Mechanical property and cracking mechanism of Cr-coated Zr alloy under tensile and compressive loading

Abstract

Cr coatings are considered as the most promising candidate materials for accident-tolerant fuel cladding coatings. In combination with optical microscopy, focus ion beam, energy dispersive spectrum and universal testing machine, the effect of Cr coating on hoop tensile and compressive mechanical properties of Zr alloy was studied and the underlying cracking failure mechanism of Cr coating was revealed. The results indicate that Cr coating slightly increases tensile/compressive yield strength and tensile strength compared with those of uncoated Zr alloy. This is attributed to high hardness/elastic modulus of Cr coating. Interrupted hoop tensile and compressive tests show that Cr coating cracking concentrates in the range of 0-30°. Further, crack initiation of the coating occurs either at the surface or at the interface between Cr coating and the matrix. Compared with the former, the latter is dominant cracking mode, irrespective of tensile and compressive loading. In lights of the advanced microstructural analysis, one possible reason, i.e., the existence of nano-scaled Al-enriched loose layer between the interface, in which nano-cracks and voids exist, is proposed to explain the interfacial cracking. The result indicates that the optimization of the interfaces can improve the resistance of coating failure. In addition, crystallographic study demonstrates that the crack propagation in the Cr coating is along the high-angle grain boundaries with the misorientation of 50°.