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

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 withγ-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 C14structure，and the Laves phase can homogeneously precipitates in theγmatrix 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_&lt；22&gt；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}和{101}due to its tetragonality(a_&lt；DO22&gt；&lt；a_&lt；Al&gt；&lt；c_&lt；DO 22&gt；)。We identified the formation mechanism and its composition region of MVS。The morphology of the Al/DO_&lt；22&gt；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.

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.