Temperature-dependent deformation and cracking behavior in Cr coating for accident tolerant fuel cladding: An in situ SEM study

Chromium (Cr)-coated zircaloy has been regarded as a promising candidate for developing accident tolerant fuel (ATF) claddings. The deformation and failure of Cr coatings under tensile stress were studied by a novel in-situ SEM testing technique, which indicated that the plastic deformation and cracking of Cr was remarkably temperature-dependent. It is shown that Cr coating experienced a brittle-to-ductile transition with temperature increasing from 25 degrees C to 500 degrees C. The tensile strength kept decreasing with temperature, but the elongation to rupture showed an evident valley value at about 450 degrees C, which approximately corresponded to the brittle-toductile transition temperature (BDTT) in this study. At 25 degrees C, the coating failed in a brittle manner displaying multiple dense channel cracks on the coating surface and cleavage facets on the facture surface. As the temperature increased, the onset of cracking was delayed and the saturated crack density declined in Cr coating. At temperatures near the BDTT, channel cracks and separated cracks coexisted in Cr coating. Under severe plastic deformation, the grain morphologies were found to change from columnar to equiaxial, indicating dynamic recrystallization had occurred locally in Cr coating. At temperatures above the BDTT, Cr coating deformed and failed in a complete ductile manner. Both the strength and ductility of the coated Zr-4 sample were improved compared with those uncoated, indicating an enhancement effect of Cr coating at temperatures above the BDTT.