Formation Mechanism of Laves Phase with Two Morphologies in a Novel 9Cr-3W-3Co-1CuVNbB Steel During Creep Process

Coarse Laves precipitates can strongly deteriorate the creep rupture strength of tempered martensite ferritic heat-resistant steels. In this study, we demonstrated two independent paths, i.e., isolated at the interface and neighboring phase-assisted, of Laves phase precipitation during the creep process in G115 steel. In particular, blocky-like Laves phase particles adjacent to the M23C6 and Cu-rich precipitate (CRP) are firstly captured. Most of the Laves phases are found to precipitate at the high-angle grain boundaries (HAGBs) whereas only a small amount precipitate at the low-angle grain boundaries (LAGBs). Among them, the M 23 C 6 are more preferential nucleation sites for the Laves phase. Crystallographic analysis indicates that the Laves phase obeys a specific orientation relationship (OR) with the M23C6, i.e., (110)M23C6// (10‾13)Laves and [1‾13] M23C6 // [ 03‾31 ]Laves. Solute segregation at grain boundaries and interphase boundaries (M23C6/ferrite and CRP/ferrite) is expected to be the main cause for the heterogeneous precipitation of Laves phase. The growth of the isolated Laves phase is dominated by the grain boundary diffusion mechanism, while that of Laves phase next to theM23C6 and CRP is controlled by the trans-interface diffusion controlled (TIDC) mechanism, which results in two morphologies of Laves phase. The work may improve the understanding of roles of M23C6 and CRP in precipitation mechanism of Laves phase, and provide a more accurate guide for the development of heat-resistant steels with superior creep strength.