On the nucleation of Laves phase particles during high-temperature exposure and creep of tempered martensite ferritic steels

This paper reports on the formation of an Mo-rich Laves phase during high-temperature exposure and creep of a tempered martensite ferritic steel with 12 wt.% Cr and 1 wt.% Mo. The material was exposed to 550 °C for time intervals between 864 and 81,984 h. For comparison, a few creep tests were carried out at 550 °C and 120 MPa (duration between 864 and 12,456 h). All tests were interrupted after specific time periods and microstructures were investigated using transmission electron microscopy and atom probe tomography. Laves phase formation occurs during both heat treatment and creep. Creep stress and strain have no significant effect on the early stages of Laves phase formation. In the present work we show that prior to Laves phase nucleation Si and Mo segregate to micrograin boundaries, where subsequently Laves phase particles appear next to M23C6 carbides.