The precipitation of the γ’ (Ll2) and γ" (DO22) phases has been studied in four alloys Fe-Ni-Cr-Ti-Al-Nb containing a higher Ti + Al/Nb ratio than that of the INCONEL 718 alloy. For these alloys, the precipitation microstructure varies rapidly with aging temperature and composition. Bct γ"particles have always been found to precipitate on γ’ phase. Moreover, by aging three alloys above a critical temperature, a “compact ntorphology” has been observed: cube-shaped γ’ particles coated on their six faces with a shell of γ" precipitate. This microstructure has proved to be very stable on prolonged aging. A thermal stability better than that encountered in nominal INCONEL 718 alloy can thus be achieved. The influence of composition and aging temperature on the conditions that bring about this “compact morphology” has been investigated. A minimal Ti + Al/Nb ratio between 0.9 and 1 has been determined, allowing the “compact morphology” to be obtained.

Morphology of y’ and y” precipitates and thermal stability of inconel 718 type alloys

Scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD) were employed to study the μ phase in a directionally solidified experimental nickel base alloy. It was observed that the μ phase distribute at interdendritic area and precipitate mainly from the γ' phase in this experimental alloy. Orientation relationship between the y' and the μ phase was found as (111) γ ' ∥ (0001) μ and (110) γ ' ∥ (1120) μ . Detrimental effect of the μ phase on oxidation resistance of the alloys was also found.

https://www.jstage.jst.go.jp/article/matertrans/46/1/46_1_54/_pdf

μ Phase in a Nickel Base Directionally Solidified Alloy

The structure of tetrahedrally close-packed phases (known also as Frank-Kasper phases) in superalloys has recently been extensively studied at the atomic level by high-resolution electron microscopy (HREM) in the Laboratory of Atom Image of Solids, Academia Sinica. In addition to a magnitude of domain structures, no less than six new phases have been found. This is a resume of the study. First, a brief description of the crystal structures of these phases is given to illustrate the capability of HREM in revealing the building of complex structures from simple structural units. Secondly, the crystallographic features of the new phases and microdomain structures are discussed. Finally, the principle of juxtaposition of parallel oriented coordination polyhedra in these tetrahedrally close-packed phases is presented and this not only explains the existing structures of such phases but may also be important in the study of the quasi-crystalline state in rapidly quenched amorphous alloys.

Tetrahedrally close-packed phases in superalloys: new phases and domain structures observed by high-resolution electron microscopy

Planar faults occur abundantly on (001), (110) and (11 1) planes in the C14 Laves phase precipitated in a Fe-based superalloy, and these faults will divide the latter into many parallel domains. The structure of the domain boundaries has been studied by HREM and no disorder has been found. Pentagonal antiprisms are reorganized at these boundaries, forming other tetrahedrally close-packed phases, such as differently oriented μ or C phases on (110) and (11 1) boundaries. The domain boundary structure has been discussed from the point of view of the intergrowth of various tetrahedrally close-packed structures with juxtaposed pentagonal antiprisms.

Domain structures of tetrahedrally close-packed phases with juxtaposed pentagonal antiprisms II. Domain boundary structures of the CI4 Laves phase

Phase equilibria in the Fe–Al–Mo system - Part I: Stability of the Laves phase Fe2Mo and isothermal section at 800 °C

To investigate crystal structure and two-dimensional faults of the precipitates in Fe-Mo alloys, electron microscopic method was used. The crystal structure of the precipitates in Fe-Mo alloys aged above 1000°C was found to be rhombohedral, while it was generally hexagonal in the alloys aged bellow 900°C. Both kinds of precipitates were formed in approximately the same amount when aged at 940°C. The precipitates of both structures were found to have numerous two-dimensional faults. The faults were found on {00.1}hex, {10.1}hex and {10.0}hex planes in the precipitates of the hexagonal structure and on {111}rhomb and {100}rhomb planes in the precipitates of the rhombohedral structure. In the hexagonal precipitates in the Fe-W alloy aged at 950°C, faults were observed also on {00.1}hex and {10.1}hex planes.

Kōhei Yagisawa

Papers

Fine Faults and Crystal Structures of Precipitates in Fe-Mo and Fe-W Alloys