Phase-Field Simulation Study on Phase Decomposition Behavior of U-Nb Alloys

The phase decomposition of U-Nb alloys exhibits significant microstructure changes with composition, aging temperature, and holding time. To study the physical mechanism of complex phase decompositions behavior of U-Nb alloys, systematic phase-field simulations were conducted. The results show that continuous and discontinuous precipitations may have the same thermodynamic condition. When the volume diffusion is inhibited and the interface diffusion plays a leading role, the phase decomposition is more inclined to discontinuous precipitation. It is speculated that the obvious difference of continuous precipitates between U-5Nb alloy and U-13Nb alloy is caused by different phase transformation mechanisms. U-5Nb alloy exhibits typical continuous precipitation, while U-13Nb alloy first undergoes miscibility gap decomposition within the γ phase, followed by the precipitate of α phase. The free energy relationship of γ phase in the middle Nb content range has an important influence on the occurrence of miscibility gap decomposition and the composition of the Nb-rich phase in the discontinuous precipitation product.