Abstract
In a previous paper (Trans. JIM. 3 (1962), P. 154∼163), it was reported that the creep rupture properties of modified iron-base heat-resisting alloys hardened by precipitation were comparable to those of Nimonic 80 A and M-252. In the present investigation, the structural stability at high temperatures of various γ′ precipitation hardening alloys containing Ti 2% and Al 2% was examined. The matrix of specimens was varied from Ni-rich to Fe rich compositions. Forged bars of 16 mmφ were used for age-hardening, electron-microscopic and X-ray diffraction tests. MC, M7C3, M23C6, γ′, σ and β phases were identified by X-ray diffraction tests on the electrolytic extracts from those specimens aged for 1000 hr. High Ni alloys had remarkable age-hardenability by the precipitation of γ′ phase. On the other hand, low-Ni high-Fe alloys had no remarkacle age-hardenability and showed an overaging phenomenon after a long time aging treatment. The deterioration in age-hardenability in these low Ni high Fe alloys is caused by the precipitation of massive β phases. The structure of β phase is a body-centered cubic, CsCl, B2 type, and is isotypic with NiAl in the Ni-Al binary system. Nickel can be partially substituted by iron or cobalt and aluminum by titanium, so that the general composition of β phase is probably (Ni, Fe) (Al, Ti) or (Ni, Fe, Co) (Al, Ti). By studying pseudo-ternary systems of Fe-Ni-Cr-Co at 732°C and 815°C, it was made possible to determine the matrix composition regions in which the β phase was precipitated. Various intermetallic compounds in quarternary systems of NiAl-NiTi-FeTi-FeAl and NiAl-NiTi-CoTi-CoAl were synthesized by the vacuum-arc melting technique, and these synthetic β phases demonstrated to be crystallographically analogous to the precipitated β phases. The behavior of the precipitated β phase was investigated by determining the pseudo-quarternary diagrams of the Ni-Fe-Al-Ti and Ni-Co-Al-Ti systems.
