Microstructure Evolution of Fe-Ni-Based Alloy HR6W during Isothermal Aging

The age-hardening behavior of the Fe-Ni-based alloy HR6W was investigated in the temperature range between 973 K and 1073 K. A two-step increase of hardness was detected for the alloy at every aging temperature; the first increase of hardness results from the precipitation of M23C6 carbides, and the second increase corresponds to precipitation of the C14-Fe2W Laves phase. The time-temperature-precipitation diagram for the alloy was established on the basis of the results of hardness measurements and microstructure observations, where the precipitation of the C14-Fe2W Laves phase was slower than that of the M23C6 carbides by three orders of magnitude and the nose temperature of the Laves phase was greater than 1073 K. The M23C6 carbides precipitated with a plate-like morphology along grain boundaries at the early stage of aging, followed by the precipitation of the C14-Fe2W Laves phase with a granular morphology with increasing aging time. The M23C6 carbides and C14-Fe2W Laves phase are aligned under the stress condition because of their precipitation on the dislocations introduced during creep deformation.