Alloy Design for Austenitic Heat-resisting Steels Strengthened by Intermetallic Compounds

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This research has focused on the phase equilibria, phase transformations and precipitation behavior of TCP and GCP compounds in Fe-Ni-M(M : transition metals) ternary systems at elevated temperatures, in order to get insight into a new concept for alloy development of austenitic heat resisting steels strengthened by intermetallic phases. As strengthening species, Fe_2Nb Laves(TCP) and Ni_3V(GCP) phases are used.Among Fe-Ni-M ternary systems, Fe_2Nb with C14 crystal structure has an extended composition homogeneity region and it can be in equilibrium with γ-Fe phase (Al). For example, Ni atoms can substitute for more than a half of Fe sublattice, and Zr atoms for twp-third of the Nb sublattice sites. Thus, solid solution alloying controls the lattice parameters of C14 structure, and the Laves phase can homogeneously precipitates in the γ matrix when its lattice parameter ratio (c/a) becomes 1.633. Based on these findings, we proposed a model Fe-Ni-Cr-Nb alloy.The Ni_3V with DO_<22> structure quenched from the high-temperature disordered Al (fcc) phase region consists of an unique multi-variant structure (MVS) with two habits planes of {010}と{101} due to its tetragonality (a_<DO22><a_<Al><c_<DO22>). We identified the formation mechanism and its composition region of MVS. The morphology of the Al/DO_<22> two-phase MVS can be intentionally changed to either "Maze" or "Chessboard", by controlling the misfit. We extended these findings to multi-component system by substituting Ni with Fe and V with Nb, and showed ways to develop austenitic steels strengthened by the intermetallic phases.