Revealing the mechanism for enhancing the creep property by adding Ta/Zr elements in RAFM steel: Experimental and modeling study

Abstract

A continuum damage mechanics (CDM) creep model was developed based on the microstructure, which could precisely delineate the evolution of mobile dislocations, dipole dislocations, boundary dislocations, and martensitic laths in the RAFM steel during creep process. The addition of Ta/Zr elements promoted the precipitation of MX carbide particles, which could pin the mobile dislocations, and restrain the transformation of mobile dislocations into dipole dislocations, thereby slowing the decrease in statistically stored dislocation (SSD) density during creep. A large number of fine MX and M₂₃C₆ carbide particles sourcing from the addition of Ta/Zr elements could effectively delay the reduction in geometrically necessary dislocation (GND) density, and restrict GNDs transforming into sub-grain boundaries. By manipulating single-factor variables, the increase in precipitate damage factors strongly affected the steady creep stage and accelerated creep stage, especially for the precipitate damage factor of M₂₃C₆, which significantly accelerates the onset of the accelerated creep stage. The higher coarsening rate of M₂₃C₆ in RAFM steel without Ta/Zr was one of the reasons for its premature creep failure, as comparing with RAFM steel with Ta/Zr. During short-term (< 1000 h) creep, fine Laves phase functions similarly to M₂₃C₆ particles, serving the purpose of precipitation strengthening. In the intermediate-term (< 10,000 h) creep process, the Laves phase undergoes a certain degree of coarsening, but the coarsening-induced cavities damage is still not the primary cause of creep fracture. Thus, it was inferred that the depletion of W elements in the matrix sourcing from the coarsening of Laves phase is the main reason for the premature creep failure. In the intermediate-term creep of RAFM steel, the ability of Ta/Zr elements to significantly reduce the coarsening rate of Laves is a key factor for contributing to the significant extension of creep rupture time for RAFM steel.