The Influence of a Second Phase on the Yielding Characteristics of Zirconium

Abstract

Tensile tests have been carried out to investigate the yielding characteristics of two-phase zirconium-hydrogen alloys. The tests were performed at room temperature at a strain rate of 3.75 × 10⁻⁵ sec⁻¹. Discontinuous yielding was observed at fine grain sizes (< 0.006 mm grain dia.) in the pure zirconium, and for the same grain size the upper yield stress and the magnitude of the yield drop were larger in transverse than in longitudinal specimens. The addition of hydrogen (0–210 ppm) to form zirconium hydride precipitates affected the discontinuous yielding behaviour of fine-grained specimens. Increasing the hydrogen content increased the precipitate size and progressively decreased the upper yield stress, eventually resulting in complete elimination of a yield drop or yield inflection. Air-cooling rather than furnace-cooling specimens from the hydriding temperature increased the number of precipitates and was also more effective in removing abrupt yield. The role of the zirconium hydride precipitates in the yielding process is considered in terms of their ability to act as preferential nucleation centres for pre-yield plastic regions.