Phase equilibria in Fe-Ni-Nb ternary system at elevated temperatures have been examined, in order to identify the two-phase region of γ-Fe (austenite) and ε-Fe2Nb (C14). The ε single phase region exists in the range of 27.5 to 35.5 at.% Nb in the Fe-Nb binary system, and it extends toward the equi-niobium concentration direction up to 44 at.% Ni in the ternary system at 1473 K, indicating that more than half of the Fe atoms in Fe2Nb can be replaced with Ni. Thus, the γ+ε two-phase region exists extensively, and the solubility of Nb in γ phase increases from 1.5 to 6.0 at.% with increase in Ni content. The lattice parameters of a and c in the C14 Laves phase decrease with increasing Ni content. The change in a axis is in good agreement with calculation based on Vegard's law, whereas that of c axis is much larger than the calculated value. The result suggests that atomic size effect is responsible for a-axis change and the binding energy is dominant factor for the c-axis change. To extend these findings to development of new class of austenitic steels strengthened by Laves phase, an attempt has been made to control the c/a ratio by alloying. The addition of Cr is effective to make the c/a ratio close to the cubic symmetry value (1.633).

Phase equilibria in Ni-Nb-Fe ternary and Ni-Nb-Fe-Cr quaternary systems at elevated temperatures have been examined. In the ternary system. NijNb-6 (DO;,) single phase field expands along the equi-niobium concentration up to 10% Fe at 1473 K. The 6 phase becomes in equilibrium with (Fe. Ni),Nb-r (C14 Laves) phase with a composition of Ni-30Fe-26Nb and y phase with its composition of Ni-35Fe-1ONb. A new intermetallic phase is found at compositions around Ni-22Nb-20Fe. and the crystal structure is identified to be ordered hexagonal (hP24 with stacking sequence of abcbcb). This phase is in equilibrium with y and 6 phases. and no 6+y+r three-phase coexisting region exists in this system. In Fe-2OCr-(25-30)Ni-(2-6)Nb quaternary system, four intermetallic phases of 6 . C14 Laves (r), C15 Laves (Cr,Nb) and (Ni. Fe),Nb,-p phases are found to exist in y matrix, and the isothermal tetrahedron at 1373 K in this system is constructed. The y-Ni solid solution phase becomes in equilibrium with C 14 Laves phase in Ni poor side (less than 30%). whereas it is in equilibrium with C15 Laves phase in Ni rich side (-40%). At the very limited region between them, there is a small window opened where the y phase can be in equilibriunl with p phase. However, at 1073 K, only 6 and C14 phases become in equilibrium with y phase. The changes in the phase equilibria with temperature is caused by the occurrence of two transition peritectoid-type reactions: (I) y + C 15 +