Benefit or harm of accident tolerant coatings on the low-cycle fatigue properties of Zr-4 cladding alloy: in-situ studies at 400°C

Abstract The present work is the first study investigating the effects of accident tolerant coatings on the low-cycle fatigue life of the Zr-4 alloy. Zr-4 alloy with different accident tolerant coatings, i.e., a Cr coating or a TiCrAlN coating by multi-arc ion plating, were studied at 400°C by in-situ fatigue testing with a scanning electron microscope (SEM). It was surprisingly found that the Cr coating improved the fatigue life of the Zr-4 alloy significantly, whereas the TiCrAlN coating decreased the fatigue life dramatically. The fatigue life variation was closely related to the ratcheting strain of each type of sample in this study. Further, in-situ SEM observations indicated that the Cr coating exhibited a good capacity for plastic deformation with the Zr-4 substrate until the late stage of fatigue life. In contrast, for the TiCrAlN coating, multiple cracks formed during the early stage of fatigue life. The distinct effects of the two coatings were confirmed by analysis of the fracture surfaces and longitudinal section microstructure. Hence, variations in the fatigue life of the coated Zr-4 samples was attributed to the cyclic deformation and cracking behavior of each coating. Finite element simulations based on the cohesive element method indicated that the pre-failure of the brittle TiCrAlN coating (with its low fracture toughness) promoted early crack initiation in the Zr-4 substrate, leading to a short fatigue life. For the Cr coating (with its high fracture toughness and good plasticity), the crack initiation in the substrate was postponed, leading to a longer fatigue life.