M <sub>23</sub>C <sub>6</sub>-Assisted Laves Phase Growth Mechanism in a Novel 9Cr-3W-3Co-1CuVNbB Steel Under Creep Deformation

Abstract

Laves phase particles are an important precipitate in tempered martensite ferritic steels that can strongly affect the microstructure and mechanical properties. We systematically investigated the evolution of Laves phase in G115 steel. Experimental results showed that the amount of Laves phases increases, whereas that of M23C6 carbides decreases as creep deformation proceeds. Further characterizations were conducted to thoroughly explore the evolutionary mechanism of Laves phase. Interestingly, a hybrid Laves-M23C6 particle was observed, and Cr segregation occurred near the remaining M23C6 carbides. Further, we found that C and Cr were also segregated near a Laves phase that was formed. The Cr concentration of the Laves phase was much lower than that of M23C6 carbide, but slightly higher than that of the ferrite matrix. The result confirms that Laves phase is transformed from M23C6 carbide, and it can also explain reasonably well the decrease in M23C6 carbide during creep deformation. We also proposed a M23C623C6 carbides, which is considered to be an interface-controlled M23C623C6-assisted Laves phase growth model was established assuming that the Gibbs energy is balanced at the M23C6/Laves phase interface. Furthermore, a guideline for development of novel tempered martensite ferritic steels was proposed. Decreasing the Si content of the material can effectively postpone Laves phase nucleation. In addition, the fraction of the Laves phase decreases, which helps improve the creep strength.