The effect of cold work on the transformation kinetics and texture of a zirconium alloy during fast thermal cycling

Abstract

The effect of cold-work on the transformation kinetics and texture evolution after fast temperature cycling were studied in a dilute zirconium alloy. It was found that cold-work delays the onset of phase transformation and helps randomise the texture after the transformation. Samples of Zircaloy-4 in two conditions, cold-rolled to 70% reduction and fully recrystallised, were heated above the β-transus at a fast rate of 100 °C s−1 using resistive heating and without constraint. Electrical resistivity was used to measure the phase fraction during heating and electron back-scatter diffraction was used to measure the texture before and after the thermal cycle. Whereas previous work on titanium suggested that cold-work leads to texture strengthening after transformation, these new experiments show that, when the heating rate is fast enough, recrystallisation is incomplete on heating, which slows down the start of transformation, leading to a random α-texture after cooling. The texture in the recrystallised material after β heat-treatment is non-random but different from the start, a consequence of both a stronger β-texture after grain growth and stronger variant selection on cooling. This variant selection can be mostly explained by α-nucleation at special β-grain boundaries. These results demonstrate that prior deformation has a strong effect on the textures produced after β-heat treatment when heating rates are fast. This has implications for the anisotropy of nuclear cladding components after loss-of-coolant (LOCA) and reactivity-initiated accidents (RIA) but also more widely for the manufacturing of zirconium and titanium using welding or additive layer manufacturing where the heating rates are high.