Development of Ultrafine Grained Zircaloy-2 by Room Temperature Cross Rolling

Abstract

Effect of change in strain path by cross rolling up to a true strain of 1.89 has been studied in the present work. The Zircaloy-2 was subjected to solutionising heat treatment at 800 °C in argon environment for 2 h and subsequently quenched in mercury prior to cross rolling at room temperature. The fragmentation of near basal grains due to change in strain path is evident from the EBSD micrographs. The dislocation density in the crossrolled alloy increases with true strain as calculated from the XRD and EBSD data and it is found to be 2.806453 × 10¹⁶/m². $\left\{ {10\bar{1}2} \right\}$ extension twins are observed initially up to 25% reduction, with the further reduction in thickness, near basal grains are oriented toward the normal direction. These basal grains undergone fragmentation due to changes in strain path upon cross-rolling as observed from KAM and EBSD images. TEM results of the cross-rolled sample confirm the formation of ultrafine and nanograins in the alloy due to orientation of incidental dislocation boundaries in the direction of macroscopic plastic flow and post-annealing treatment of the deformed alloy. A tensile strength of 991 MPa with 7.5% ductility is observed in the 85% cross-rolled alloy. The cross-rolled alloy upon annealing at 400 °C for 30 min improves ductility to 11%.