Ziyan Pan’s research while affiliated with Sun Yat-sen University and other places

In this study, the fracture mechanisms of Cr-coated Zr4 alloy samples were studied by in-situ tensile testing with high-resolution observations. Both original sample and pre-oxidized sample were studied to study the effects of pre-oxidation on the cracking and failure behavior. For the Cr-coated Zr4 sample, with the increase of tensile strain, multiple surface cracks were dominant and less interfacial cracks were formed, indicating good interfacial strength of Cr coating. For the pre-oxidized samples, there was a thin oxide layer formed on the Cr coating surface, revealing improved oxidation resistance and protection effects. However, a brittle ZrCr 2 diffusion layer was formed in the same while at the Cr/Zr4 interface underneath the Cr coating, which would lead to earlier micro-cracks formed under tensile stress and evidently degrade the interfacial strength. The findings in the study indicated the importance of optimizing coating microstructure in future study to avoid forming the above-mentioned brittle diffusion interlayer and the associated premature failure.

In-situ three-point bending tests and finite element modeling based on the cohesive zone model were developed to study the stress evolution and cracking behavior of the Cr coated Zr-4 alloy for accident tolerant fuel claddings. The initiation and propagation of micro-cracks were captured by in-situ observation and predicted by the numerical simulation. The results showed that vertical cracks first initiated from the coating surface and propagated to the Cr/Zr4 interface. Under larger bending strain, interfacial cracks began to initiate from the vertical crack tips driven by large local stress concentration.

Chromium (Cr)-coated zirconium alloys have been considered as a promising candidate material for accident-tolerant fuel (ATF) cladding for nuclear reactors because of their superior oxidation resistance under accident conditions. However, the oxidation and diffusion behaviours that occur in the Cr coating–Zr substrate system at high temperatures significantly affect the microstructure and mechanical properties of the coating, leading to cracking modes that are distinct from those of the as-deposited coating. To understand the effects of oxidation and inter-diffusion on the fracture mechanisms of Cr-coated Zry-4 alloys, in situ three-point bending tests were conducted in this study. Crack initiation and propagation in the oxidised and vacuum-annealed coatings were observed in real time. The results showed that high-temperature exposure led to recrystallisation of the Cr coating (columnar grains transformed into equiaxed grains), which greatly enhanced the crack resistance of the Cr coating. However, a diffusion-induced intermetallic ZrCr2 layer and an α-Zr(O) layer (which transformed from β-Zr owing to oxygen transportation) formed simultaneously at the coating/substrate interface. The micro-cracks formed in these brittle layers rapidly penetrated all the layers under external load, leading to premature failure of the coated sample.