EXPERIMENTAL AND COMPUTATIONAL STUDY ON GRAIN BOUNDARY AND GRAIN INTERIOR PRECIPITATION OF FE_2NB LAVES PHASE IN FE-CR-NI-NB AUSTENITIC HEAT RESISTANT STEELS

摘要

The precipitation behavior of Fe_2Nb Laves phase (C14 structure) on grain boundary (GB) and grain interior (GI), and Ni_3Nb metastable γ" phase and stable 8 phase on GI in the austenitic heat-resistant model steels is studied experimentally in different temperatures, and it is reproduced by thermo-kinetic calculation. The steel studied was prepared by arc melting, and cold rolled by 65%. Then, the sample was solution treated at γ single phase region, in order to control the grain size to approximately 150 μm. Solution-treated sample was subsequently aged at 973 K to 1473 K for up to 3600 h. Microstructure was observed by FE-SEM and the chemical composition of y matrix and precipitates of Laves and δ phase was analyzed by EPMA. The precipitation modeling was performed using MatCalc software. The thermodynamic database constructed in our research group was used for the calculation of chemical potential of each phase. Classical nucleation theory was used for the nucleation, and SFFK model was used for the growth and coarsening stages. Grain boundary and grain interior Laves phase is defined as different phases, and morphology of all precipitates is assumed as spherical in the calculation. Precipitation start time is defined as the time when the precipitate fraction is reached to 1%. Experimental results show that, above 973 K, the Laves phase firstly nucleates on grain boundaries and then nucleates within the grain interior. The nose temperature is located at around 1273 K. In order to reproduce the experimentally determined TTP diagram, first, interaction parameters among elements is modified. Then, by introducing the lower interfacial energy between γ matrix and Laves phase, TTP diagram is reproduced by calculation, suggestion that the interface is relatively stable.