Revealing the Stabilization Mechanism of Coupling Effect of Ta/Zr on M₂₃C₆ and Laves Precipitations in Low-Carbon 9Cr Ferrite/Martensite (F/M) Steels: Experiment and Ab Initio Molecular Dynamics

Through analyzing the evolution behaviors of transition carbides, cementite, M₂₃C₆, and laves phase, then calculating the interface energies of lave/α-Fe interface models (with or without Ta, Zr, and Ta/Zr doped) by ab initio molecular dynamics (AIMD) method, the stabilization mechanism of the coupling effect of Ta/Zr on the stability of M₂₃C₆ and laves (C14-Fe₂W type) precipitations in low-carbon 9Cr F/M steel (9Cr F/M steel) during long-term aging (5000, 10,000, and 20,000 hours) at 600 °C was revealed. The pre-existing carbides before the tempering process significantly influence the size and morphology of M₂₃C₆ carbide during the tempering process. The refinement mechanism for the coupling effect of Ta/Zr on M₂₃C₆ carbide precipitations relates to the dilution effect of carbide and grain boundary on the carbon concentration during the tempering process. But the grain boundary is not the main factor for stabilizing M₂₃C₆ and laves particles during long-term aging process at 600 °C. The initial refinement morphology of M₂₃C₆ carbide is the main ingredient for decreasing the coarsening rate of M₂₃C₆ carbide during long-term aging process. The calculated results of the lave/α-Fe interfacial energies at 873 K (600 °C) reveals that the coupling effect of Ta/Zr enhances the stability of laves at elevated temperatures. This enhancement is attributed to the significant decrease of the lave/α-Fe interfacial energy caused by Zr, while Ta does not contribute to the stability of lave/α-Fe interface at elevated temperatures.