Ductile-to-brittle transition behavior and mechanism of Cr-coated Zr-1Nb alloys after 1000–1200 °C steam oxidation

Mechanical properties, particularly ductility, are crucial for evaluating the safety threshold and limits of Cr-coated zirconium claddings at loss of coolant accident (LOCA). The mechanical properties of Cr-coated Zr-1Nb alloy plates after 1000–1200 °C steam oxidation are investigated through uniaxial tensile tests at 135 °C, with uncoated specimens as comparison. The results show that the ductile-to-brittle transition of Cr-coated samples is significantly affected by oxidation temperatures. After 2 h oxidation at 1000 °C, Cr-coated samples remain good ductile with 16.2 % elongation compared to 0.4 % for uncoated ones. While, Cr-coated samples exhibit brittle fracture with 0.2 % elongation after 2 h oxidation at 1150 °C, and the tensile strength diminishes to 321 MPa, a reduction of 18 % compared to before oxidation. The ductile-to-brittle transition is associated with the protective structure evolution of Cr coating, which essentially determines the formation of brittle α-Zr(O) in Zr substrate. At 1000 °C, good oxygen diffusion barrier of Cr coating significantly reduces α-Zr(O) layer formation. However, accelerated oxidation of Cr coating results in the generation of α-Zr(O) phase. The critical α-Zr(O) intrusion depth for brittle fracture is about 85 % of Zr substrate, and the content of O and Cr increased by 0.96 % (wt.) and 1.84 % (wt.), respectively. Formation of brittle α-Zr(O) dominates the ductility degradation, and O and Cr element diffusion into substrate also have a certain effect. The findings can provide insights for performance degradation modeling of Cr-coated Zr alloys under LOCA conditions.