Resistance to embrittlement in ferritic/martensitic steel with TaWVCr coating in oxygen-saturated lead‑bismuth eutectic at 350 °C

Ferritic/martensitic (F/M) steels are potentially applied as structural materials in the lead cooled fast reactor, a promising fourth-generation reactor that adopts lead bismuth eutectic (LBE) as coolant. However, the F/M steels normally suffer from a LBE sensitivity, showing a liquid metal embrittlement when exposed to the LBE. Here in present work, TaWVCr complex concentrated alloy coating was deposited by using magnetron sputtering to protect the F/M steels from liquid metal embrittlement. For this purpose, 12Cr F/M steel coated and uncoated TaWVCr were tested in a static oxygen-saturated LBE at 350 °C by corrosion testing and slow strain rate tension testing, respectively, for comparison. The TaWVCr-coated 12Cr F/M steel exhibited a tensile elongation of ∼19.6 %, over three times of the uncoated 12Cr F/M steel one (< 5.1 %). It was demonstrated that the TaWVCr coating, even in thickness of about 5 μm, is effective in securing the F/M steel against embrittling. Comprehensive microstructural characterizations were conducted, to reveal the mechanisms responsible for embrittlement resistance. It was revealed that the embrittlement of 12Cr F/M steel is primarily associated with cracking of the surface oxide scale, which propagates into the matrix and hence provides a channel for LBE to wet the matrix. On the contrary, the TaWVCr coating substantially preserves the 12Cr F/M steel from being punctured by oxidation cracks and concomitantly reduces the sensitivity to liquid metal embrittlement. As a result, the fracture pattern changes from a brittle fracture to ductile fracture with the introduction of TaWVCr coating.