS A
separation of the particle-matrix interface. In fact, it has been pointed out by T h o m p s o n and Weihrauch 6 that even in isolated cases, where void nucleation appeared to be homogeneous, a very fine, undetected precipitate dispersion could have been responsible. These kinds of details must be taken into account in developing any ductile fracture model, but studies of the fracture mechanism in Zr-2.5 pct Nb have not yet reached this level. In Zr-2.5 pct Nb two phases emerge as likely void initiation sites, the fl-zirconium phase and zirconium hydrides. N o study has been made on the role of fl-zirconium on the fracture process except to show that it can undergo a ductility reduction through an agehardening process when annealed between 300 and 400 ~ Since all pressure tubes receive an autoclaving treatment at 400 ~ for 24 h, and see temperatures up to 300 ~ during service, the fl phase could be embrittled sufficiently for it to act as a void nucleation site. Zirconium hydride has received much more attention as a fracture initiator. Because of the low solubility of hydrogen in zirconium at 20 ~ ( ~ 1/tg/g), hydri
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