Laves phase evolution in a modified P911 heat resistant steel during creep at 923 K

The evolution of Fe2(W, Mo) Laves phase in a 3%Co modified P911 heat resistant steel was examined during creep tests at 923 K. The tempered martensite lath structure evolved after heat treatment was characterized by dispersion of MX carbonitrides and M23C6 carbides. Appearance of Laves phase particles was recorded after a creep strain of 1%. The mean size of Laves phase particles increased from 190 to 265 nm with increasing strain to 18%. The Laves phase particles were spaced on various boundaries including low-angle boundaries (LABs) of laths/subgrains, but most of these particles (about 90%) were located on high-angle boundaries (HABs) at all strains studied. The size of Laves phase particles located on HABs was larger and their coarsening kinetics was faster than those precipitated on LABs. It is assumed that the evolution of Laves phase during creep is controlled by the grain boundary diffusion of tungsten and molybdenum.

► Laves phase particles appear mainly on high-angle boundaries.
► Volume fraction of Laves phase increases to saturation during creep for about 3000 h.
► Sizes of Laves phase particles are expressed by power law functions of creep time.
► Grow exponents of 6.7 and 8.3 are obtained for particles located on HABs and LABs.