Precipitation behavior of Laves phase in 10%Cr steel X12CrMoWVNbN10-1-1 during short-term creep exposure

Precipitation behavior of Laves phase in steel X12CrMoWVNbN10-1-1 during short-term (<= 400 h) creep exposure was investigated using a transmission electron microscope (TEM) and an ultra high resolution thermal field emission scanning electron microscope (UHR-FESEM). The results demonstrate that Laves phase precipitates are detected in the creep specimens exposed for such a short-term of 400 h at 600 degrees C and 204 h at 650 degrees C. Stacking faults are observed in them and the selected area electron diffraction patterns of the phase are characteristic of streaked. There are two mechanisms of the nucleation and growth for Laves phase, one of which is that Laves phases occur alone on martensite lath boundaries and they are coherent with a grain but grow into the adjacent grain which has no rational orientation relationship with them. The other one is that Laves phases are formed in the regions adjacent to M23C6 particles, and then they grow at the expense of the Cr-rich M23C6 carbides in close vicinity. It is established that the former one is the dominant formation mechanism for Laves phase in steel X12CrMoWVNbN10-1-1. Laves phases prefer to locate on the prior austenite grain boundaries and the martensite lath boundaries. The mean diameter and the number density of the phase show significant change with temperature, but slightly change with stress. Moreover, it is proved that the formation of Laves phase in this steel is a thermodynamically possible process. The nucleation process has not finished after heat-treatment and the number of nuclei for Laves phase formed in the steel decreases with raising the temperature. (C) 2010 Elsevier B.V. All rights reserved.