Co-segregation of Y and Zr in W-Cr-Y-Zr alloys: First-principles modeling at finite temperature and application to SMART materials

Spinodal phase separation in SMART (Self-passivating Metal Alloys with Reduced Thermo-oxidation) materials based on binary W-Cr with alloying elements Y and Zr is systematically investigated by a combination of Density Functional Theory with Cluster Expansion Hamiltonian and large-scale Monte Carlo simulations with thermodynamic integration. Comparing alloys of Zr with those of Y in ternary compositions with W and Cr, it is shown that there is a significant difference in the average short-range order between W and Zr, which changes from positive to negative from 500 K to higher temperatures in W70Cr29Zr1 alloys compared with the positive SRO (Short-Range Order) between W and Y in W70Cr29Y1 alloys at all temperatures. This change, however, does not affect the segregation behavior between W and Cr in the whole range of temperature between these alloys. Importantly, it is found that co-segregation of Y and Zr due to the negative SRO between them, in W70Cr29Y0.5Zr0.5 and W70Cr28Y1Zr1 considered alloys, increases the W-Cr positive SRO closer to the binary W70Cr30. Our modeling results reveal a significant impact on spinodal phase segregation of W and Cr in designing SMART materials for DEMO devices of future fusion power plants. Findings from recent experimental studies validate the modeling results, particularly the high-temperature behavior of the W-Cr-Zr alloys and the microstructure evolution of the W-Cr-Y-Zr alloys. The results provide a fundamental understanding of the radiation resistance phenomena observed in recent experiments on neutron-irradiated self-passivating alloys.