STP633

    Factors Controlling Hydrogen Assisted Subcritical Crack Growth in Zr-2.5Nb Alloys

    Published: Jan 1977


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    Abstract

    The delayed failure of cold-worked Zr-2.5Nb pressure tube. material has been studied using static load tests on compact-tension specimens containing hydrogen within the range ∼ 10 to 400 μg/g. The experimental approach was to measure crack velocity (V) as a function of crack tip stress intensity factor (K), temperature and hydrogen content, relate these data to fractographic and metallographic observations, and compare the results with recent models of hydrogen embrittlement. Slow crack growth was observed at all temperatures between 25 and 325 °C and at K values between ∼ 10 and 50 MPam. Below 250°C, the V-K relationships exhibited two-stage behavior; at K > 15 to 20 MPam, the crack velocity was only weekly dependent on stress intensity, whereas at smaller K values, the crack velocity decreased rapidly with K, an indication of a threshold value of K ∼ 5 to 10 MPam. The crack velocity increased with increase in temperature, although because of scatter in the data, this could not be expressed quantitatively. At 250°C and above, slow crack growth was not reproducible except after a thermal cycle. The thermal cycle produced a region of reoriented hydrides concentrated at the crack tip which significantly reduced the incubation period for crack growth and confirmed the important role of the hydride phase in the fracture process. Fractography showed that the features of the slow growth fracture were similar at all temperatures studied. The main observations were of ductile striations, or stretch zones, parallel to the crack front, with brittle, plate-like regions, some of which contained cleavage features, between the striations. A fracture mechanism is suggested which involves the repeated precipitation of hydride at the crack tip, followed by crack advance through this embrittled region, and crack arrest in the more ductile matrix, leading to discontinuous crack growth. This general mode of crack growth has been considered in a recent model for embrittlement in hydride-forming materials, the predictions of which show good agreement with the results from this study.

    Keywords:

    zirconium, zirconium alloys, fracture (materials), cracks, velocity, stresses, hydrides, incubation period, striations, discontinuous crack growth, embrittlement


    Author Information:

    Simpson, LA
    Research officers, Materials Science Branch, Whiteshell Nuclear Research Establishment, Atomic Energy of Canada Limited, Pinawa, Manitoba,

    Nuttall, K
    Research officers, Materials Science Branch, Whiteshell Nuclear Research Establishment, Atomic Energy of Canada Limited, Pinawa, Manitoba,


    Paper ID: STP35594S

    Committee/Subcommittee: B10.02

    DOI: 10.1520/STP35594S


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