The challenge of acquiring a satisfactory EBSD result of CWSR Zircaloy-4 cladding tube

Abstract

This work presents a methodology combining SEM, EDS, conventional EBSD, and transmission-EBSD to analyse a recrystallised Zircaloy-4 sheet and cold-worked stress-relieved (CWSR) Zircaloy-4 cladding in unprecedented detail. Second-phase precipitates (SPPs) in Zircaloy-4 specimens were revealed after chemical polishing using a solution containing hydrofluoric acid (HF). Pitting corrosion of Zircaloy-4 specimens was revealed after electropolishing using an electrolyte containing HClO₄ . A zirconium coupon without SPPs was used to confirm the chemical response of SPPs on surface morphology. Intrinsic features of cold-worked Zircaloy-4 such as relatively small grain sizes, high dislocation density, and complex microstructure make it significantly more difficult to collect excellent EBSD results compared to recrystallised Zircaloy-4. The fine hydride structure of as-hydrided CWSR Zircaloy-4 cladding further increases the level of challenge on EBSD analysis.

Lay description: We present a methodology combining multiple microscopic methods to analyse a recrystallised Zircaloy-4 sheet and cold-worked stress-relieved (CWSR) Zircaloy-4 cladding, important alloys of structural materials widely used in nuclear application, and emphasis on the challenge of acquiring a satisfactory electron backscatter diffraction (EBSD) result of CWSR Zircaloy-4 cladding material in great details. EBSD is a powerful technique to characterise the crystallographic distribution and lattice type of conductive crystalline materials, especially for a highly textured material like CWSR Zircaloy-4 alloy. However, zirconium alloys are known to be one of the most difficult materials to prepare for EBSD characterisation. We point out that the configuration of the microstructure of the specimen cause the challenge in the EBSD sample preparations. Moreover, the occurrence of tiny zirconium hydride precipitates in Zircaloy-4 increases the difficulty. We believe that the information of the EBSD sample preparation related results in this paper can provide researchers and scientists in this community a useful reference to speed up the EBSD sample preparation of CWSR Zircaloy-4 cladding material and to expect the corresponding EBSD results.

Keywords: CWSR Zircaloy-4; EBSD; electropolishing; grain size; zirconium hydride.