Determination of fracture strength of δ-zirconium hydrides embedded in zirconium matrix at high temperatures

摘要

The fracture strength of δ-zirconium hydrides embedded in a zirconium matrix was determined at temperatures between 25 °C and 250 °C by ring tensile tests using Zircaloy-2 tubes. Essentially all of the present hydrides in the tubes were re-oriented in the radial direction by a temperature cycling treatment and then tensile stress was applied perpendicular to the hydrides to ensure that brittle fracture would occur at the hydrides. The hydrides failed in a brittle manner below 100 °C where-as the zirconium matrix itself underwent ductile fracture without hydride cracking at temperatures above 200 °C under plane stress condition. Brittle fracture of the hydrides continued to occur at temperatures up to 250 °C under plane strain condition, suggesting that the upper limit temperature for hydride fracture, T _(upper), was raised by the triaxial stress state under the plane strain condition. The apparent fracture strength of the hydrides, σhydridef, was determined at temperatures below T_(upper) from the measured fracture strength of the tubes, making a correction for the compressive transformation stress in the hydrides. σhydridef was about 710 MPa at temperatures between 25 °C and 250 °C at both plane stress and plane strain conditions. The temperature dependency was very small in this temperature range. T_(upper) was almost equivalent to the cross-over temperature between σhydridef and the ultimate tensile strength (UTS), which suggests that, at temperatures above T_(upper), the zirconium matrix would undergo ductile fracture before the stress in the hydride is raised above σhydridef, since UTS is smaller than σhydridef.