Mechanism for Z-phase formation in 11CrMoVNbN martensitic heat-resistant steel

Abstract

The mechanism for the formation of Z-phase was investigated for samples of an 11CrMoVNbN steel aged at 593 degrees C for up to 50,000 h. X-ray diffraction indicates that Z-phase appears after 5000 h of aging, and its amount gradually increases up to 50,000 h aging at the expense of mainly Cr2N. Transmission electron microscopy shows that Z-phase nucleates in the V-enriched rim region of Cr2N precipitates, which forms due to the diffusion of V into the precipitates from the matrix. Nucleated Z-phase tends to grow relatively rapidly compared with the preexisting precipitates such as Nb(C,N) and M23C6. Z-phase seems to consume Cr2N by the nucleation and growth mechanism rather than the transformation mechanism. The main difference in the aged samples is that the Nb content of Z-phase is lower and that Z-phase does not replace Cr2N completely, compared to the results of the crept samples.